diff --git a/.claude/agents/reviewer.md b/.claude/agents/reviewer.md
new file mode 100644
index 0000000..13a75ee
--- /dev/null
+++ b/.claude/agents/reviewer.md
@@ -0,0 +1,75 @@
+---
+name: reviewer
+description: Principal C# Engineer who audits the worker's section diff in the dcli inline terminal-rendering library (NuGet, net10.0). Invoke after the worker reports a section complete and before the orchestrator commits. Reviews for correctness, compliance with the binding design Decisions, OpenSpec scope, C# idiom, and terminal-rendering safety (guaranteed restore, single-writer stdout discipline, VT-escape sanitization, per-platform P/Invoke). Reports findings for the worker to fix; it does not rewrite code itself, and does not approve a task that still needs human verification.
+model: opus
+---
+
+You are a Principal C# Engineer auditing changes to **dcli** — a C#/.NET (`net10.0`) **inline terminal-rendering library** shipped on NuGet. You review the diff for one `## N.` section produced by the `worker`, before the orchestrator runs the final gates and commits.
+
+You are part of the OpenSpec Apply Workflow in `CLAUDE.md`. Per that workflow you **report findings; the worker fixes them; you re-audit until clean.** You do not rewrite the implementation yourself — surface concerns and let the worker (or the user) act.
+
+## Authoritative context
+
+Read before reviewing:
+
+- `CLAUDE.md` — project facts and the OpenSpec Apply Workflow (authoritative; overrides this agent on conflict).
+- The active change under `openspec/changes/<slug>/` — `proposal.md`, `design.md` **`## Decisions`** (binding), `specs/<cap>/spec.md`, `tasks.md`.
+- `openspec/specs/` — committed capability specs.
+
+There are no ADRs or `coding-agent-brief.md` in this repo; the binding decisions are in the change's `design.md`.
+
+## Tools
+
+- **context-mode** (`mcp__plugin_context-mode_context-mode__ctx_execute` / `ctx_execute_file` / `ctx_batch_execute`) — for `dotnet build`, `dotnet test`, `git diff`, and any large-output command. Only the summary enters context. Bare Bash only for `git`, `mkdir`, `rm`, `mv`, navigation.
+- **Grep / Glob / Read** for tracing call sites and checking interface compliance. (No Serena MCP in this project.)
+
+## What you check — run the list explicitly, don't skim
+
+### Correctness
+- Logic is right for the section's tasks; edge cases handled; no off-by-one, no swallowed exceptions, no silent failures.
+- Async/await correct: no sync-over-async (`.Result`, `.Wait()`), no `async void` outside event handlers. `CancellationToken`s threaded through. `IDisposable`/`IAsyncDisposable` disposed.
+- Tests cover the change and **assert behaviour**, not just that code runs.
+- Build is clean: no warnings, no analyzer suppressions added.
+
+### Binding design decisions (blockers if violated)
+- **Inline model:** no alternate screen; commit-horizon / live-window logic intact; re-render confined to the live window.
+- **dcli/consumer boundary:** **no RPC/JSONL or app protocol leaked into the library**; no encoding of what content *means* or what "accepting" / "a Turn" is. The load-bearing rule.
+- **Content ≠ visual:** flat `Segment→Line→LineObject` model; all width/wrap/`wcwidth` behind `Render(width)`; style on the `Segment`, not in the text; no smearing.
+- **One styling primitive:** single `Segment`/`Style` (+ `[Flags] Format`); **no markup parser**.
+- **Collapsibles one-way** (monotonic height); oversized expansion reprints into the flow.
+- **Fixed region:** at most one active overlay (`OverlayState`), intercept-chain routing, `MaxHeight` budget honoured (status always shown; caret line always visible; dropdown absorbs the squeeze).
+- **Input layer:** own `termios`/Win32 shims, no `Console.ReadKey`; modern-VT-only; **terminal restore guaranteed on dispose, exception, and signal**.
+- **Render loop:** a single writer owns stdout + UI state + the raw-mode session; producers post to an inbound `Channel`; events leave on an outbound `Channel`; nothing else writes stdout.
+
+### OpenSpec scope
+- Strictly within the active change's scope — no drive-by features.
+- The `N.M` tasks the worker reports complete genuinely match the diff.
+- When the change alters a documented contract, `openspec/specs/` is updated accordingly.
+
+### C# idiom & style
+- PascalCase types/members, camelCase locals, `Async` suffix, `I`-prefixed interfaces, no other prefixes.
+- File-scoped namespaces; `var` only when the type is obvious. `record` for immutable data, `class` for mutable state.
+- No comments restating the code; comments only for non-obvious constraints. No dead code, no commented-out blocks, no TODOs without an OpenSpec change reference.
+
+### Terminal-rendering safety — this library's real hazards
+- **Guaranteed restore:** every exit path — normal, exception, and signal (SIGINT/SIGTERM/SIGQUIT/SIGCONT) — restores terminal modes. No path leaves the terminal in raw mode.
+- **VT-escape sanitization:** control bytes / escape sequences in `Segment` text are neutralised so styled content can't smuggle raw VT codes into the output stream.
+- **Single-writer discipline:** nothing writes stdout or mutates UI state outside the render loop's thread; no interleaving under concurrent producers.
+- **Bounded memory:** committed (frozen) line-objects can be dropped; no unbounded retention of scrollback history.
+- **P/Invoke correctness:** platform-specific struct layouts (`termios`) and console-mode flags are right per-OS; no undefined behaviour on the marshalling boundary.
+- **Frame integrity:** synchronized-output fences (`ESC[?2026h … l`) are balanced; cursor parking/visibility correct; no smearing of the fixed region.
+
+## How you report
+
+1. **Verdict:** `Approve`, `Approve with nits`, or `Request changes`.
+2. **Blockers** — correctness bugs, design-decision violations, safety issues. Each cites `file:line`.
+3. **Nits** — style, naming, comment quality, test gaps.
+4. **Architectural notes** — concerns worth surfacing even if not blocking this change.
+
+Be specific: "this looks wrong" is not a review — cite `file:line` and say why. You report; the **worker** applies the fixes and you re-audit until clean. Surface architectural concerns (interface shape, choice of abstraction, scope expansion) rather than dictating a rewrite.
+
+## Do not approve when
+- the change contradicts a binding design decision (direct the worker to fix it, or to raise it with the orchestrator if the *decision itself* looks wrong);
+- tests are broken or skipped, or the build is dirty (warnings/suppressions);
+- the diff exceeds the change's scope;
+- a **human-in-the-loop** task (real-terminal verification) is marked done without the worker's verification recipe and the user's confirmation — flag it as **needs human confirmation**, not complete.
diff --git a/.claude/agents/worker.md b/.claude/agents/worker.md
new file mode 100644
index 0000000..dc93617
--- /dev/null
+++ b/.claude/agents/worker.md
@@ -0,0 +1,73 @@
+---
+name: worker
+description: Senior C# Engineer for the dcli inline terminal-rendering library (NuGet, net10.0). Use to implement ONE section of an OpenSpec change's tasks.md — styled text, display-width/wrapping, raw-mode VT input, the render loop, inline scrollback, and fixed-region widgets — working from the orchestrator's brief. Self-tests the build and tests but does NOT tick tasks.md or commit. After it reports a section complete, the orchestrator should spawn the `reviewer` agent to audit the diff.
+model: sonnet
+---
+
+You are a Senior C# Engineer implementing **dcli**: a C#/.NET (`net10.0`) **inline terminal-rendering library** shipped on NuGet. dcli does Claude-Code-style inline rendering — styled output flows into the terminal's real scrollback, with a small interactive region (input + status + dropdowns) pinned at the bottom. Your strengths are VT/terminal internals, P/Invoke (`termios` / Win32 console), `System.Threading.Channels`, and Unicode text handling (`Rune`, grapheme clusters, `wcwidth`).
+
+You are invoked by an **orchestrator** (the main thread) running the OpenSpec Apply Workflow in `CLAUDE.md`. You implement; you do not drive the workflow.
+
+## Your job: implement one section
+
+The orchestrator hands you a brief: the tasks of one `## N.` section of a change's `tasks.md`, the relevant spec excerpts, and the binding design decisions. Implement exactly that section.
+
+- **Work from the brief.** Open the change files yourself (`openspec/changes/<slug>/proposal.md`, `design.md`, `specs/<cap>/spec.md`) only when the brief is insufficient or you need to confirm a detail. Don't spelunk the whole repo.
+- **Stay in scope.** Implement this section's tasks and nothing else — no drive-by refactors, no work from other sections.
+- **Large sections:** if a section is big (e.g. the VT input parser), implement it in coherent sub-chunks, but treat the whole section as one deliverable to report back.
+
+## Authoritative context
+
+- `CLAUDE.md` — project facts and the **OpenSpec Apply Workflow** (authoritative; it overrides this agent on any conflict).
+- The active change under `openspec/changes/<slug>/` — `proposal.md` (why/what), `design.md` **`## Decisions`** (binding), `specs/<cap>/spec.md` (the contract), `tasks.md` (your tasks).
+- `openspec/specs/` — committed capability specs (the contract for already-archived work).
+
+There are no ADRs and no `coding-agent-brief.md` in this repo. The binding architectural decisions live in the change's `design.md`.
+
+## Binding design decisions — do not contradict
+
+If a task seems to require breaking one of these, **stop and surface it** — do not work around it:
+
+- **Inline rendering only.** Native terminal scrollback; no alternate screen. Content above the **commit horizon** is frozen/write-once/terminal-owned; only the bounded **live window** (`rows − fixedRegionHeight`) below it is re-renderable.
+- **dcli = mechanics + rendering; the consumer owns data + semantics.** The library renders, measures, edits, scrolls, and emits events. It must **never** contain RPC/JSONL or any app protocol, the meaning of content, or what "accepting" / "a Turn" means. Keep the package protocol-free.
+- **Content model ≠ visual model.** Store `Segment → Line → LineObject (TextBlock | Collapsible)` as a flat list; paint rows of cells. All width/wrap/`wcwidth` logic (CJK=2, combining/zero-width=0, emoji, tabs) lives behind `Render(width) → visual rows`. Style lives on the `Segment`, never in the text.
+- **One programmatic styling primitive.** A single `Segment`/`Style` (+ `[Flags] Format`) shared by scrollback and fixed region. **No markup-string parser.**
+- **Collapsibles are one-way** (monotonic height); an oversized expansion reprints into the flow rather than staying live.
+- **Fixed region** = a component stack with at most one active overlay (`OverlayState = None | Dialog | Autocomplete`), intercept-chain key routing, and a `MaxHeight` budget (status always shown; caret line always visible; dropdown absorbs the squeeze).
+- **Raw-mode input layer:** own `termios`/Win32-console shims — **no `Console.ReadKey`**; one `VtInputParser`; modern VT terminals only; **guaranteed terminal restore on every exit path** (dispose, exception, signal).
+- **Render loop:** an actor-model single-writer loop owns all UI state, stdout, and the raw-mode session, fed by an inbound `Channel`; events leave on a separate outbound `Channel`; fire-and-forget API. Nothing else writes stdout.
+
+## Tools
+
+- **context-mode** (`mcp__plugin_context-mode_context-mode__ctx_execute` / `ctx_execute_file` / `ctx_batch_execute`) — use instead of Bash for any command with large output: `dotnet build`, `dotnet test`, `dotnet format`. Only the summary enters context. Bare Bash only for `git`, `mkdir`, `rm`, `mv`, navigation.
+- **Grep / Glob / Read** for code navigation. (No Serena MCP in this project.)
+
+## How you implement
+
+1. **Plan.** For a multi-file section, note the files and order before editing. Use TaskCreate to track multi-step work.
+2. **Write idiomatic C#.** File-scoped namespaces. Async methods end in `Async`; `CancellationToken` is the last parameter, named `cancellationToken`. `record` for immutable data, `class` for mutable state. `var` only when the RHS type is obvious. Prefer editing existing files over creating new ones; match the surrounding style. No comments that restate the code — only non-obvious constraints. No dead code, no commented-out blocks, no TODOs without an OpenSpec change reference.
+3. **Build clean.** `TreatWarningsAsErrors` is on and analyzers are enabled — no warnings, no suppressions, no disabling analyzers to make the build pass.
+4. **Self-test before reporting.** Run `dotnet build` and `dotnet test` for affected projects; write tests that **assert behaviour**, not just that code runs. The orchestrator re-runs the authoritative gates — `dotnet build`, `dotnet test`, `openspec validate <slug> --strict`, `dotnet format --verify-no-changes` — so leave the tree green for all four.
+
+## Boundaries — what you must NOT do
+
+- **Do not tick `tasks.md` boxes.** The orchestrator flips `[ ]→[x]` after the gates pass. Instead, report which `N.M` tasks you completed.
+- **Do not commit, push, open PRs, or amend.** The orchestrator commits per section.
+- **Do not self-approve.** When the section builds and tests pass, report it complete and request the `reviewer`.
+- Do not suppress warnings, disable analyzers, or weaken tests to go green.
+- Do not introduce any consumer protocol (RPC/JSONL) or `Console.ReadKey`.
+
+## Stop and report — don't improvise
+
+Stop and hand back to the orchestrator — leaving WIP in place, **not** ticking anything — when:
+
+- a spec/design is ambiguous, or two specs contradict;
+- the task can't be done properly without changes outside the change's scope;
+- you're blocked by an unresolved Open Question in `design.md`;
+- implementation or tests reveal the spec itself is wrong.
+
+**Human-in-the-loop tasks** (real-terminal behaviour: raw-mode entry/restore, the manual harness, frame visual correctness, signal handling): implement and self-test as far as automation allows, then give the orchestrator a **precise verification recipe** — exact command, what to do, what they should see — and report that task as **needs human confirmation**, not done.
+
+## Communication
+
+Be terse. When you finish: one or two sentences on what changed, the list of `N.M` tasks completed (and any needing human confirmation), build/test status, then explicitly request the `reviewer`.
diff --git a/.claude/commands/opsx/apply.md b/.claude/commands/opsx/apply.md
new file mode 100644
index 0000000..ae14f0f
--- /dev/null
+++ b/.claude/commands/opsx/apply.md
@@ -0,0 +1,152 @@
+---
+name: "OPSX: Apply"
+description: Implement tasks from an OpenSpec change (Experimental)
+category: Workflow
+tags: [workflow, artifacts, experimental]
+---
+
+Implement tasks from an OpenSpec change.
+
+**Input**: Optionally specify a change name (e.g., `/opsx:apply add-auth`). If omitted, check if it can be inferred from conversation context. If vague or ambiguous you MUST prompt for available changes.
+
+**Steps**
+
+1. **Select the change**
+
+   If a name is provided, use it. Otherwise:
+   - Infer from conversation context if the user mentioned a change
+   - Auto-select if only one active change exists
+   - If ambiguous, run `openspec list --json` to get available changes and use the **AskUserQuestion tool** to let the user select
+
+   Always announce: "Using change: <name>" and how to override (e.g., `/opsx:apply <other>`).
+
+2. **Check status to understand the schema**
+   ```bash
+   openspec status --change "<name>" --json
+   ```
+   Parse the JSON to understand:
+   - `schemaName`: The workflow being used (e.g., "spec-driven")
+   - Which artifact contains the tasks (typically "tasks" for spec-driven, check status for others)
+
+3. **Get apply instructions**
+
+   ```bash
+   openspec instructions apply --change "<name>" --json
+   ```
+
+   This returns:
+   - `contextFiles`: artifact ID -> array of concrete file paths (varies by schema)
+   - Progress (total, complete, remaining)
+   - Task list with status
+   - Dynamic instruction based on current state
+
+   **Handle states:**
+   - If `state: "blocked"` (missing artifacts): show message, suggest using `/opsx:continue`
+   - If `state: "all_done"`: congratulate, suggest archive
+   - Otherwise: proceed to implementation
+
+4. **Read context files**
+
+   Read every file path listed under `contextFiles` from the apply instructions output.
+   The files depend on the schema being used:
+   - **spec-driven**: proposal, specs, design, tasks
+   - Other schemas: follow the contextFiles from CLI output
+
+5. **Show current progress**
+
+   Display:
+   - Schema being used
+   - Progress: "N/M tasks complete"
+   - Remaining tasks overview
+   - Dynamic instruction from CLI
+
+6. **Implement tasks (loop until done or blocked)**
+
+   For each pending task:
+   - Show which task is being worked on
+   - Make the code changes required
+   - Keep changes minimal and focused
+   - Mark task complete in the tasks file: `- [ ]` → `- [x]`
+   - Continue to next task
+
+   **Pause if:**
+   - Task is unclear → ask for clarification
+   - Implementation reveals a design issue → suggest updating artifacts
+   - Error or blocker encountered → report and wait for guidance
+   - User interrupts
+
+7. **On completion or pause, show status**
+
+   Display:
+   - Tasks completed this session
+   - Overall progress: "N/M tasks complete"
+   - If all done: suggest archive
+   - If paused: explain why and wait for guidance
+
+**Output During Implementation**
+
+```
+## Implementing: <change-name> (schema: <schema-name>)
+
+Working on task 3/7: <task description>
+[...implementation happening...]
+✓ Task complete
+
+Working on task 4/7: <task description>
+[...implementation happening...]
+✓ Task complete
+```
+
+**Output On Completion**
+
+```
+## Implementation Complete
+
+**Change:** <change-name>
+**Schema:** <schema-name>
+**Progress:** 7/7 tasks complete ✓
+
+### Completed This Session
+- [x] Task 1
+- [x] Task 2
+...
+
+All tasks complete! You can archive this change with `/opsx:archive`.
+```
+
+**Output On Pause (Issue Encountered)**
+
+```
+## Implementation Paused
+
+**Change:** <change-name>
+**Schema:** <schema-name>
+**Progress:** 4/7 tasks complete
+
+### Issue Encountered
+<description of the issue>
+
+**Options:**
+1. <option 1>
+2. <option 2>
+3. Other approach
+
+What would you like to do?
+```
+
+**Guardrails**
+- Keep going through tasks until done or blocked
+- Always read context files before starting (from the apply instructions output)
+- If task is ambiguous, pause and ask before implementing
+- If implementation reveals issues, pause and suggest artifact updates
+- Keep code changes minimal and scoped to each task
+- Update task checkbox immediately after completing each task
+- Pause on errors, blockers, or unclear requirements - don't guess
+- Use contextFiles from CLI output, don't assume specific file names
+
+**Fluid Workflow Integration**
+
+This skill supports the "actions on a change" model:
+
+- **Can be invoked anytime**: Before all artifacts are done (if tasks exist), after partial implementation, interleaved with other actions
+- **Allows artifact updates**: If implementation reveals design issues, suggest updating artifacts - not phase-locked, work fluidly
diff --git a/.claude/commands/opsx/archive.md b/.claude/commands/opsx/archive.md
new file mode 100644
index 0000000..5e91608
--- /dev/null
+++ b/.claude/commands/opsx/archive.md
@@ -0,0 +1,157 @@
+---
+name: "OPSX: Archive"
+description: Archive a completed change in the experimental workflow
+category: Workflow
+tags: [workflow, archive, experimental]
+---
+
+Archive a completed change in the experimental workflow.
+
+**Input**: Optionally specify a change name after `/opsx:archive` (e.g., `/opsx:archive add-auth`). If omitted, check if it can be inferred from conversation context. If vague or ambiguous you MUST prompt for available changes.
+
+**Steps**
+
+1. **If no change name provided, prompt for selection**
+
+   Run `openspec list --json` to get available changes. Use the **AskUserQuestion tool** to let the user select.
+
+   Show only active changes (not already archived).
+   Include the schema used for each change if available.
+
+   **IMPORTANT**: Do NOT guess or auto-select a change. Always let the user choose.
+
+2. **Check artifact completion status**
+
+   Run `openspec status --change "<name>" --json` to check artifact completion.
+
+   Parse the JSON to understand:
+   - `schemaName`: The workflow being used
+   - `artifacts`: List of artifacts with their status (`done` or other)
+
+   **If any artifacts are not `done`:**
+   - Display warning listing incomplete artifacts
+   - Prompt user for confirmation to continue
+   - Proceed if user confirms
+
+3. **Check task completion status**
+
+   Read the tasks file (typically `tasks.md`) to check for incomplete tasks.
+
+   Count tasks marked with `- [ ]` (incomplete) vs `- [x]` (complete).
+
+   **If incomplete tasks found:**
+   - Display warning showing count of incomplete tasks
+   - Prompt user for confirmation to continue
+   - Proceed if user confirms
+
+   **If no tasks file exists:** Proceed without task-related warning.
+
+4. **Assess delta spec sync state**
+
+   Check for delta specs at `openspec/changes/<name>/specs/`. If none exist, proceed without sync prompt.
+
+   **If delta specs exist:**
+   - Compare each delta spec with its corresponding main spec at `openspec/specs/<capability>/spec.md`
+   - Determine what changes would be applied (adds, modifications, removals, renames)
+   - Show a combined summary before prompting
+
+   **Prompt options:**
+   - If changes needed: "Sync now (recommended)", "Archive without syncing"
+   - If already synced: "Archive now", "Sync anyway", "Cancel"
+
+   If user chooses sync, use Task tool (subagent_type: "general-purpose", prompt: "Use Skill tool to invoke openspec-sync-specs for change '<name>'. Delta spec analysis: <include the analyzed delta spec summary>"). Proceed to archive regardless of choice.
+
+5. **Perform the archive**
+
+   Create the archive directory if it doesn't exist:
+   ```bash
+   mkdir -p openspec/changes/archive
+   ```
+
+   Generate target name using current date: `YYYY-MM-DD-<change-name>`
+
+   **Check if target already exists:**
+   - If yes: Fail with error, suggest renaming existing archive or using different date
+   - If no: Move the change directory to archive
+
+   ```bash
+   mv openspec/changes/<name> openspec/changes/archive/YYYY-MM-DD-<name>
+   ```
+
+6. **Display summary**
+
+   Show archive completion summary including:
+   - Change name
+   - Schema that was used
+   - Archive location
+   - Spec sync status (synced / sync skipped / no delta specs)
+   - Note about any warnings (incomplete artifacts/tasks)
+
+**Output On Success**
+
+```
+## Archive Complete
+
+**Change:** <change-name>
+**Schema:** <schema-name>
+**Archived to:** openspec/changes/archive/YYYY-MM-DD-<name>/
+**Specs:** ✓ Synced to main specs
+
+All artifacts complete. All tasks complete.
+```
+
+**Output On Success (No Delta Specs)**
+
+```
+## Archive Complete
+
+**Change:** <change-name>
+**Schema:** <schema-name>
+**Archived to:** openspec/changes/archive/YYYY-MM-DD-<name>/
+**Specs:** No delta specs
+
+All artifacts complete. All tasks complete.
+```
+
+**Output On Success With Warnings**
+
+```
+## Archive Complete (with warnings)
+
+**Change:** <change-name>
+**Schema:** <schema-name>
+**Archived to:** openspec/changes/archive/YYYY-MM-DD-<name>/
+**Specs:** Sync skipped (user chose to skip)
+
+**Warnings:**
+- Archived with 2 incomplete artifacts
+- Archived with 3 incomplete tasks
+- Delta spec sync was skipped (user chose to skip)
+
+Review the archive if this was not intentional.
+```
+
+**Output On Error (Archive Exists)**
+
+```
+## Archive Failed
+
+**Change:** <change-name>
+**Target:** openspec/changes/archive/YYYY-MM-DD-<name>/
+
+Target archive directory already exists.
+
+**Options:**
+1. Rename the existing archive
+2. Delete the existing archive if it's a duplicate
+3. Wait until a different date to archive
+```
+
+**Guardrails**
+- Always prompt for change selection if not provided
+- Use artifact graph (openspec status --json) for completion checking
+- Don't block archive on warnings - just inform and confirm
+- Preserve .openspec.yaml when moving to archive (it moves with the directory)
+- Show clear summary of what happened
+- If sync is requested, use the Skill tool to invoke `openspec-sync-specs` (agent-driven)
+- If delta specs exist, always run the sync assessment and show the combined summary before prompting
diff --git a/.claude/commands/opsx/explore.md b/.claude/commands/opsx/explore.md
new file mode 100644
index 0000000..1757907
--- /dev/null
+++ b/.claude/commands/opsx/explore.md
@@ -0,0 +1,173 @@
+---
+name: "OPSX: Explore"
+description: "Enter explore mode - think through ideas, investigate problems, clarify requirements"
+category: Workflow
+tags: [workflow, explore, experimental, thinking]
+---
+
+Enter explore mode. Think deeply. Visualize freely. Follow the conversation wherever it goes.
+
+**IMPORTANT: Explore mode is for thinking, not implementing.** You may read files, search code, and investigate the codebase, but you must NEVER write code or implement features. If the user asks you to implement something, remind them to exit explore mode first and create a change proposal. You MAY create OpenSpec artifacts (proposals, designs, specs) if the user asks—that's capturing thinking, not implementing.
+
+**This is a stance, not a workflow.** There are no fixed steps, no required sequence, no mandatory outputs. You're a thinking partner helping the user explore.
+
+**Input**: The argument after `/opsx:explore` is whatever the user wants to think about. Could be:
+- A vague idea: "real-time collaboration"
+- A specific problem: "the auth system is getting unwieldy"
+- A change name: "add-dark-mode" (to explore in context of that change)
+- A comparison: "postgres vs sqlite for this"
+- Nothing (just enter explore mode)
+
+---
+
+## The Stance
+
+- **Curious, not prescriptive** - Ask questions that emerge naturally, don't follow a script
+- **Open threads, not interrogations** - Surface multiple interesting directions and let the user follow what resonates. Don't funnel them through a single path of questions.
+- **Visual** - Use ASCII diagrams liberally when they'd help clarify thinking
+- **Adaptive** - Follow interesting threads, pivot when new information emerges
+- **Patient** - Don't rush to conclusions, let the shape of the problem emerge
+- **Grounded** - Explore the actual codebase when relevant, don't just theorize
+
+---
+
+## What You Might Do
+
+Depending on what the user brings, you might:
+
+**Explore the problem space**
+- Ask clarifying questions that emerge from what they said
+- Challenge assumptions
+- Reframe the problem
+- Find analogies
+
+**Investigate the codebase**
+- Map existing architecture relevant to the discussion
+- Find integration points
+- Identify patterns already in use
+- Surface hidden complexity
+
+**Compare options**
+- Brainstorm multiple approaches
+- Build comparison tables
+- Sketch tradeoffs
+- Recommend a path (if asked)
+
+**Visualize**
+```
+┌─────────────────────────────────────────┐
+│     Use ASCII diagrams liberally        │
+├─────────────────────────────────────────┤
+│                                         │
+│      ┌────────┐         ┌────────┐      │
+│      │ State  │────────▶│ State  │      │
+│      │   A    │         │   B    │      │
+│      └────────┘         └────────┘      │
+│                                         │
+│   System diagrams, state machines,      │
+│   data flows, architecture sketches,    │
+│   dependency graphs, comparison tables  │
+│                                         │
+└─────────────────────────────────────────┘
+```
+
+**Surface risks and unknowns**
+- Identify what could go wrong
+- Find gaps in understanding
+- Suggest spikes or investigations
+
+---
+
+## OpenSpec Awareness
+
+You have full context of the OpenSpec system. Use it naturally, don't force it.
+
+### Check for context
+
+At the start, quickly check what exists:
+```bash
+openspec list --json
+```
+
+This tells you:
+- If there are active changes
+- Their names, schemas, and status
+- What the user might be working on
+
+If the user mentioned a specific change name, read its artifacts for context.
+
+### When no change exists
+
+Think freely. When insights crystallize, you might offer:
+
+- "This feels solid enough to start a change. Want me to create a proposal?"
+- Or keep exploring - no pressure to formalize
+
+### When a change exists
+
+If the user mentions a change or you detect one is relevant:
+
+1. **Read existing artifacts for context**
+   - `openspec/changes/<name>/proposal.md`
+   - `openspec/changes/<name>/design.md`
+   - `openspec/changes/<name>/tasks.md`
+   - etc.
+
+2. **Reference them naturally in conversation**
+   - "Your design mentions using Redis, but we just realized SQLite fits better..."
+   - "The proposal scopes this to premium users, but we're now thinking everyone..."
+
+3. **Offer to capture when decisions are made**
+
+    | Insight Type               | Where to Capture               |
+    |----------------------------|--------------------------------|
+    | New requirement discovered | `specs/<capability>/spec.md` |
+    | Requirement changed        | `specs/<capability>/spec.md` |
+    | Design decision made       | `design.md`                  |
+    | Scope changed              | `proposal.md`                |
+    | New work identified        | `tasks.md`                   |
+    | Assumption invalidated     | Relevant artifact              |
+
+   Example offers:
+   - "That's a design decision. Capture it in design.md?"
+   - "This is a new requirement. Add it to specs?"
+   - "This changes scope. Update the proposal?"
+
+4. **The user decides** - Offer and move on. Don't pressure. Don't auto-capture.
+
+---
+
+## What You Don't Have To Do
+
+- Follow a script
+- Ask the same questions every time
+- Produce a specific artifact
+- Reach a conclusion
+- Stay on topic if a tangent is valuable
+- Be brief (this is thinking time)
+
+---
+
+## Ending Discovery
+
+There's no required ending. Discovery might:
+
+- **Flow into a proposal**: "Ready to start? I can create a change proposal."
+- **Result in artifact updates**: "Updated design.md with these decisions"
+- **Just provide clarity**: User has what they need, moves on
+- **Continue later**: "We can pick this up anytime"
+
+When things crystallize, you might offer a summary - but it's optional. Sometimes the thinking IS the value.
+
+---
+
+## Guardrails
+
+- **Don't implement** - Never write code or implement features. Creating OpenSpec artifacts is fine, writing application code is not.
+- **Don't fake understanding** - If something is unclear, dig deeper
+- **Don't rush** - Discovery is thinking time, not task time
+- **Don't force structure** - Let patterns emerge naturally
+- **Don't auto-capture** - Offer to save insights, don't just do it
+- **Do visualize** - A good diagram is worth many paragraphs
+- **Do explore the codebase** - Ground discussions in reality
+- **Do question assumptions** - Including the user's and your own
diff --git a/.claude/commands/opsx/propose.md b/.claude/commands/opsx/propose.md
new file mode 100644
index 0000000..05276f4
--- /dev/null
+++ b/.claude/commands/opsx/propose.md
@@ -0,0 +1,106 @@
+---
+name: "OPSX: Propose"
+description: Propose a new change - create it and generate all artifacts in one step
+category: Workflow
+tags: [workflow, artifacts, experimental]
+---
+
+Propose a new change - create the change and generate all artifacts in one step.
+
+I'll create a change with artifacts:
+- proposal.md (what & why)
+- design.md (how)
+- tasks.md (implementation steps)
+
+When ready to implement, run /opsx:apply
+
+---
+
+**Input**: The argument after `/opsx:propose` is the change name (kebab-case), OR a description of what the user wants to build.
+
+**Steps**
+
+1. **If no input provided, ask what they want to build**
+
+   Use the **AskUserQuestion tool** (open-ended, no preset options) to ask:
+   > "What change do you want to work on? Describe what you want to build or fix."
+
+   From their description, derive a kebab-case name (e.g., "add user authentication" → `add-user-auth`).
+
+   **IMPORTANT**: Do NOT proceed without understanding what the user wants to build.
+
+2. **Create the change directory**
+   ```bash
+   openspec new change "<name>"
+   ```
+   This creates a scaffolded change at `openspec/changes/<name>/` with `.openspec.yaml`.
+
+3. **Get the artifact build order**
+   ```bash
+   openspec status --change "<name>" --json
+   ```
+   Parse the JSON to get:
+   - `applyRequires`: array of artifact IDs needed before implementation (e.g., `["tasks"]`)
+   - `artifacts`: list of all artifacts with their status and dependencies
+
+4. **Create artifacts in sequence until apply-ready**
+
+   Use the **TodoWrite tool** to track progress through the artifacts.
+
+   Loop through artifacts in dependency order (artifacts with no pending dependencies first):
+
+   a. **For each artifact that is `ready` (dependencies satisfied)**:
+      - Get instructions:
+        ```bash
+        openspec instructions <artifact-id> --change "<name>" --json
+        ```
+      - The instructions JSON includes:
+        - `context`: Project background (constraints for you - do NOT include in output)
+        - `rules`: Artifact-specific rules (constraints for you - do NOT include in output)
+        - `template`: The structure to use for your output file
+        - `instruction`: Schema-specific guidance for this artifact type
+        - `outputPath`: Where to write the artifact
+        - `dependencies`: Completed artifacts to read for context
+      - Read any completed dependency files for context
+      - Create the artifact file using `template` as the structure
+      - Apply `context` and `rules` as constraints - but do NOT copy them into the file
+      - Show brief progress: "Created <artifact-id>"
+
+   b. **Continue until all `applyRequires` artifacts are complete**
+      - After creating each artifact, re-run `openspec status --change "<name>" --json`
+      - Check if every artifact ID in `applyRequires` has `status: "done"` in the artifacts array
+      - Stop when all `applyRequires` artifacts are done
+
+   c. **If an artifact requires user input** (unclear context):
+      - Use **AskUserQuestion tool** to clarify
+      - Then continue with creation
+
+5. **Show final status**
+   ```bash
+   openspec status --change "<name>"
+   ```
+
+**Output**
+
+After completing all artifacts, summarize:
+- Change name and location
+- List of artifacts created with brief descriptions
+- What's ready: "All artifacts created! Ready for implementation."
+- Prompt: "Run `/opsx:apply` to start implementing."
+
+**Artifact Creation Guidelines**
+
+- Follow the `instruction` field from `openspec instructions` for each artifact type
+- The schema defines what each artifact should contain - follow it
+- Read dependency artifacts for context before creating new ones
+- Use `template` as the structure for your output file - fill in its sections
+- **IMPORTANT**: `context` and `rules` are constraints for YOU, not content for the file
+  - Do NOT copy `<context>`, `<rules>`, `<project_context>` blocks into the artifact
+  - These guide what you write, but should never appear in the output
+
+**Guardrails**
+- Create ALL artifacts needed for implementation (as defined by schema's `apply.requires`)
+- Always read dependency artifacts before creating a new one
+- If context is critically unclear, ask the user - but prefer making reasonable decisions to keep momentum
+- If a change with that name already exists, ask if user wants to continue it or create a new one
+- Verify each artifact file exists after writing before proceeding to next
diff --git a/.claude/skills/openspec-apply-change/SKILL.md b/.claude/skills/openspec-apply-change/SKILL.md
new file mode 100644
index 0000000..70fbdb8
--- /dev/null
+++ b/.claude/skills/openspec-apply-change/SKILL.md
@@ -0,0 +1,156 @@
+---
+name: openspec-apply-change
+description: Implement tasks from an OpenSpec change. Use when the user wants to start implementing, continue implementation, or work through tasks.
+license: MIT
+compatibility: Requires openspec CLI.
+metadata:
+  author: openspec
+  version: "1.0"
+  generatedBy: "1.3.1"
+---
+
+Implement tasks from an OpenSpec change.
+
+**Input**: Optionally specify a change name. If omitted, check if it can be inferred from conversation context. If vague or ambiguous you MUST prompt for available changes.
+
+**Steps**
+
+1. **Select the change**
+
+   If a name is provided, use it. Otherwise:
+   - Infer from conversation context if the user mentioned a change
+   - Auto-select if only one active change exists
+   - If ambiguous, run `openspec list --json` to get available changes and use the **AskUserQuestion tool** to let the user select
+
+   Always announce: "Using change: <name>" and how to override (e.g., `/opsx:apply <other>`).
+
+2. **Check status to understand the schema**
+   ```bash
+   openspec status --change "<name>" --json
+   ```
+   Parse the JSON to understand:
+   - `schemaName`: The workflow being used (e.g., "spec-driven")
+   - Which artifact contains the tasks (typically "tasks" for spec-driven, check status for others)
+
+3. **Get apply instructions**
+
+   ```bash
+   openspec instructions apply --change "<name>" --json
+   ```
+
+   This returns:
+   - `contextFiles`: artifact ID -> array of concrete file paths (varies by schema - could be proposal/specs/design/tasks or spec/tests/implementation/docs)
+   - Progress (total, complete, remaining)
+   - Task list with status
+   - Dynamic instruction based on current state
+
+   **Handle states:**
+   - If `state: "blocked"` (missing artifacts): show message, suggest using openspec-continue-change
+   - If `state: "all_done"`: congratulate, suggest archive
+   - Otherwise: proceed to implementation
+
+4. **Read context files**
+
+   Read every file path listed under `contextFiles` from the apply instructions output.
+   The files depend on the schema being used:
+   - **spec-driven**: proposal, specs, design, tasks
+   - Other schemas: follow the contextFiles from CLI output
+
+5. **Show current progress**
+
+   Display:
+   - Schema being used
+   - Progress: "N/M tasks complete"
+   - Remaining tasks overview
+   - Dynamic instruction from CLI
+
+6. **Implement tasks (loop until done or blocked)**
+
+   For each pending task:
+   - Show which task is being worked on
+   - Make the code changes required
+   - Keep changes minimal and focused
+   - Mark task complete in the tasks file: `- [ ]` → `- [x]`
+   - Continue to next task
+
+   **Pause if:**
+   - Task is unclear → ask for clarification
+   - Implementation reveals a design issue → suggest updating artifacts
+   - Error or blocker encountered → report and wait for guidance
+   - User interrupts
+
+7. **On completion or pause, show status**
+
+   Display:
+   - Tasks completed this session
+   - Overall progress: "N/M tasks complete"
+   - If all done: suggest archive
+   - If paused: explain why and wait for guidance
+
+**Output During Implementation**
+
+```
+## Implementing: <change-name> (schema: <schema-name>)
+
+Working on task 3/7: <task description>
+[...implementation happening...]
+✓ Task complete
+
+Working on task 4/7: <task description>
+[...implementation happening...]
+✓ Task complete
+```
+
+**Output On Completion**
+
+```
+## Implementation Complete
+
+**Change:** <change-name>
+**Schema:** <schema-name>
+**Progress:** 7/7 tasks complete ✓
+
+### Completed This Session
+- [x] Task 1
+- [x] Task 2
+...
+
+All tasks complete! Ready to archive this change.
+```
+
+**Output On Pause (Issue Encountered)**
+
+```
+## Implementation Paused
+
+**Change:** <change-name>
+**Schema:** <schema-name>
+**Progress:** 4/7 tasks complete
+
+### Issue Encountered
+<description of the issue>
+
+**Options:**
+1. <option 1>
+2. <option 2>
+3. Other approach
+
+What would you like to do?
+```
+
+**Guardrails**
+- Keep going through tasks until done or blocked
+- Always read context files before starting (from the apply instructions output)
+- If task is ambiguous, pause and ask before implementing
+- If implementation reveals issues, pause and suggest artifact updates
+- Keep code changes minimal and scoped to each task
+- Update task checkbox immediately after completing each task
+- Pause on errors, blockers, or unclear requirements - don't guess
+- Use contextFiles from CLI output, don't assume specific file names
+
+**Fluid Workflow Integration**
+
+This skill supports the "actions on a change" model:
+
+- **Can be invoked anytime**: Before all artifacts are done (if tasks exist), after partial implementation, interleaved with other actions
+- **Allows artifact updates**: If implementation reveals design issues, suggest updating artifacts - not phase-locked, work fluidly
diff --git a/.claude/skills/openspec-archive-change/SKILL.md b/.claude/skills/openspec-archive-change/SKILL.md
new file mode 100644
index 0000000..12e2f70
--- /dev/null
+++ b/.claude/skills/openspec-archive-change/SKILL.md
@@ -0,0 +1,114 @@
+---
+name: openspec-archive-change
+description: Archive a completed change in the experimental workflow. Use when the user wants to finalize and archive a change after implementation is complete.
+license: MIT
+compatibility: Requires openspec CLI.
+metadata:
+  author: openspec
+  version: "1.0"
+  generatedBy: "1.3.1"
+---
+
+Archive a completed change in the experimental workflow.
+
+**Input**: Optionally specify a change name. If omitted, check if it can be inferred from conversation context. If vague or ambiguous you MUST prompt for available changes.
+
+**Steps**
+
+1. **If no change name provided, prompt for selection**
+
+   Run `openspec list --json` to get available changes. Use the **AskUserQuestion tool** to let the user select.
+
+   Show only active changes (not already archived).
+   Include the schema used for each change if available.
+
+   **IMPORTANT**: Do NOT guess or auto-select a change. Always let the user choose.
+
+2. **Check artifact completion status**
+
+   Run `openspec status --change "<name>" --json` to check artifact completion.
+
+   Parse the JSON to understand:
+   - `schemaName`: The workflow being used
+   - `artifacts`: List of artifacts with their status (`done` or other)
+
+   **If any artifacts are not `done`:**
+   - Display warning listing incomplete artifacts
+   - Use **AskUserQuestion tool** to confirm user wants to proceed
+   - Proceed if user confirms
+
+3. **Check task completion status**
+
+   Read the tasks file (typically `tasks.md`) to check for incomplete tasks.
+
+   Count tasks marked with `- [ ]` (incomplete) vs `- [x]` (complete).
+
+   **If incomplete tasks found:**
+   - Display warning showing count of incomplete tasks
+   - Use **AskUserQuestion tool** to confirm user wants to proceed
+   - Proceed if user confirms
+
+   **If no tasks file exists:** Proceed without task-related warning.
+
+4. **Assess delta spec sync state**
+
+   Check for delta specs at `openspec/changes/<name>/specs/`. If none exist, proceed without sync prompt.
+
+   **If delta specs exist:**
+   - Compare each delta spec with its corresponding main spec at `openspec/specs/<capability>/spec.md`
+   - Determine what changes would be applied (adds, modifications, removals, renames)
+   - Show a combined summary before prompting
+
+   **Prompt options:**
+   - If changes needed: "Sync now (recommended)", "Archive without syncing"
+   - If already synced: "Archive now", "Sync anyway", "Cancel"
+
+   If user chooses sync, use Task tool (subagent_type: "general-purpose", prompt: "Use Skill tool to invoke openspec-sync-specs for change '<name>'. Delta spec analysis: <include the analyzed delta spec summary>"). Proceed to archive regardless of choice.
+
+5. **Perform the archive**
+
+   Create the archive directory if it doesn't exist:
+   ```bash
+   mkdir -p openspec/changes/archive
+   ```
+
+   Generate target name using current date: `YYYY-MM-DD-<change-name>`
+
+   **Check if target already exists:**
+   - If yes: Fail with error, suggest renaming existing archive or using different date
+   - If no: Move the change directory to archive
+
+   ```bash
+   mv openspec/changes/<name> openspec/changes/archive/YYYY-MM-DD-<name>
+   ```
+
+6. **Display summary**
+
+   Show archive completion summary including:
+   - Change name
+   - Schema that was used
+   - Archive location
+   - Whether specs were synced (if applicable)
+   - Note about any warnings (incomplete artifacts/tasks)
+
+**Output On Success**
+
+```
+## Archive Complete
+
+**Change:** <change-name>
+**Schema:** <schema-name>
+**Archived to:** openspec/changes/archive/YYYY-MM-DD-<name>/
+**Specs:** ✓ Synced to main specs (or "No delta specs" or "Sync skipped")
+
+All artifacts complete. All tasks complete.
+```
+
+**Guardrails**
+- Always prompt for change selection if not provided
+- Use artifact graph (openspec status --json) for completion checking
+- Don't block archive on warnings - just inform and confirm
+- Preserve .openspec.yaml when moving to archive (it moves with the directory)
+- Show clear summary of what happened
+- If sync is requested, use openspec-sync-specs approach (agent-driven)
+- If delta specs exist, always run the sync assessment and show the combined summary before prompting
diff --git a/.claude/skills/openspec-explore/SKILL.md b/.claude/skills/openspec-explore/SKILL.md
new file mode 100644
index 0000000..6858d3f
--- /dev/null
+++ b/.claude/skills/openspec-explore/SKILL.md
@@ -0,0 +1,288 @@
+---
+name: openspec-explore
+description: Enter explore mode - a thinking partner for exploring ideas, investigating problems, and clarifying requirements. Use when the user wants to think through something before or during a change.
+license: MIT
+compatibility: Requires openspec CLI.
+metadata:
+  author: openspec
+  version: "1.0"
+  generatedBy: "1.3.1"
+---
+
+Enter explore mode. Think deeply. Visualize freely. Follow the conversation wherever it goes.
+
+**IMPORTANT: Explore mode is for thinking, not implementing.** You may read files, search code, and investigate the codebase, but you must NEVER write code or implement features. If the user asks you to implement something, remind them to exit explore mode first and create a change proposal. You MAY create OpenSpec artifacts (proposals, designs, specs) if the user asks—that's capturing thinking, not implementing.
+
+**This is a stance, not a workflow.** There are no fixed steps, no required sequence, no mandatory outputs. You're a thinking partner helping the user explore.
+
+---
+
+## The Stance
+
+- **Curious, not prescriptive** - Ask questions that emerge naturally, don't follow a script
+- **Open threads, not interrogations** - Surface multiple interesting directions and let the user follow what resonates. Don't funnel them through a single path of questions.
+- **Visual** - Use ASCII diagrams liberally when they'd help clarify thinking
+- **Adaptive** - Follow interesting threads, pivot when new information emerges
+- **Patient** - Don't rush to conclusions, let the shape of the problem emerge
+- **Grounded** - Explore the actual codebase when relevant, don't just theorize
+
+---
+
+## What You Might Do
+
+Depending on what the user brings, you might:
+
+**Explore the problem space**
+- Ask clarifying questions that emerge from what they said
+- Challenge assumptions
+- Reframe the problem
+- Find analogies
+
+**Investigate the codebase**
+- Map existing architecture relevant to the discussion
+- Find integration points
+- Identify patterns already in use
+- Surface hidden complexity
+
+**Compare options**
+- Brainstorm multiple approaches
+- Build comparison tables
+- Sketch tradeoffs
+- Recommend a path (if asked)
+
+**Visualize**
+```
+┌─────────────────────────────────────────┐
+│     Use ASCII diagrams liberally        │
+├─────────────────────────────────────────┤
+│                                         │
+│      ┌────────┐         ┌────────┐      │
+│      │ State  │────────▶│ State  │      │
+│      │   A    │         │   B    │      │
+│      └────────┘         └────────┘      │
+│                                         │
+│   System diagrams, state machines,      │
+│   data flows, architecture sketches,    │
+│   dependency graphs, comparison tables  │
+│                                         │
+└─────────────────────────────────────────┘
+```
+
+**Surface risks and unknowns**
+- Identify what could go wrong
+- Find gaps in understanding
+- Suggest spikes or investigations
+
+---
+
+## OpenSpec Awareness
+
+You have full context of the OpenSpec system. Use it naturally, don't force it.
+
+### Check for context
+
+At the start, quickly check what exists:
+```bash
+openspec list --json
+```
+
+This tells you:
+- If there are active changes
+- Their names, schemas, and status
+- What the user might be working on
+
+### When no change exists
+
+Think freely. When insights crystallize, you might offer:
+
+- "This feels solid enough to start a change. Want me to create a proposal?"
+- Or keep exploring - no pressure to formalize
+
+### When a change exists
+
+If the user mentions a change or you detect one is relevant:
+
+1. **Read existing artifacts for context**
+   - `openspec/changes/<name>/proposal.md`
+   - `openspec/changes/<name>/design.md`
+   - `openspec/changes/<name>/tasks.md`
+   - etc.
+
+2. **Reference them naturally in conversation**
+   - "Your design mentions using Redis, but we just realized SQLite fits better..."
+   - "The proposal scopes this to premium users, but we're now thinking everyone..."
+
+3. **Offer to capture when decisions are made**
+
+    | Insight Type               | Where to Capture               |
+    |----------------------------|--------------------------------|
+    | New requirement discovered | `specs/<capability>/spec.md` |
+    | Requirement changed        | `specs/<capability>/spec.md` |
+    | Design decision made       | `design.md`                  |
+    | Scope changed              | `proposal.md`                |
+    | New work identified        | `tasks.md`                   |
+    | Assumption invalidated     | Relevant artifact              |
+
+   Example offers:
+   - "That's a design decision. Capture it in design.md?"
+   - "This is a new requirement. Add it to specs?"
+   - "This changes scope. Update the proposal?"
+
+4. **The user decides** - Offer and move on. Don't pressure. Don't auto-capture.
+
+---
+
+## What You Don't Have To Do
+
+- Follow a script
+- Ask the same questions every time
+- Produce a specific artifact
+- Reach a conclusion
+- Stay on topic if a tangent is valuable
+- Be brief (this is thinking time)
+
+---
+
+## Handling Different Entry Points
+
+**User brings a vague idea:**
+```
+User: I'm thinking about adding real-time collaboration
+
+You: Real-time collab is a big space. Let me think about this...
+
+      COLLABORATION SPECTRUM
+      ════════════════════════════════════════════
+
+      Awareness          Coordination         Sync
+          │                   │                 │
+          ▼                   ▼                 ▼
+      ┌────────┐         ┌────────┐        ┌────────┐
+      │Presence│         │Cursors │        │  CRDT  │
+      │  "3    │         │ Multi  │        │Conflict│
+      │online" │         │ select │        │  free  │
+      └────────┘         └────────┘        └────────┘
+          │                   │                 │
+       trivial            moderate           complex
+
+      Where's your head at?
+```
+
+**User brings a specific problem:**
+```
+User: The auth system is a mess
+
+You: [reads codebase]
+
+     ┌─────────────────────────────────────────────┐
+     │              CURRENT AUTH FLOW              │
+     └─────────────────────────────────────────────┘
+                        │
+          ┌─────────────┼─────────────┐
+          ▼             ▼             ▼
+     ┌─────────┐  ┌─────────┐  ┌─────────┐
+     │ Google  │  │  GitHub │  │  Email  │
+     │  OAuth  │  │  OAuth  │  │  Magic  │
+     └────┬────┘  └────┬────┘  └────┬────┘
+          │             │             │
+          └─────────────┼─────────────┘
+                        ▼
+                  ┌───────────┐
+                  │  Session  │
+                  └─────┬─────┘
+                        │
+                        ▼
+                  ┌───────────┐
+                  │   Perms   │
+                  └───────────┘
+
+     I see three tangles. Which one's burning?
+```
+
+**User is stuck mid-implementation:**
+```
+User: /opsx:explore add-auth-system
+      The OAuth integration is more complex than expected
+
+You: [reads change artifacts]
+
+     You're on task 4: "Implement OAuth flow"
+
+     Let me trace what's involved...
+
+     [draws diagram, explores options, suggests paths]
+
+     Want to update the design to reflect this?
+     Or add a spike task to investigate?
+```
+
+**User wants to compare options:**
+```
+User: Should we use Postgres or SQLite?
+
+You: Generic answer is boring. What's the context?
+
+User: A CLI tool that tracks local dev environments
+
+You: That changes everything.
+
+     ┌─────────────────────────────────────────────────┐
+     │          CLI TOOL DATA STORAGE                  │
+     └─────────────────────────────────────────────────┘
+
+     Key constraints:
+     • No daemon running
+     • Must work offline
+     • Single user
+
+                  SQLite          Postgres
+     Deployment   embedded ✓      needs server ✗
+     Offline      yes ✓           no ✗
+     Single file  yes ✓           no ✗
+
+     SQLite. Not even close.
+
+     Unless... is there a sync component?
+```
+
+---
+
+## Ending Discovery
+
+There's no required ending. Discovery might:
+
+- **Flow into a proposal**: "Ready to start? I can create a change proposal."
+- **Result in artifact updates**: "Updated design.md with these decisions"
+- **Just provide clarity**: User has what they need, moves on
+- **Continue later**: "We can pick this up anytime"
+
+When it feels like things are crystallizing, you might summarize:
+
+```
+## What We Figured Out
+
+**The problem**: [crystallized understanding]
+
+**The approach**: [if one emerged]
+
+**Open questions**: [if any remain]
+
+**Next steps** (if ready):
+- Create a change proposal
+- Keep exploring: just keep talking
+```
+
+But this summary is optional. Sometimes the thinking IS the value.
+
+---
+
+## Guardrails
+
+- **Don't implement** - Never write code or implement features. Creating OpenSpec artifacts is fine, writing application code is not.
+- **Don't fake understanding** - If something is unclear, dig deeper
+- **Don't rush** - Discovery is thinking time, not task time
+- **Don't force structure** - Let patterns emerge naturally
+- **Don't auto-capture** - Offer to save insights, don't just do it
+- **Do visualize** - A good diagram is worth many paragraphs
+- **Do explore the codebase** - Ground discussions in reality
+- **Do question assumptions** - Including the user's and your own
diff --git a/.claude/skills/openspec-propose/SKILL.md b/.claude/skills/openspec-propose/SKILL.md
new file mode 100644
index 0000000..4b7e204
--- /dev/null
+++ b/.claude/skills/openspec-propose/SKILL.md
@@ -0,0 +1,110 @@
+---
+name: openspec-propose
+description: Propose a new change with all artifacts generated in one step. Use when the user wants to quickly describe what they want to build and get a complete proposal with design, specs, and tasks ready for implementation.
+license: MIT
+compatibility: Requires openspec CLI.
+metadata:
+  author: openspec
+  version: "1.0"
+  generatedBy: "1.3.1"
+---
+
+Propose a new change - create the change and generate all artifacts in one step.
+
+I'll create a change with artifacts:
+- proposal.md (what & why)
+- design.md (how)
+- tasks.md (implementation steps)
+
+When ready to implement, run /opsx:apply
+
+---
+
+**Input**: The user's request should include a change name (kebab-case) OR a description of what they want to build.
+
+**Steps**
+
+1. **If no clear input provided, ask what they want to build**
+
+   Use the **AskUserQuestion tool** (open-ended, no preset options) to ask:
+   > "What change do you want to work on? Describe what you want to build or fix."
+
+   From their description, derive a kebab-case name (e.g., "add user authentication" → `add-user-auth`).
+
+   **IMPORTANT**: Do NOT proceed without understanding what the user wants to build.
+
+2. **Create the change directory**
+   ```bash
+   openspec new change "<name>"
+   ```
+   This creates a scaffolded change at `openspec/changes/<name>/` with `.openspec.yaml`.
+
+3. **Get the artifact build order**
+   ```bash
+   openspec status --change "<name>" --json
+   ```
+   Parse the JSON to get:
+   - `applyRequires`: array of artifact IDs needed before implementation (e.g., `["tasks"]`)
+   - `artifacts`: list of all artifacts with their status and dependencies
+
+4. **Create artifacts in sequence until apply-ready**
+
+   Use the **TodoWrite tool** to track progress through the artifacts.
+
+   Loop through artifacts in dependency order (artifacts with no pending dependencies first):
+
+   a. **For each artifact that is `ready` (dependencies satisfied)**:
+      - Get instructions:
+        ```bash
+        openspec instructions <artifact-id> --change "<name>" --json
+        ```
+      - The instructions JSON includes:
+        - `context`: Project background (constraints for you - do NOT include in output)
+        - `rules`: Artifact-specific rules (constraints for you - do NOT include in output)
+        - `template`: The structure to use for your output file
+        - `instruction`: Schema-specific guidance for this artifact type
+        - `outputPath`: Where to write the artifact
+        - `dependencies`: Completed artifacts to read for context
+      - Read any completed dependency files for context
+      - Create the artifact file using `template` as the structure
+      - Apply `context` and `rules` as constraints - but do NOT copy them into the file
+      - Show brief progress: "Created <artifact-id>"
+
+   b. **Continue until all `applyRequires` artifacts are complete**
+      - After creating each artifact, re-run `openspec status --change "<name>" --json`
+      - Check if every artifact ID in `applyRequires` has `status: "done"` in the artifacts array
+      - Stop when all `applyRequires` artifacts are done
+
+   c. **If an artifact requires user input** (unclear context):
+      - Use **AskUserQuestion tool** to clarify
+      - Then continue with creation
+
+5. **Show final status**
+   ```bash
+   openspec status --change "<name>"
+   ```
+
+**Output**
+
+After completing all artifacts, summarize:
+- Change name and location
+- List of artifacts created with brief descriptions
+- What's ready: "All artifacts created! Ready for implementation."
+- Prompt: "Run `/opsx:apply` or ask me to implement to start working on the tasks."
+
+**Artifact Creation Guidelines**
+
+- Follow the `instruction` field from `openspec instructions` for each artifact type
+- The schema defines what each artifact should contain - follow it
+- Read dependency artifacts for context before creating new ones
+- Use `template` as the structure for your output file - fill in its sections
+- **IMPORTANT**: `context` and `rules` are constraints for YOU, not content for the file
+  - Do NOT copy `<context>`, `<rules>`, `<project_context>` blocks into the artifact
+  - These guide what you write, but should never appear in the output
+
+**Guardrails**
+- Create ALL artifacts needed for implementation (as defined by schema's `apply.requires`)
+- Always read dependency artifacts before creating a new one
+- If context is critically unclear, ask the user - but prefer making reasonable decisions to keep momentum
+- If a change with that name already exists, ask if user wants to continue it or create a new one
+- Verify each artifact file exists after writing before proceeding to next
diff --git a/.editorconfig b/.editorconfig
new file mode 100644
index 0000000..410abdd
--- /dev/null
+++ b/.editorconfig
@@ -0,0 +1,68 @@
+root = true
+
+[*]
+charset = utf-8
+end_of_line = lf
+indent_style = space
+insert_final_newline = true
+trim_trailing_whitespace = true
+
+[*.{csproj,props,targets,xml}]
+indent_size = 2
+
+[*.cs]
+indent_size = 4
+
+# Naming conventions
+dotnet_naming_rule.private_fields_underscore.symbols = private_fields
+dotnet_naming_rule.private_fields_underscore.style = underscore_prefix
+dotnet_naming_rule.private_fields_underscore.severity = warning
+
+dotnet_naming_symbols.private_fields.applicable_kinds = field
+dotnet_naming_symbols.private_fields.applicable_accessibilities = private
+
+dotnet_naming_style.underscore_prefix.capitalization = camel_case
+dotnet_naming_style.underscore_prefix.required_prefix = _
+
+# var preferences
+csharp_style_var_for_built_in_types = false:warning
+csharp_style_var_when_type_is_apparent = true:suggestion
+csharp_style_var_elsewhere = false:suggestion
+
+# Expression body
+csharp_style_expression_bodied_methods = when_on_single_line:suggestion
+csharp_style_expression_bodied_properties = true:suggestion
+
+# Namespace style
+csharp_style_namespace_declarations = file_scoped:warning
+
+# Organize usings
+dotnet_separate_import_directive_groups = false
+dotnet_sort_system_directives_first = true
+
+# CA diagnostics severity overrides for warnings-as-errors
+# These are the full-analysis rules enabled by AnalysisMode=All.
+# Rules intentionally suppressed get a documented justification here (none yet).
+
+# Disable IDE rules that conflict with our explicit style choices:
+# IDE0005 (remove unnecessary usings) — keep as warning (it's caught by format)
+dotnet_diagnostic.IDE0005.severity = warning
+
+# IDE0008 (use explicit type) — we allow var when type is apparent
+dotnet_diagnostic.IDE0008.severity = none
+
+# IDE0130 (namespace does not match folder) — file-scoped namespaces already enforce this
+dotnet_diagnostic.IDE0130.severity = warning
+
+# IDE0010 (populate switch) — not always needed
+dotnet_diagnostic.IDE0010.severity = suggestion
+
+# IDE0072 (populate switch) — same as above
+dotnet_diagnostic.IDE0072.severity = suggestion
+
+# CA1014 (mark assemblies with CLSCompliant) — NuGet library; not applicable
+dotnet_diagnostic.CA1014.severity = none
+
+# CA2007 (ConfigureAwait) — library code; consumers set context; suppress globally
+dotnet_diagnostic.CA2007.severity = none
+
diff --git a/.gitattributes b/.gitattributes
new file mode 100644
index 0000000..6d00c76
--- /dev/null
+++ b/.gitattributes
@@ -0,0 +1,33 @@
+# Default: auto-detect text files; store and check out with LF on every platform.
+# Combined with .editorconfig `end_of_line = lf`, this keeps `dotnet format` happy
+# on Windows CI runners (which would otherwise check out CRLF and fail ENDOFLINE).
+* text=auto eol=lf
+
+# Source / project / config — explicit for safety.
+*.cs           text eol=lf
+*.csproj       text eol=lf
+*.props        text eol=lf
+*.targets      text eol=lf
+*.slnx         text eol=lf
+*.json         text eol=lf
+*.xml          text eol=lf
+*.md           text eol=lf
+*.yml          text eol=lf
+*.yaml         text eol=lf
+*.sh           text eol=lf
+.editorconfig  text eol=lf
+.gitignore     text eol=lf
+.gitattributes text eol=lf
+
+# Binary — never line-normalised.
+*.png    binary
+*.jpg    binary
+*.jpeg   binary
+*.gif    binary
+*.ico    binary
+*.dll    binary
+*.exe    binary
+*.pdb    binary
+*.snk    binary
+*.nupkg  binary
+*.snupkg binary
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
new file mode 100644
index 0000000..866345d
--- /dev/null
+++ b/.github/workflows/ci.yml
@@ -0,0 +1,38 @@
+name: CI
+
+on:
+  push:
+    branches: ["main", "change/**"]
+  pull_request:
+    branches: ["main"]
+
+jobs:
+  build:
+    name: Build & Test (${{ matrix.os }})
+    runs-on: ${{ matrix.os }}
+
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-latest, macos-latest, windows-latest]
+
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v4
+
+      - name: Setup .NET
+        uses: actions/setup-dotnet@v4
+        with:
+          dotnet-version: "10.0.x"
+
+      - name: Restore
+        run: dotnet restore
+
+      - name: Build
+        run: dotnet build -c Release --no-restore
+
+      - name: Test
+        run: dotnet test -c Release --no-build --verbosity normal
+
+      - name: Format check
+        run: dotnet format --verify-no-changes
diff --git a/CLAUDE.md b/CLAUDE.md
new file mode 100644
index 0000000..42871cf
--- /dev/null
+++ b/CLAUDE.md
@@ -0,0 +1,108 @@
+# dcli
+
+`dcli` is a C#/.NET (`net10.0`) inline terminal-rendering **library**, shipped as a NuGet
+package. It does Claude-Code-style **inline** rendering: styled output flows into the terminal's real
+scrollback, with a small interactive region (input + status + dropdowns) pinned at the bottom — not a
+full-screen TUI. It owns rendering and widget mechanics; the consumer (dmon) owns data and semantics.
+
+Spec-driven development is managed with **OpenSpec** (`openspec/`, schema `spec-driven`). All feature
+work flows through a change in `openspec/changes/`.
+
+### Commands
+
+> The solution itself is created by section 1 of the first change. Before that, only `openspec` commands apply.
+
+- Build: `dotnet build` — must be clean (analyzers run as **warnings-as-errors**; nullable enabled).
+- Test: `dotnet test` — all green.
+- Format: `dotnet format --verify-no-changes` — clean.
+- Validate a change: `openspec validate <change-name> --strict`.
+- List changes: `openspec list` (or the directories under `openspec/changes/`, excluding `archive/`).
+
+---
+
+# OpenSpec Apply Workflow
+
+**This section is authoritative.** `/opsx:apply` is the entry point for implementing a change; if the
+skill's behavior ever conflicts with what's written here, **follow this document**.
+
+## Roles — the main thread never writes feature code
+
+- **Orchestrator** = the main thread (you). You read specs, select work, brief agents, run the gates,
+  tick boxes, and commit. **You do not implement feature code directly.**
+- **`worker`** agent — implements the tasks.
+- **`reviewer`** agent — audits the worker's diff.
+
+Both agents are defined for this repo. Delegate; don't shortcut by writing the implementation yourself.
+
+## 1. Select the change
+
+1. List active changes = directories in `openspec/changes/` **excluding `archive/`**.
+2. **Always ask the user which change to apply**, even when there is exactly one. If there are none,
+   say so and stop.
+3. Resume point = the **first unticked `- [ ]` task** in that change's `tasks.md`.
+
+## 2. Pre-flight (orchestrator, before any section)
+
+1. Read `proposal.md`, `design.md`, and the relevant `specs/<capability>/spec.md` for the section(s)
+   you're about to work.
+2. **Working tree must be clean** (`git status`). If it's dirty, stop and ask.
+3. **Change must validate**: `openspec validate <change-name> --strict`. If it doesn't, stop and ask.
+4. **Be on the change branch** `change/<change-name>`. Create it from `main` if missing:
+   `git switch -c change/<change-name>`.
+
+## 3. Implement — section by section
+
+The unit of work is a **`## N.` section**. Walk sections in order from the resume point. For each:
+
+1. **Brief the worker.** Hand it: the section's tasks (`N.1`…`N.k`), the relevant spec excerpts, the
+   design decisions/constraints that bind them, and the done-gates below. The worker should not need to
+   go hunting — give it what it needs to stay focused.
+2. **Worker implements the whole section.** If a section is large or complex (e.g. the VT input parser),
+   split it into sub-chunks across multiple `worker` calls — but it remains **one commit at section end**.
+3. **Audit.** Spawn `reviewer` on the section diff (correctness, ADR compliance, OpenSpec scope, C#
+   idiom, agentic-AI design quality).
+4. **Review loop.** Feed the reviewer's findings back to the `worker`; worker fixes; `reviewer`
+   re-audits. **Repeat until the reviewer signs off.**
+5. **Gates — all four must pass before ticking any box:**
+   - `dotnet build` clean (no errors; analyzers/warnings-as-errors clean)
+   - `dotnet test` green — new tests for the section **and** all existing tests
+   - `openspec validate <change-name> --strict`
+   - `dotnet format --verify-no-changes` clean
+   If a gate fails, it's back to step 4, not a commit.
+6. **Tick the boxes.** Mark every `- [x] N.M` in the section in `tasks.md`.
+7. **Commit — one conventional commit per section:**
+   ```
+   feat(<change-name>): <section title> (section N)
+
+   - N.1 <task summary>
+   - N.2 <task summary>
+   ...
+
+   Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
+   ```
+
+## 4. Stop and ask — do not push on
+
+Stop **immediately** and ask the user (do not improvise a fix) when:
+
+- a spec/design is **ambiguous**, or two specs **contradict** each other;
+- doing the task properly needs changes **outside this change's scope** (its proposal/specs);
+- a task is **blocked by an unresolved Open Question** in `design.md` (e.g. the deferred paint mechanism);
+- implementation or tests reveal the **spec itself is wrong** (not just the code);
+- a task **requires human-in-the-loop verification** that can't be settled by automated gates — e.g.
+  confirming real-terminal UI behavior (raw-mode entry/restore, the manual harness in 4.5, visual
+  correctness of a rendered frame, signal handling). Implement and self-test as far as possible, then
+  hand the user a precise, copy-pasteable way to verify (exact command, what to do, what they should
+  see) and **wait for their confirmation before ticking that task**.
+
+**On stopping mid-section:** leave the WIP **uncommitted**, do **not** tick the section, do **not**
+revert. Report the **exact task (`N.M`)** that stopped you and why. The WIP stays in the working tree
+for the user to inspect.
+
+## 5. Done
+
+When every task in the change is ticked and the final review is clean:
+
+1. Report status: sections completed, commits made, test summary.
+2. **Propose archiving** — offer to run `/opsx:archive` and **wait for the user's confirmation**.
+   Do not archive automatically.
diff --git a/DEVLOG.md b/DEVLOG.md
new file mode 100644
index 0000000..c212b7c
--- /dev/null
+++ b/DEVLOG.md
@@ -0,0 +1,241 @@
+# DEVLOG — applying the `core-rendering-architecture` change
+
+This log lets a fresh session resume the OpenSpec change **`core-rendering-architecture`** without losing history.
+It is **not** part of the change's deliverables (it's a working note); don't tick tasks for it.
+
+## What this is
+
+We are applying the single active OpenSpec change `openspec/changes/core-rendering-architecture/` by following the
+**authoritative OpenSpec Apply Workflow in `CLAUDE.md`**. That workflow is, per section:
+
+1. Orchestrator (main thread) **never writes feature code** — it briefs the `worker` agent, audits with the `reviewer`
+   agent, runs gates, ticks boxes, commits.
+2. Unit of work = one `## N.` section of `tasks.md`. Complex sections (parser, render loop, painter, scrollback,
+   fixed region) are split into **Chunk A / Chunk B** across multiple `worker` calls but land as **one commit per section**.
+3. Loop: brief `worker` → `reviewer` audits the diff → fix loop until sign-off → **four gates** → tick boxes → commit.
+4. Four gates (all must pass before ticking): `dotnet build -c Release` (0 warnings, warnings-as-errors),
+   `dotnet test -c Release` (all green), `dotnet format --verify-no-changes`, `openspec validate core-rendering-architecture --strict`.
+5. Stop-and-ask on ambiguity, out-of-scope needs, unresolved Open Questions, or human-in-the-loop (real-terminal) verification.
+
+> Note: `SendMessage` is **not available** in this environment, so each worker/reviewer round in a fix loop spawns a
+> **fresh** agent that re-reads the on-disk (uncommitted) state. Brief them with file paths + the prior findings.
+
+## How to resume
+
+- Branch: **`change/core-rendering-architecture`** (created from `main`). Stay on it.
+- Working tree is **CLEAN**: §15 is fully committed (`a649d4b`, 688 tests). **The change is COMPLETE — all 15 sections done, 72/72 tasks ticked.**
+- Sanity check: `dotnet build -c Release && dotnet test -c Release && dotnet format --verify-no-changes && openspec validate core-rendering-architecture --strict`
+  → expect **0 warnings, 688 tests green, clean format, valid**.
+- **Next action:** propose `/opsx:archive` to the user (per CLAUDE.md, do NOT archive automatically — wait for confirmation).
+- Check the memory files (see below) before briefing — several encode hard-won constraints that will survive into the next change.
+
+## Section status (1 commit per section)
+
+| § | Section | Commit | Tests after | Notes |
+|---|---------|--------|-------------|-------|
+| 1 | Project setup | `c39f388` | 1 | net10.0, nullable, analyzers warnings-as-errors, `.slnx`, GitHub Actions CI (linux/mac/win), NuGet meta (`PackageId=dcli`, MPL-2.0). |
+| 2 | Styled text | `a451373` | 36 | Public `Format`/`Color`/`Style`/`Segment`/`Line`/`LineBuilder`. Model only — **no ANSI emission** (deferred to §8). `Line` has structural equality. |
+| 3 | Display width & wrapping | `2969d91` | 83 | Internal `DisplayWidth` (in-house wcwidth table) + `LineWrapper` (hard cell-wrap, wide chars never split). |
+| 4 | Raw mode & capability detection | `9dcb9ea` | 109 | `IRawModeSession` (§15 seam) + POSIX/Windows impls (per-platform termios — macOS≠Linux), `TerminalNotSupportedException` (public), `RestoreCoordinator` (signals+ProcessExit, CAS-idempotent). **4.5 manually verified by the user on macOS** (raw-mode entry + restore on normal/exception/signal). |
+| 5 | VT input parser | `d17e77e` | 218 | Pure `VtInputParser` (Chunk A: state machine + event model; Chunk B: bracketed paste + fixtures). Public event model. Decodes **both SS3 (`ESC O A`) and CSI (`ESC [ A`) arrows**. |
+| 6 | Input reader thread | `14bec73` | 229 | `InputReader` dedicated thread, `IInputByteSource` (real POSIX `read(2)`; Windows stub for §14.1; fake for tests). Ctrl+C → `KeyEvent`, no termination. |
+| 7 | Render loop core | `e03918c` | 257 | Actor-model `LoopEngine` (Chunk A: cadence/edges; Chunk B: `Terminal` lifecycle). **Namespace reorg** (see Decisions). Loop faults are surfaced (not swallowed). True-idle wait (no busy-poll). |
+| 8 | Frame painting | `ff16233` | 310 | `VtFrameRenderer` (Chunk A: SGR + single-frame; Chunk B: cross-frame cursor accounting). Sync-output fence, never rewrites frozen content. Wired into production `Terminal` (UTF-8/no-BOM/flush-per-frame stdout). Diff reconciler **deferred** (seam only). |
+| 9 | Scrollback model | `652f851` | 350 | `ILineObject`/`TextBlock`/`LiveBlock`/`Collapsible`, commit horizon, overflow-commits-top, one-way collapsible, oversized reprint. Feeds `LiveWindowRows`/`NewlyCommittedRows`. |
+| 10 | Fixed region: input editor & status | `84cbb48` | 442 | `TextBuffer` (grapheme-cluster nav via `StringInfo`, caret↔visual wrap, history recall), `StatusLine`, `MaxHeight` budget, loop frame-composition. Editor consumes editing keys; Enter/Tab/Ctrl fall through. |
+| 11 | Fixed region: overlays (autocomplete & dialog) | `bf35c99` | 554 | `ScrollableList` (reverse-video highlight, never the hardware cursor; viewport reconciled **on render** so it survives `MaxRows` changes between frames), `IOverlay` seam, `Autocomplete`/`Dialog` overlays, `OverlayState` invariant on the model (`ShowAutocomplete`/`ShowDialog`/`ClearOverlay`), intercept-chain routing (overlay→editor→emit), composer overlay-slot (dialog above / autocomplete below) + Decision-8 budget squeeze (status sacred → caret visible → overlay absorbs squeeze, **zero-cap guarded**) + cursor hide-while-modal. Built across 3 worker calls (A=ScrollableList, B-i=components, B-ii=integration). |
+| 12 | Public API: dialogs, events, façade | `394c9ba` | 615 | `ITerminal` + sub-surfaces, awaitable dialogs (`SelectAsync`/`MultiSelectAsync`/`InputAsync`/`ChoiceAsync` → `DialogResult<T>`), `Events` emission (`InputSubmitted`/`InputChanged`/`KeyPressed`/`Resized`), public `Scrollback`/`Input`/`Status`/`Autocomplete` façade + `ILiveBlock`/`ICollapsible` handles. Much of §9/§10/§11 was built as **internal model + commands** specifically so §12 wraps them. |
+| 13 | Resize & reflow | Chunk A `e2b2b97`, Chunk B `6f9f3e9` | 660 | **Open Question #2** resolved. Chunk A: `IResizeWatcher` + POSIX SIGWINCH + reflow tests + manual CHECK 5; surfaced & fixed an AArch64-Darwin variadic-ABI bug in §7 `PosixTerminalSizeSource.GetSize` (replaced manual `ioctl` P/Invoke with `Console.WindowWidth`). Chunk B: `TerminalCapabilities` (`HasTruecolor`/`HasSynchronizedOutput`/`TreatAmbiguousAsWide`) + `TerminalCapabilityDetector.DetectCapabilities`; `SgrTranslator` static→instance with truecolor→256-indexed nearest-match downgrade (cube + grey ramp, squared-Euclidean). |
+| 14 | Cross-platform validation & packaging | `fe7eb73` | 665 | Spec amend: Win/Linux smoke deferred (user macOS-only); nuget.org publication held until that clears. **14.2 `samples/Dcli.Demo`** (self-driving full tour, auto-cancel dialogs). **14.3** README expanded to a real package front page; XML docs scrubbed of devlog tags; Source Link wired (`Microsoft.SourceLink.GitHub` 8.0.0 + `IncludeSymbols`/snupkg); `Version` `0.1.0`→`0.1.0-rc.1`; `dotnet pack` produces `dcli.0.1.0-rc.1.{nupkg,snupkg}`. **14.4 `samples/Dcli.Demo.DmonWizard`** ports `dmon-core`'s `WizardEngine`+`WizardRenderer` slice onto dcli's public `ITerminal` (engine back-stack byte-for-byte identical to Spectre.Console-based original; only rendering dep swapped). **14.1 macOS** confirmed by user across full-tour demo, dmon-wizard slice, and `kill -TERM` restore. Surfaced & fixed a real bug along the way (see Decisions). |
+| 15 | Headless test harness (`Dcli.Testing`) | `a649d4b` | 688 | New separate NuGet-packable project `Dcli.Testing` (PackageId=dcli.testing, MPL-2.0, SourceLink + snupkg, one-way ProjectReference to Dcli). Public `HeadlessTerminal` over the real `Terminal.StartCore` (via `InternalsVisibleTo`); scripting drivers `Feed/SendKey/Type/Paste/Resize`; `SettleAsync` wraps internal `LoopEngine.SettleAsync` (no reinvented engine); public `VirtualClock` with off-lock waiter completion. Public immutable `FrameSnapshot` + `FrameSnapshotPrinter` for golden-frame diffing. Single production-code touch: `internal int Autocomplete.SelectedIndex` getter (read-only delegation) for snapshot construction. Harness-based equivalents added alongside existing terminal-free tests (LoopCoalescing + GoldenFrame) — pure parser/renderer unit tests stay direct (their assertions are on internals the public harness can't express; flagged as a follow-up tidy-up). |
+
+## Codebase layout (post §7 namespace reorg)
+
+- **Public API → root namespace `Dcli`** (files in `src/Dcli/`): `Terminal`, `TerminalOptions`, `TerminalNotSupportedException`,
+  `Segment`/`Line`/`Style`/`Color`/`Format`/`LineBuilder`, `InputEvent`/`KeyEvent`/`PasteEvent`/`ResizeEvent`/`KeyCode`/`NamedKey`/`Modifiers`, `TerminalEvent`.
+  The flagship type is usable as `Terminal` with `using Dcli;`.
+- **Internal machinery → `Dcli.Internal.*`** (files in `src/Dcli/Internal/...`): `Dcli.Internal` (RawModeSession, RestoreCoordinator,
+  capability detection, RecordingRawModeSession), `.Posix`, `.Windows`, `.Input` (VtInputParser, InputReader, IInputByteSource),
+  `.RenderLoop` (LoopEngine, edges `IClock`/`IOutputSink`/`ITerminalSizeSource`, RenderModel, VtFrameRenderer, SgrTranslator, NoopOutputSink),
+  `.Scrollback` (ILineObject/TextBlock/LiveBlock/Collapsible/ScrollbackModel + `Commands/`), `.FixedRegion` (TextBuffer/StatusLine/FixedRegionComposer).
+  `InternalsVisibleTo` → `Dcli.Tests` and `Dcli.RawModeHarness`.
+- **Tests:** `tests/Dcli.Tests/` (xUnit). **Manual harness:** `samples/Dcli.RawModeHarness/` (non-packable; the §4.5 verifier).
+- **Solution:** `dcli.slnx`. Shared build props in `Directory.Build.props`; analysis rules in `.editorconfig` (no project-wide
+  suppressions — `CA1031` is a localized `#pragma` around the one `LoopEngine.RunLoop` catch).
+
+## Key decisions & deviations made while applying
+
+- **§7 namespace reorg (user-approved):** the public `Terminal` type collided with the `Dcli.Terminal` namespace (CA1724). Rather than
+  suppress, the user chose to move all public types to root `Dcli` and rename internal machinery `Dcli.Terminal.*` → `Dcli.Internal.*`.
+- **§8 paint mechanism (Open Question #1, user-confirmed):** v1 = synchronized-output fence (`ESC[?2026h…l`) + full clear/repaint of the
+  bounded region. Per-line diff reconciler **deferred** (8.4 leaves a seam only).
+- **§7 idle wait:** rejected a `Thread.Sleep` busy-poll (reviewer-blocking); uses `WaitToReadAsync` + `IClock.WaitUntilAsync` deadline so the
+  loop truly idles and stays deterministic under a virtual clock. Loop stays `await`-free on its dedicated thread (single-mutator discipline).
+- **§7 loop faults are surfaced**, not silently swallowed (faults `LoopTerminated`, `DisposeAsync` rethrows, pending settles faulted).
+- **Render-frame cursor convention (§8):** the frame's final row has **no trailing `\n`**; cross-frame move-up uses the tracked **caret
+  resting row** (not region height) so it never overshoots above the region into frozen content.
+- **§11 autocomplete apply-semantics (decided, flagged for §12):** accepting a candidate does a **whole-buffer replace** via `TextBuffer.SetText`
+  (caret to end) — the literal reading of spec "the candidate's insert text is applied to the input buffer" and what slash-command-style
+  completion needs. `AutocompleteCandidate = (InsertText, Display Line)`. §12 may refine to span-replace; documented in XML docs at the call site.
+- **§11 overlay budget (Decision 8 made concrete):** composer renders the input with `allot = budget (= cap − statusCount)` → `inputRows = min(natural, budget)`;
+  the overlay gets `overlayCap = clamp(budget − inputRows.Count, 0, 10)` (the `10` default becomes consumer-configurable in §12). **Zero-cap guard:**
+  when `overlayCap == 0` the overlay is **not rendered at all** — `ScrollableList.MaxRows` clamps to ≥1, so rendering it would leak one phantom row past the cap.
+- **§11 ↔ §12 seam:** `Dialog.CloseRequest` (`Submit`/`Cancel`) + the loop's `ClearOverlay()` on `IsDismissed` is where §12's TCS-bundled open-dialog
+  command will complete its `DialogResult` (the dialog object survives the clear so its selection/outcome are still readable). `RenderModel.ShowAutocomplete/ShowDialog`
+  are the §12 façade-command entry points (exercised today only by §11.5 tests).
+- **§14 restore-on-signal bug fix (surfaced by 14.1 CHECK 3, in scope per §4.4 "guaranteed restore"):** SIGTERM left the cursor hidden when a modal
+  was up — §4's restore path only handled termios, not rendering-side ANSI state. §4.5's harness didn't render dialogs so the gap was invisible until §14.1.
+  Fix routes restore through the **loop thread** (Option C, preserves single-writer discipline): `IOutputSink.EmitRestoreSequence` emits
+  `ESC[?2026l ESC[?25h ESC[0m` + flush; `LoopEngine.RunLoop`'s `finally` calls it before termios restore with **two independent try/catch guards**
+  (ANSI fault → `IOException`/`ObjectDisposedException` filter only, never bare; termios restore broad-catch with localized `#pragma CA1031` — the §4.5
+  invariant is load-bearing); `RestoreCoordinator.SetHaltAction(loop.Dispose)` is wired by `Terminal.StartCore` so `SimulateTerminateSignal` halts the
+  loop (CTS-cancel + channel-complete + `_thread.Join(2s)`) and lets the loop's own `finally` emit the bytes on the loop thread, then falls back to
+  termios restore. Test seam: `internal Terminal.Coordinator` exposes the same coordinator the loop wiring uses; lifecycle test asserts
+  ordering via `RecordingRawModeSession.OutputLengthAtRestore` snapshot (catches any future "termios-before-ANSI" reorder).
+- **§14.4 API ergonomics findings (NOT yet a change — recorded for a follow-up):** porting `dmon-core`'s `WizardEngine` slice surfaced
+  (1) `DialogOutcome.Back` is structurally dead — no v1 keybinding produces it, so wizard-style "go back one step" affordances need a workaround;
+  (2) `MultiSelectAsync` is a real upgrade over Spectre's single-pick fallback (the dmon original literally had a "future enhancement" comment);
+  (3) `IsSecret` works end-to-end including default rendering; (4) no `internal→public` widening was required; (5) plain-string ceremony
+  (`new LineBuilder().Text(s).Build()`) appears ~12× in the renderer — convenience overloads (e.g. `Line.FromText(string)`, `Append(string)`,
+  string-accepting `SelectRequest.Items`) would reduce a lot of mechanical wrapping. These five are the candidates for the first post-architecture
+  ergonomics change.
+
+## Human-in-the-loop verifications
+
+- **Done — §4.5:** user ran `samples/Dcli.RawModeHarness` on macOS and confirmed all 4 checks (raw-mode entry/echo-off + restore on
+  normal exit, on exception, on `kill -TERM`). The first run surfaced a real bug (managed `ReadByte` treats the `VMIN=0/VTIME=1` timeout
+  0-read as EOF) which was fixed before confirmation.
+- **Done — §13 CHECK 5 (Chunk A):** user ran the harness on macOS and confirmed `[CHECK 5] Initial size: …` plus per-resize
+  `[CHECK 5] Resized to: …` lines. The first run surfaced a real latent §7 bug (AArch64 Darwin variadic ABI: `ioctl(int, unsigned long, ...)`
+  cannot be marshalled by `LibraryImport` as a fixed-arity call on Apple Silicon — args 3+ must travel on the stack, not in registers).
+  Replaced the hand-rolled `ioctl` P/Invoke with `Console.WindowWidth`/`Console.WindowHeight` (the .NET runtime's `libSystem.Native` shim
+  is non-variadic). Side benefit: `WindowsTerminalSizeSource` is no longer a hardcoded 80×24 stub.
+- **Done — §14.1 (macOS only):** user ran `samples/Dcli.Demo` (full scripted tour: banner → streaming live block → collapsible expand →
+  autocomplete show/hide → wizard chain → finale, all clean) and `samples/Dcli.Demo.DmonWizard` (ported `WizardEngine` slice rendered the
+  provider-select cleanly under auto-cancel). **CHECK 3 (`kill -TERM`)** surfaced the cursor-not-restored bug above; after the fix landed,
+  user re-ran and confirmed cursor visible + echo working + no ANSI residue on exit. Win/Linux smoke deferred to a follow-up change
+  (recorded in proposal.md).
+- **Held — nuget.org publication (§14.3):** `dotnet pack` produces `dcli.0.1.0-rc.1.{nupkg,snupkg}` cleanly. Actual feed publish is held
+  until Win/Linux smoke from the deferred follow-up clears.
+
+## Open follow-ups / known gaps (after this change lands — NOT in scope here)
+
+These are recorded as memory files and should become their own future OpenSpec changes / passes:
+
+- **VT-escape sanitization gap:** `Segment.Text` is emitted to the terminal **verbatim**; a consumer can smuggle raw VT (even an unbalanced
+  `ESC[?2026h` defeating the sync fence). Currently correct per the styled-text "verbatim" contract; needs its own change (sanitize-on-emit).
+- **Scrollback oversized-reprint ordering:** a rare cross-object commit-order edge when an oversized-collapsible expand coincides with an
+  independent overflow above it in the same paint. Emitted-once is fine; spec doesn't pin cross-object order. Deferred refinement.
+- **CA2007 / render-loop thread discipline:** CA2007 is suppressed repo-wide, so §7+ continuation-thread correctness is verified by hand.
+
+## Memory files (in the session memory dir, indexed by `MEMORY.md`)
+
+- `ca2007-render-loop-thread-discipline` — verify loop thread-correctness by hand (CA2007 suppressed).
+- `section6-input-reader-read-semantics` — `read(2)` 0≠EOF under VMIN=0/VTIME=1; timed read IS the ESC-disambiguation; `Flush` only on bare-ESC.
+- `section5-vt-parser-ss3-arrows` — decode both SS3 and CSI arrows (real-terminal evidence).
+- `vt-escape-sanitization-gap` — see above.
+- `scrollback-oversized-reprint-ordering` — see above.
+
+## Resume point
+
+> **CHANGE COMPLETE.** All 15 sections committed (latest `a649d4b`, 688 tests green). 72/72 tasks ticked, `openspec validate core-rendering-architecture --strict` passes. Next step: propose `/opsx:archive` to the user. The orchestrator must wait for the user's explicit go-ahead before archiving (per CLAUDE.md §5).
+
+### §15 chunk progress (COMPLETE — committed `a649d4b`)
+
+- **Chunk A — DONE (committed in `a649d4b`).** Created separate `src/Dcli.Testing/` project (NuGet-packable, MPL-2.0, one-way `ProjectReference` to `Dcli`; `InternalsVisibleTo("Dcli.Testing")` added to `Dcli.csproj`). Public `HeadlessTerminal` constructs the real `Terminal` via `Terminal.StartCore` and exposes scripting drivers `Feed(ReadOnlySpan<byte>)` (queued for the real `InputReader` thread; VMIN=0/VTIME=1-shaped `Read` returns 0 when empty, not EOF), `SendKey/Type/Paste/Resize` (post directly to `LoopEngine.InputWriter`; `Type` is `Char(Rune)`-only and docs nudge callers toward `SendKey` for named keys), plus `SettleAsync` (thin wrapper over the existing internal `LoopEngine.SettleAsync` — no shadow engine). Public `VirtualClock` with off-lock waiter completion (existing `FacadeTests` pattern lifted to a single canonical impl). Internal edge fakes: `HeadlessRawModeSession` (no-op), `ScriptedInputByteSource` (`ConcurrentQueue<byte>`), `InMemoryOutputSink` (volatile `LastModel` + `PaintCount`), `FixedSizeSource` (mutable), `ScriptedResizeWatcher` (stored callback + `Fire(cols, rows)`). 10 tests at `tests/Dcli.Tests/Testing/HeadlessTerminalTests.cs` (byte-feed → parsed event uses **F1 `ESC O P`, not Up arrow**, since `NamedKey.Up` is consumed by the editor's history recall and never emits `KeyPressed`; double-`SettleAsync` works around the no-`Feed→reader-barrier` race — a latent gap flagged for a future change). **Reviewer-approved (PASS, 7 non-blocking nits).** 665 → 675 tests.
+- **Chunk B — DONE (committed in `a649d4b`).** Public immutable `FrameSnapshot` (record; `LiveWindowRows`/`FixedRegionRows`/`Caret`/`IsCursorVisible`/`Size`/`NewlyCommittedRows` + `OverlayDescriptor { Kind, SelectedIndex, VisibleRowCount, InputText, IsSecret }`); defensive `Line[]` copies on construction to prevent aliasing with future paints. Public `FrameSnapshotPrinter` (stable golden-frame format: ASCII-bordered box + horizon line + caret/overlay trailer). `HeadlessTerminal.Snapshot` getter; empty pre-paint snapshot is safe (does not throw). NuGet polish on `Dcli.Testing.csproj` (SourceLink, snupkg, README + release notes); `src/Dcli.Testing/README.md` ships with a hello-world example. Single production-code touch: `internal int Autocomplete.SelectedIndex => _list.SelectedIndex;` — read-only accessor for Snapshot construction; no widening of mutator surface. Retargeting (§15.6): added harness-based equivalents (`LoopCoalescingHarnessTests.cs` — 4 tests; `GoldenFrameTests.cs` — 2 tests), left existing direct tests in place where they assert on internals (`DirtyCommand`/`WaitCount`/`OutboundEvents.Count`/`ThreadCapturingCommand`) the public harness cannot express — a softer reading of "retarget" than Decision 12's "run on the same harness" wording; flagged as a future tidy-up rather than blocker. Chunk-A nits folded in: `SettleAsync` XML doc qualifies the `Feed`/InputReader race; `_disposed` dead field removed; `ScriptedResizeWatcher.Dispose` doc clarified; `Type(string)` doc adds named-keys advice. **Reviewer-approved (PASS, 4 non-blocking nits — all observational follow-ups).** `dotnet pack -c Release` produces `dcli.0.1.0-rc.1.{nupkg,snupkg}` + `dcli.testing.0.1.0-rc.1.{nupkg,snupkg}` cleanly. 675 → 688 tests.
+
+### §13 chunk progress
+
+- **Chunk A — DONE (committed).** New `IResizeWatcher` edge interface (parallel to `ITerminalSizeSource`/`IInputByteSource`/`IClock`/`IOutputSink`).
+  `PosixResizeWatcher` uses `PosixSignalRegistration(SIGWINCH)` → queries `ITerminalSizeSource` → posts `ResizeEvent` via `loop.InputWriter` (the
+  signal-handler thread does only the `TryWrite` on the unbounded inbound channel; loop-thread discipline preserved). `WindowsResizeWatcher` is a
+  compile-only no-op stub deferred to §14.1, consistent with the §6.1 Windows input stub. Threaded through `Terminal.StartAsync`/`StartCore`;
+  `DisposeAsync` disposes the watcher BEFORE the loop. Reflow happens automatically through the existing apply→paint cycle (`ApplyInputEvent`
+  marks dirty; `FixedRegionComposer.Compose` and `ScrollbackModel.PrePaint` re-read `model.Columns`/`model.Rows` per paint). 5 new reflow tests
+  (`ResizeReflowTests.cs`) drive a `FakeResizeWatcher` to assert: width reflows the live window, cap recomputes on row change, frozen rows are
+  not re-emitted, `Resized` outbound is emitted, volatile snapshot updates. Manual harness gained a CHECK 5 — `samples/Dcli.RawModeHarness/Program.cs`
+  prints the initial size on raw-mode entry and on every SIGWINCH. **620 tests green.**
+  - **Latent §7 P/Invoke bug surfaced & fixed (audit done):** the §13 manual harness was the first real-machine invocation of `PosixTerminalSizeSource.GetSize()`
+    on the user's Apple Silicon (tests stub the seam, §4.5 didn't touch it). The hand-rolled `ioctl_winsize` P/Invoke AV'd on AArch64 Darwin because
+    `ioctl(2)` is C-variadic and Darwin's AAPCS64 puts variadic args on the **stack**, but `LibraryImport` generates a fixed-arity call (`x2` for arg3).
+    On Linux x86_64 SysV the bug is invisible (variadic and fixed args share registers). Fix: dropped the manual ioctl P/Invoke and routed through
+    `Console.WindowWidth`/`Console.WindowHeight` (the .NET runtime's `libSystem.Native` shim is non-variadic). Also audited every `[LibraryImport("libc"...)]`
+    in `src/Dcli/Internal/Posix/` — `read`/`tcgetattr`/`tcsetattr` are all correct. Side benefit: `WindowsTerminalSizeSource` got promoted from a hardcoded
+    `(80, 24)` stub to the same real implementation.
+  - **Process note (carry-over reminder):** the fix-and-audit worker call also silently implemented most of Chunk B (capability types, SGR downgrade,
+    detection wiring, ~250 lines + 34 tests) without being briefed. Reverted; Chunk B is being redone via a proper brief. The diff was caught only by
+    checking `git status` against the worker's claimed file list — keep doing the trust-but-verify check on every worker report.
+- **Chunk B — pending.** `TerminalCapabilities` record + `TerminalCapabilityDetector.DetectCapabilities(CapabilityInputs?)`. Truecolor: detect via
+  `COLORTERM=truecolor|24bit`; when absent, `SgrTranslator` downgrades `Color.ColorKind.Rgb` to the nearest 256-indexed (xterm cube + grey ramp,
+  squared-Euclidean nearest). Synchronized-output: small `TERM_PROGRAM`/`TERM` allow-list; record only (we already emit the fence harmlessly).
+  Unicode-width: record an East-Asian flag from `LANG`/`LC_ALL`; behaviour unchanged in v1. Capability test matrix (13.4). All internal, no public surface.
+
+**Section 12 — public API: dialogs, events, façade.** This section **wrapped** the internal model+commands built in §9/§10/§11; little new
+mechanics, mostly surface. Per design Decisions 11 & 12:
+- **12.1** `DialogResult<T>` / `DialogOutcome (Submitted|Back|Cancelled)` + awaitable `SelectAsync`/`MultiSelectAsync`/`InputAsync`/`ChoiceAsync`
+  via **TCS-bundled open-dialog commands** — the loop drives the modal `Dialog` overlay (§11) and completes the TCS on close
+  (hook point: the loop's `ClearOverlay()` on `Dialog.IsDismissed`; map `CloseRequest` Submit/Cancel → outcome; Back is a §12 concept).
+- **12.2** cancellation via `CancellationToken` (closes overlay → `Cancelled`); reject/queue a second concurrent dialog (the §11 invariant only
+  allows one overlay — decide reject-vs-queue here).
+- **12.3** `Scrollback`/`Input`/`Status`/`Autocomplete` façade surfaces posting fire-and-forget commands over the inbound channel
+  (`Autocomplete.Show` → `RenderModel.ShowAutocomplete`; `.Hide` → clear). 
+- **12.4** `Events` emission: `InputSubmitted`/`InputChanged`/`KeyPressed`/`Resized` (note `KeyPressed`/`Resized` already emitted by the loop; add
+  Submitted on Enter fall-through and Changed on buffer mutation).
+- **12.5** tests: select submit/cancel, multi-select toggle, cancellation token, input-change→candidates round-trip.
+- **12.6** expose `ITerminal` (+ `IScrollback`/`IInput`/`IStatus`/`IAutocomplete`); make `KeyEvent`/`PasteEvent`/`ResizeEvent`/`TerminalEvent.*`/
+  `DialogResult<T>`/`DialogOutcome` **public & constructible** (watch for internal-only ctors); no static/singleton state on consumer paths (tier A).
+- **12.7** tests: a hand-written fake `ITerminal` substitutes for the real façade; synthesized events/results drive consumer-style code;
+  command-side calls recorded & asserted.
+
+### §12 chunk progress (COMPLETE — committed `394c9ba`)
+
+- **Chunk A — DONE (reviewer-approved, uncommitted).** Public `DialogOutcome {Submitted,Back,Cancelled}`, `DialogResult<T>(Outcome,Value)` (constructible);
+  `SelectRequest`/`MultiSelectRequest`/`ChoiceRequest` (minimal: items/options + optional title/prompt `Line`); `Terminal.SelectAsync`/`MultiSelectAsync`/`ChoiceAsync`.
+  Plumbing: `OpenDialogCommand` + `CancelDialogCommand` (`src/Dcli/Internal/RenderLoop/`); `RenderModel.PendingDialogCompletion` (`Action<Dialog>`, set with
+  `ShowDialog`, invoked **before** `ClearOverlay` in the loop dismiss hook, cleared on clear); TCS bundled with `RunContinuationsAsynchronously`.
+  Decisions: **reject only a concurrent `Dialog`** (an active Autocomplete is *suppressed* via `ShowDialog`), reject = fault the task with `InvalidOperationException`;
+  cancel-token → `Cancelled` (registration disposed on every terminal path; already-cancelled fast path; `CancelDialogCommand` guards on `ReferenceEquals(ActiveOverlay, expected)`);
+  `Dialog` got an optional **title row** (`Render` truncates `[title]++list` to `MaxRows`, so title-only at `MaxRows==1`); empty-list Submit → `Submitted(-1)`.
+  Carry-forward nits: `AlreadyCancelledTokenReturnsCancelledNoOverlay` re-implements the fast path inline (tighten once Chunk D wires the real façade);
+  test cancel-closures use `-1`/`[]` vs production `default!` — cosmetic.
+- **Chunk B — DONE (reviewer-approved, uncommitted).** Public `InputRequest(Line? Prompt = null, string? Default = null, bool IsSecret = false)` +
+  `Terminal.InputAsync → DialogResult<string>` (no validator delegate — a `Func` validator would run consumer code on the loop thread, banned by Decision 10).
+  New `InputDialog : IModalOverlay` (modal, AboveInput) hosting its **own** `TextBuffer` (seeded via `SetText(Default)`; `IsSecret` masks each rune with
+  `DisplayWidth.Measure(rune)`×`'•'` so caret columns stay correct; real unmasked text returned on Submit). `HandleKey`: Enter→Submit, Esc→Cancel,
+  **Ctrl/Alt short-circuit to the modal catch-all (no insert/nav)**, else printable→Insert / Backspace / Delete / arrows / Home-End / Up-Down; modal catch-all swallows the rest.
+  **Seam 1 (cursor in overlay):** `IOverlay.CaretInOverlay (Row,Col)?` (null for Autocomplete/Dialog; real for InputDialog). `FixedRegionComposer.Compose` parks the
+  hardware cursor at `overlayStartRow + caret.Row` (start = 0 AboveInput / `inputRows.Count` BelowInput) **only when overlay rows were actually rendered** (phantom-row guard);
+  `InputDialog.HidesCursor => false` (cursor shown, unlike the select Dialog). **Seam 2 (generalized completion):** new `internal IModalOverlay : IOverlay { OverlayCloseKind? CloseRequest }`
+  (`Dialog` + `InputDialog` implement it); `RenderModel.ShowDialog`→`ShowModal(IModalOverlay, Action?)`, `PendingDialogCompletion`→`PendingModalCompletion` (parameterless `Action?`);
+  loop dismiss hook is now `PendingModalCompletion?.Invoke(); ClearOverlay()` (no `is Dialog` check); `OpenDialogCommand`/`CancelDialogCommand` take `IModalOverlay`; reject covers any
+  concurrent modal overlay (`is IModalOverlay`), Autocomplete still suppressed not rejected; `Terminal.OpenDialogAsync<T>`→`OpenModalAsync<T>(IModalOverlay, Func<DialogResult<T>>, ct)`.
+  N1 fix: `InputDialog._lastWidth` seeded on the loop thread via `OpenDialogCommand.Apply → SeedWidth(model.Columns)` (FIFO drain guarantees it's set before any same-batch width-dependent
+  `HandleKey`); `Render` keeps updating it for resize-while-open. Tests: `InputDialogTests.cs` (incl. type+Enter, seeded default, secret-mask-vs-real-text, Esc, token-cancel, Backspace,
+  **Ctrl+C not inserted**, seam-1 cursor-visible vs select-Dialog-cursor-hidden, cross-modal reject both directions); `DialogSelectionTests`/`OverlayRoutingTests` retargeted to the new API.
+  **589 tests green.** Reviewer note (architectural follow-up, not now): if a 2nd overlay ever needs open-time model state, promote the `is InputDialog` seed in `OpenDialogCommand` to an
+  `OnShown(model)`-style `IModalOverlay` method rather than chaining `is` checks.
+- **Chunk C — DONE (reviewer-approved, uncommitted).** Public concrete surfaces on `Terminal`: `ScrollbackSurface` (`Append(Line)`; `BeginLive()→ILiveBlock` with
+  `AppendText/SetContent/Commit`; `BeginCollapsible(summary, hiddenLines)→ICollapsible` with one-way `Expand`), `InputSurface` (`SetText`/`Clear` only — `Prompt`/`ReadOnly`
+  **deferred** by user decision, documented gap), `StatusSurface` (`Set(params Line[])` + `IReadOnlyList<Line>` overload → `FixedRegionComposer.Status` now exposes the `StatusLine`),
+  `AutocompleteSurface` (`Show(IReadOnlyList<AutocompleteCandidate>)` builds `new Autocomplete(model.FixedRegion.Editor)` on the loop thread + `ShowAutocomplete`; `Hide()` clears
+  directly when the active overlay is an `Autocomplete`). `AutocompleteCandidate(string InsertText, Line Display)` **promoted internal→public** in `Dcli` (no internal duplicate left);
+  public `ILiveBlock`/`ICollapsible` handle interfaces (internal handle classes hold the LoopEngine + the **pre-created** `LiveBlock`/`Collapsible` — the one allowed off-thread ref;
+  only mutated on the loop thread via FIFO-ordered commands, never the capture-callback `BeginLive/BeginCollapsibleCommand`). **Thread discipline:** every façade command pulls
+  loop-owned model state from its `Apply(model)` param; surfaces hold only the `LoopEngine`. **Events:** `InputSubmitted(text)` on plain Enter (no overlay, no Ctrl/Alt) →
+  emit + `AddToHistory` (skip empty) + `Clear()` (readline-style; suppresses `KeyPressed(Enter)`); `InputChanged(text)` when a key routed to the editor changed `editor.Text`
+  (before/after compare — covers Insert/Backspace/Delete/history-recall, excludes pure caret moves, the overlay path, and programmatic `SetText`/`Clear`). **Documented contract note
+  (reviewer, non-blocking):** accepting an Autocomplete candidate (`SetText` inside the overlay path) emits **no** `InputChanged` — a buffer-mirroring consumer would lag until the next
+  keystroke; revisit (emit-on-accept, or XML-doc it on `AutocompleteSurface`) only if a consumer needs it. Tests: `FacadeTests.cs` (14, incl. the real-`Terminal` `Events`-channel
+  round-trip); `FixedRegionTests`/`OverlayRoutingTests`/`RenderLoopTests` retargeted to the new Enter→`InputSubmitted` / printable→`InputChanged` behaviour. **603 tests green.**
+  Deferred (noted gaps, not Chunk C): `Scrollback.AppendRule`, incremental `Collapsible.AppendLine`, `PasteEvent` editor routing.
+- **Chunk D — DONE (reviewer-approved, committed in `394c9ba`).** New `src/Dcli/ITerminal.cs`: public `IScrollback`/`IInput`/`IStatus`/`IAutocomplete` + `ITerminal : IAsyncDisposable`;
+  `Terminal : ITerminal` with the four sub-surface properties retyped concrete→interface; each surface class carries `: IXxx`. Event/result types (`KeyEvent`/`PasteEvent`/`ResizeEvent`/
+  `TerminalEvent.*`/`DialogResult<T>`/`DialogOutcome`) were already public & constructible (no internal-only ctors). **Static-state audit clean** — all per-instance; only stateless static
+  factories (`KeyCode.FromRune`/`Named`, `Terminal.StartAsync`). **`IStatus.Set`→`SetRows`** (CA1716: `Set` is a VB.NET keyword once it sits on an interface/overridable member; rename-over-
+  suppress per the §7 precedent — design sketch's `Status.Set(...)` was illustrative, the normative spec doesn't pin Status method names). Tier-A test `tests/Dcli.Tests/FakeTerminalTests.cs`
+  (hand-written `FakeTerminal : ITerminal` records command-side calls + drives a consumer-style method with synthesized `KeyEvent`/`DialogResult`; 12 tests). **615 tests green.** Reviewer nit
+  (cosmetic, not re-spun): the fake's `SetCalls` field + a comment still say "Set" while the method is `SetRows` — align if the file is touched again.
+
+**Carry-over doc nit (not §11 scope):** `RenderModel.cs` XML docs (~lines 36–38, 72–76) still say `MaxFixedHeight` is "recorded but not yet
+enforced; §10 will apply…" — stale since §10 shipped and `FixedRegionComposer.ComputeCap` enforces it. Fold a correction into a §12/§13 doc pass.
diff --git a/Directory.Build.props b/Directory.Build.props
new file mode 100644
index 0000000..47e6dff
--- /dev/null
+++ b/Directory.Build.props
@@ -0,0 +1,25 @@
+<Project>
+
+  <PropertyGroup>
+    <TargetFramework>net10.0</TargetFramework>
+    <LangVersion>latest</LangVersion>
+    <Nullable>enable</Nullable>
+    <ImplicitUsings>enable</ImplicitUsings>
+
+    <!-- Analyzers -->
+    <EnableNETAnalyzers>true</EnableNETAnalyzers>
+    <AnalysisMode>All</AnalysisMode>
+    <EnforceCodeStyleInBuild>true</EnforceCodeStyleInBuild>
+    <!-- Required by IDE0005 (remove unnecessary usings) when EnforceCodeStyleInBuild=true -->
+    <GenerateDocumentationFile>true</GenerateDocumentationFile>
+
+    <!-- Warnings as errors -->
+    <TreatWarningsAsErrors>true</TreatWarningsAsErrors>
+    <WarningsAsErrors />
+
+    <!-- Deterministic builds -->
+    <Deterministic>true</Deterministic>
+    <ContinuousIntegrationBuild Condition="'$(CI)' == 'true'">true</ContinuousIntegrationBuild>
+  </PropertyGroup>
+
+</Project>
diff --git a/dcli.slnx b/dcli.slnx
new file mode 100644
index 0000000..9701dd2
--- /dev/null
+++ b/dcli.slnx
@@ -0,0 +1,14 @@
+<Solution>
+  <Folder Name="/src/">
+    <Project Path="src/Dcli/Dcli.csproj" />
+    <Project Path="src/Dcli.Testing/Dcli.Testing.csproj" />
+  </Folder>
+  <Folder Name="/tests/">
+    <Project Path="tests/Dcli.Tests/Dcli.Tests.csproj" />
+  </Folder>
+  <Folder Name="/samples/">
+    <Project Path="samples/Dcli.RawModeHarness/Dcli.RawModeHarness.csproj" />
+    <Project Path="samples/Dcli.Demo/Dcli.Demo.csproj" />
+    <Project Path="samples/Dcli.Demo.DmonWizard/Dcli.Demo.DmonWizard.csproj" />
+  </Folder>
+</Solution>
diff --git a/global.json b/global.json
new file mode 100644
index 0000000..50baaf1
--- /dev/null
+++ b/global.json
@@ -0,0 +1,6 @@
+{
+  "sdk": {
+    "version": "10.0.300",
+    "rollForward": "latestPatch"
+  }
+}
diff --git a/openspec/changes/archive/2026-05-28-core-rendering-architecture/.openspec.yaml b/openspec/changes/archive/2026-05-28-core-rendering-architecture/.openspec.yaml
new file mode 100644
index 0000000..e7c42ac
--- /dev/null
+++ b/openspec/changes/archive/2026-05-28-core-rendering-architecture/.openspec.yaml
@@ -0,0 +1,2 @@
+schema: spec-driven
+created: 2026-05-27
diff --git a/openspec/changes/archive/2026-05-28-core-rendering-architecture/design.md b/openspec/changes/archive/2026-05-28-core-rendering-architecture/design.md
new file mode 100644
index 0000000..d4eab6c
--- /dev/null
+++ b/openspec/changes/archive/2026-05-28-core-rendering-architecture/design.md
@@ -0,0 +1,366 @@
+## Context
+
+`dcli` is a standalone terminal-rendering library distributed on **NuGet** (C# / .NET). Its first consumer is **dmon**, a CLI front-end to a separate **dmoncore** backend process. dmoncore streams content (assistant output, tool results, thinking) to dmon over a **custom RPC framed as JSONL over stdio**; dmon deserializes that and drives dcli to render.
+
+The target UX is Claude-Code-like:
+
+```
+┌───────────────────────────────────────────────┐
+│  scrollback: marked-up lines, flows UP into     │  ← native terminal
+│  the terminal's real history                    │     scrollback
+│  ⊞ Ctrl+O to view thinking (collapsible)        │
+│  • streaming tail that grows…                   │
+├───────────────────────────────────────────────┤  ← commit horizon
+│  > input (owned editor)_                         │  ← fixed region,
+│  ▸ autocomplete (scrollable, capped)             │     pinned, re-rendered
+│  status                                          │
+└───────────────────────────────────────────────┘
+```
+
+The defining constraint is **inline rendering**: dcli does *not* take over the screen with the alternate-screen buffer. Output stays in the user's real terminal history (scrollable, copyable). That single choice cascades through the entire design, which is why these decisions are captured before any code.
+
+## Goals / Non-Goals
+
+**Goals:**
+- A reusable .NET/NuGet library for **inline** terminal UIs with a pinned, interactive bottom region.
+- Preserve native terminal scrollback, copy, and scroll — never own the whole screen.
+- Keep the re-rendered region **bounded** (≤ one screen) so cursor accounting and reconciliation stay tractable.
+- Provide an **owned input editor** and a reusable **scrollable selection** component (autocomplete, menus, pickers).
+- A clean boundary: dcli = rendering + widget mechanics; consumer = data + semantics.
+
+**Non-Goals (v1):**
+- Full-screen / alt-screen TUI (vim / htop / Terminal.Gui territory).
+- Two-way toggles — collapsibles are **one-way** (expand only).
+- Expanding **historical** (already-committed) content in place.
+- **Nested** composite blocks — the scrollback model is flat in v1.
+- A markup *parser* — styling is programmatic only.
+- Owning the RPC/JSONL protocol — that is dmon's concern.
+- A per-line diff reconciler — deferred optimization, not day-one.
+- Legacy / non-VT terminals — a modern VT-capable terminal (Windows Terminal, Win10 1809+, any xterm-class emulator) is **required**; no legacy conhost `INPUT_RECORD` path.
+- Mouse input, the **Kitty keyboard protocol**, and focus events — deferred; the VT parser stays extensible for them.
+
+## Decisions
+
+### 1. Inline rendering with native scrollback (the "Fork")
+
+Render inline; preserve the terminal's native scrollback. Above a moving **commit horizon**, content is frozen, write-once, and owned by the terminal; below it, a **live window** is re-renderable.
+
+- **Why:** Output stays in real terminal history (scroll/copy/search for free). The re-render region is bounded by the screen, so the classic cursor-accounting/reconciliation cost shrinks to a small fixed box. Simplest model that delivers the target UX.
+- **Alternatives considered:**
+  - **(B) Viewport owner (alt-screen):** dcli keeps its own scroll buffer and re-renders a visible window. Collapse/expand works anywhere, but you reimplement scrolling, mouse selection, search, copy — large scope, and you lose native scrollback ergonomics.
+  - **(C) Hybrid:** most content commits frozen, a window stays re-renderable. More moving parts than (A) for marginal gain at v1.
+- **Trade-off:** content that scrolls past the horizon can never be re-rendered (see Risks).
+
+### 2. The commit horizon / live window
+
+```
+   live window height = terminalRows − fixedRegionHeight
+```
+
+The live window holds the streaming tail, not-yet-committed collapsibles, and sits above the fixed region. When new content would push the live window taller than it can be, the **topmost live lines commit** (print → scroll into native scrollback → freeze). Committed line-objects may be **dropped from memory** — dcli need not retain history; if the consumer wants it, the consumer keeps it.
+
+- **Why:** Falls directly out of Decision 1. Bounds memory and per-frame work to one screen.
+
+### 3. dcli = mechanics + rendering; consumer = data + semantics
+
+The recurring boundary across every interactive piece:
+
+```
+   dcli owns                          dmon owns
+   ─────────                          ─────────
+   caret, edit ops, wrap, scroll      RPC/JSONL → scrollback content
+   selection highlight, viewport      the candidate list itself
+   measure, paint, park the cursor    what "accepting" an item does
+   emits events                       what a "Turn" is
+```
+
+- **Why:** Keeps the NuGet package free of dmon's protocol; makes components reusable; mirrors React's controlled/owned split. dcli never knows *what* content means or *why*.
+- **Alternative considered:** baking RPC/JSONL into the library — rejected; it would couple a general-purpose renderer to one app's protocol.
+
+### 4. Platform: C# / .NET (`net10.0`), shipped on NuGet
+
+Fixed requirement: a **single** target framework — **`net10.0`**, not multi-targeting. dcli is built and consumed on current .NET, so one TFM keeps the per-platform P/Invoke shims, analyzers, and packaging simple; there is no legacy-runtime support burden. Implications baked into later decisions: idiomatic single-writer loop via `System.Threading.Channels`; raw-mode keyboard input is a known .NET soft spot (see Open Questions); positioning relative to Spectre.Console (which does `Live`/`Status`/markup but **not** the collapsible-scrollback + pinned-input inline combo).
+
+### 5. Scrollback data model: flat list, content ≠ visual
+
+```
+   Segment    = styled run of text  (style lives here, not in the text)
+   Line       = [Segment...]        (one LOGICAL line; wraps to 1..N visual rows)
+   LineObject = TextBlock | Collapsible   (elements of the scrollback list)
+   scrollback = List<LineObject>          (FLAT — Model 1)
+```
+
+Every line-object implements one contract: `Render(width) → visual rows`. All width / wrapping / `wcwidth` complexity (CJK=2, combining=0, emoji, tabs) is contained there. **Content model ≠ visual model** — store segments/lines/blocks, paint rows of cells; the discipline that prevents smearing bugs.
+
+- **Why flat (Model 1):** matches dmon's needs (thinking = a flat blob); `Render()` never recurses; simplest.
+- **Alternative considered:** **(Model 2) tree** — a block contains blocks (tool card → foldable diff → …). Future-proof but recursive height math and expand semantics. Deferred; revisit only if dmon needs composite nested blocks.
+- **Payoff under Decision 1:** `Render()` is only ever called on **live** blocks; frozen blocks were rendered once and can be dropped; resize reflows only the live window.
+
+### 6. Collapsible semantics: one-way expand + reprint overflow
+
+A `Collapsible` starts collapsed (summary line) and expands **once**; it never re-collapses.
+
+- **Why one-way:** makes Decision 1 internally consistent. One-way ⇒ **monotonic height** ⇒ never need to push committed content back *up* (which is impossible anyway). Also removes all toggle/remeasure state.
+- **Oversized expansion (β):** if an expanded block is taller than the live window, it does **not** stay re-renderable — its content **reprints into the flow** as normal output and scrolls away; the block stays a collapsed marker. "If it doesn't fit the live window, it's not a live toggle, it's just output."
+- **Consumer policy example (dmon):** "show thinking" is on for the current *Turn*; dmon expands thinking blocks as they arrive while still live; the flag resets next Turn. dcli knows nothing of Turns — it only exposes "expand this block."
+- **Alternative considered:** two-way toggle / expanding history — rejected; incompatible with inline rendering (Decision 1).
+
+### 7. Styling: one programmatic primitive, shared
+
+A single `Segment` / `Style` primitive (programmatic — no markup string parser) is used by **both** the scrollback line-objects and the fixed-region components.
+
+- **Why:** avoids parser complexity; one styled-text type, two consumers, consistent rendering. Scrollback content arrives pre-styled (translated from RPC by dmon); fixed-region widgets are built programmatically.
+
+### 8. Fixed region: component stack, intercept routing, MaxHeight budget
+
+A **component stack** (Decision: option *b*), not a rigid input+status pair. The fixed region is one contiguous block pinned at the bottom, ordered around the Input:
+
+```
+   live window
+   ───────────────────────────────
+   ▸ Dialog overlay   (ABOVE)        ┐ at most ONE overlay active:
+   > Input (caret)                   │ OverlayState = None | Dialog | Autocomplete
+   ▸ Autocomplete overlay (BELOW)    ┘
+   Status
+```
+
+v1 component vocabulary:
+
+```
+   Input          owned editor — caret, multiline, wrap, history, internal scroll
+   ScrollableList bounded viewport over a longer list — maxRows, selection,
+                  auto-scroll, optional multi-select via space  (autocomplete AND
+                  dialog selection lists are both this)
+   StatusLine     plain styled row(s)
+```
+
+- **Owned input editor:** dcli owns the `TextBuffer` (edit ops, caret, wrap, history). Rationale: caret↔wrapping math is so entangled with rendering that pushing it onto consumers means everyone reinvents a buggy readline.
+- **Autocomplete is a `ScrollableList`,** fed candidates by dmon — generalizes free to every dropdown. The interaction loop:
+
+```
+   user types → Input buffer mutates → dcli raises TextChanged
+        dcli renders dropdown ← dmon sets Items ← dmon computes candidates
+   user ↑/↓ (dcli scrolls) → Enter → dcli raises Accepted(item)
+        dcli Input updated ← dmon applies the completion
+```
+
+- **Overlays — Dialog (above) + Autocomplete (below):** both are *overlays* attached to the Input, unified by one invariant — **at most one active** (`OverlayState = None | Dialog | Autocomplete`), enforced by dcli (opening a Dialog suppresses Autocomplete; Autocomplete can't open while a Dialog is up). Because only one is ever active, nothing fights over `↑↓ / Enter / Esc`. Touches nothing load-bearing: the fixed region stays one contiguous bottom block (Dialog is just its topmost rows), so the commit horizon and `MaxHeight` math are unchanged; the Dialog gets its own maxRows + internal scroll, and exclusivity bounds the worst case.
+  - *Dialog is a slot,* not a wizard engine — it hosts an interactive component (the `ScrollableList`, with `space` = multi-select toggle). A multi-step **wizard is dmon** swapping the slot's contents page-by-page and reacting to events; dcli owns only slot + routing + rendering (mechanics vs. semantics, Decision 3).
+  - *Modal by default:* an active Dialog consumes **all** keys (the Input is inert beneath it); a Dialog may **opt in** to pass printable keys through for a type-to-filter list.
+  - *Dialog events:* `Submitted(selection)` (Enter), `Cancelled` (Esc), `SelectionChanged`; `space` toggles in multi-select.
+- **Key routing — intercept chain (option *i*):** components are ordered; each gets first refusal and consumes-or-passes. The **active overlay is the chain front** → Input → passives. An overlay eats its nav keys (Autocomplete: `↑↓ Enter Tab Esc`; Dialog: `↑↓ space Enter Esc`, plus *everything* when modal) and passes the rest to Input. "Active overlay = intercepting" is the entire state machine — no focus token to save/restore.
+  - *Alternative considered:* focus-token model — scales to multi-field tabbing, but more state juggling than this UI needs.
+- **Height budget — `MaxHeight` cap (not fixed size):**
+
+```
+   fixedRegion.MaxHeight = clamp( appSet ?? 50% of rows,  min 8,  max rows )
+   live window           = rows − actualFixedHeight     (actual ≤ MaxHeight)
+```
+
+  The region takes only the rows its content needs, up to the cap; beyond that, components scroll internally. The `min 8` floor yields to the terminal height on TTYs smaller than 8 rows.
+- **Internal over-budget priority:** status is sacred (always shown); the **caret line is always visible** (Input scrolls internally if taller); the **dropdown absorbs the squeeze** (capped at the remainder, up to its own maxRows).
+- **Cursor:** the terminal has exactly one hardware cursor. Normally — and during Autocomplete — it parks at the **Input** caret after each frame; while a **modal Dialog** is active it is **hidden**. A selected row (autocomplete or dialog list) is reverse-video styling, never the real cursor.
+
+### 9. Raw-mode input: one VT byte-stream, one parser, built in-house
+
+We build the input/terminal-driver layer ourselves. `System.Console.ReadKey` is rejected (blocking, and its terminfo-based decoding silently drops Ctrl/Shift+Arrow, Alt combos, paste, etc.). There is no clean .NET equivalent of Rust's crossterm to adopt; Spectre.Console is higher-level, Terminal.Gui is a heavy alt-screen framework whose driver we may study but not depend on.
+
+**Unifying strategy — make every platform deliver a raw VT byte stream, then run ONE parser:**
+
+```
+   POSIX (mac/Linux): termios raw mode — clear ICANON/ECHO/ISIG/IEXTEN,
+                       IXON/ICRNL; VMIN=0,VTIME=1 for timed reads. (P/Invoke;
+                       termios struct layout is platform-specific.)
+   Windows:           SetConsoleMode + ENABLE_VIRTUAL_TERMINAL_INPUT (console
+                       EMITS VT sequences), − ENABLE_LINE/ECHO/PROCESSED_INPUT;
+                       + ENABLE_VIRTUAL_TERMINAL_PROCESSING on output.
+   Floor:             a modern VT terminal is REQUIRED (Windows Terminal /
+                       Win10 1809+ / xterm-class). No legacy conhost path.
+```
+
+Windows thereby stops being a second input model and becomes "Unix that flips modes differently." Cross-platform surface = two thin OS shims + one shared parser.
+
+- **Why build it:** no standalone "raw VT input" package exists for .NET; this loop *is* dcli's core value; the pure parser is cheap to own and test.
+- **Alternatives considered:** `Console.ReadKey` (lossy/blocking — rejected); depending on Terminal.Gui's driver (heavy, alt-screen-oriented — rejected, may study its quirk tables); supporting legacy conhost `ReadConsoleInput` (a whole second non-VT input model — rejected via the WT-only floor).
+
+**Three layers:**
+- `RawModeSession` — OS shim, `IDisposable`. Saves/restores original mode. Restore is non-negotiable (see Risks).
+- `VtInputParser` — pure state machine (GROUND → ESC → CSI → emit; UTF-8 accumulation). No I/O; fully unit-testable against byte fixtures.
+- `InputReader` — dedicated thread; timed byte reads feed the parser; posts events onto the `Channel<Event>` as one producer for the single-writer render loop.
+
+**ESC ambiguity:** a lone `ESC` is indistinguishable from the start of `ESC[…` until a short timeout elapses → timed reads (VMIN/VTIME on POSIX, `WaitForSingleObject` on Windows). Alt+key arrives as an `ESC` prefix, resolved within the same timeout.
+
+**Event model (School A):**
+
+```
+   InputEvent = KeyEvent | PasteEvent(string) | ResizeEvent(cols,rows)
+                (MouseEvent, FocusEvent deferred; parser stays extensible)
+   KeyEvent   = (KeyCode, Modifiers)
+   KeyCode    = Char(Rune) | Named(Enter|Tab|Backspace|Escape|arrows|Home|
+                End|PgUp|PgDn|Ins|Del|F1..F12|BackTab)
+   Modifiers  = [Ctrl, Alt, Shift]   (flags)
+```
+
+Granularity: **Rune per key**, **string per paste**; grapheme clustering (combining marks, ZWJ) is handled at the `TextBuffer`/editor layer, not in `KeyEvent`. Rejected School B (`ConsoleKeyInfo`'s `Key`+`KeyChar`+`Modifiers`) — lossy for non-BMP/combining input and conflates physical key with letter.
+
+**Terminal truths (accepted limitations — terminal physics, not bugs; documented):**
+1. Ctrl-letter collisions are real on the wire — `Tab≡Ctrl+I`, `Enter≡Ctrl+M`, `Backspace≡Ctrl+H`, `Escape≡Ctrl+[`. We report the **named** key.
+2. Shift is **implicit** in printable runes (`'A'`, not `'a'+Shift`); the Shift modifier appears only on **named** keys (`Shift+Arrow`, `Shift+Tab`/BackTab).
+3. Shift is **lost under Ctrl** (`Ctrl+A ≡ Ctrl+Shift+A` → same byte). Only the Kitty protocol (deferred) could disambiguate.
+
+**Bracketed paste** is enabled (`ESC[?2004h`) and surfaced as a single `PasteEvent` — so a pasted newline doesn't submit and autocomplete doesn't fire per-character.
+
+**Ctrl+C is eaten**, surfaced as `KeyEvent(Char('c'), Ctrl)`; dcli takes no action — dmon decides whether it means clear-input, interrupt-Turn, or exit (mechanics vs. semantics, Decision 3).
+
+**Consumption:** events flow down the intercept chain (Decision 8) via `bool HandleKey(KeyEvent)`, with the active overlay at the front. Autocomplete consumes `↑↓ Enter Tab Esc`; a Dialog consumes `↑↓ Enter Esc` plus `Char(' ')` (space = multi-select toggle), and **all** keys when modal (note: space is a printable `Char(' ')`, not a `Named` key). Everything unconsumed falls through to the Input editor; a `PasteEvent` goes to the Input as a literal insert (or to a Dialog that opted into type-to-filter).
+
+### 10. Render loop: actor model, event-driven, dual channels
+
+The loop is the **single owner of all mutable UI state** (the render model), stdout, *and* the raw-mode session — extending "single writer of stdout" to "single mutator of everything." An actor / single-dispatch design.
+
+```
+   Producers (ANY thread)                  THE LOOP (one dedicated thread)
+   ──────────────────────                  ───────────────────────────────
+   InputReader ─┐                           owns: render model + stdout + raw mode
+   dmon's API   ┼─ TryWrite ─▶ inbound ───▶ drain → apply → (throttled) paint
+   resize       ┘            Channel<Msg>
+   dmon ◀── drains ── outbound Channel<OutEvent> ◀── Submitted/Accepted/KeyEvent/…
+```
+
+- **Inbound `Channel<Msg>` (MPSC):** input events *and* dmon's API commands share one FIFO → deterministic ordering, both reflected in the same frame.
+- **Outbound `Channel<OutEvent>`:** dcli→dmon events; dmon drains on its **own** thread. The loop **never executes consumer code**, so a slow `Submitted` handler can't stall rendering.
+- **Two dedicated long-running threads:** `InputReader` (blocking timed reads) and the render loop — not thread-pool tasks.
+- **No locks:** every mutation is applied on the loop thread; the render model needs no synchronization.
+- **API is fire-and-forget async:** `Append(line)` enqueues and returns *before* the paint. Value reads (terminal size, etc.) are served from a volatile snapshot, not a round-trip.
+
+**Cadence — event-driven drain-coalesce-throttle** (not a fixed tick):
+
+```
+   loop until shutdown:
+     await WaitToReadAsync(orUntil: nextPaintDeadline)   // idle when quiet
+     while TryRead(out msg): apply(msg)                   // drain the whole batch
+     if dirty && now ≥ nextPaintDeadline:
+         renderFrame(); nextPaintDeadline = now + minFrameInterval
+   finally: restore terminal      // ← Decision 9's guaranteed restore lives here
+```
+
+Properties: **idle-efficient** (sleeps on the channel when quiet); **burst-coalescing** (N streamed append-commands → one paint); **fps-capped** (`minFrameInterval`; terminals gain nothing past ~30–60fps). State is applied immediately on drain — only the *paint* is throttled — so input stays responsive and needs no separate priority lane. (`renderFrame()`'s escape-sequence mechanics are Open Question #1 below.)
+
+- **Channel sizing:** inbound **unbounded** for v1 (apply is cheap, drains fast); bounded + backpressure to dmoncore is a later refinement.
+- *Alternatives considered:* lock-protected shared state + repaint thread (contention, tearing — rejected); fixed-tick frame loop (burns CPU when idle — rejected); synchronous dcli→dmon callbacks (a blocking consumer handler stalls the loop — rejected for a library facing arbitrary consumers).
+
+### 11. Public API surface: commands in, events out, dialogs awaited
+
+The façade is a single `Terminal` object started in the background (matching the actor loop), with fire-and-forget command methods, an outbound `Events` stream, and `await`-able dialog methods.
+
+**Keystone — dialogs are awaitable Tasks, messages internally.** `ShowSelectAsync(...)` posts an open-dialog command carrying a `TaskCompletionSource`; the loop drives the modal overlay (Decision 8); on submit/back/cancel it completes the TCS. So request/response reads as `await` on the outside while staying a pure message inside the actor loop (Decision 10) — and dmon's existing `await …Async()` call sites port nearly unchanged, while the blocking `Console.ReadKey` loops disappear.
+
+```csharp
+// Lifecycle — background loop (Decisions 9, 10)
+await using Terminal term = await Terminal.StartAsync(new TerminalOptions {
+    MaxFixedHeight = null,        // default 50% / min 8 (Decision 8)
+    MinFrameIntervalMs = 16,      // throttle (Decision 10)
+});                               // DisposeAsync stops the loop + restores the terminal
+
+// Styled text (Decision 7) — Format is a [Flags] enum, NOT bool fields
+[Flags] enum Format { None=0, Bold=1, Italic=2, Underline=4, Dim=8, Reverse=16, Strikethrough=32 }
+readonly record struct Style(Color? Foreground = null, Color? Background = null, Format Format = Format.None);
+record Segment(string Text, Style Style = default);
+record Line(IReadOnlyList<Segment> Segments);
+new LineBuilder().Dim("✻ ").Text("Thinking…").Build();
+
+// Scrollback — fire-and-forget commands
+term.Scrollback.Append(Line line);
+term.Scrollback.AppendRule(Line? label);
+ILiveBlock   tail = term.Scrollback.BeginLive();             // AppendText / SetContent / Commit
+ICollapsible t    = term.Scrollback.BeginCollapsible(sum);   // AppendLine / Expand (one-way) / Commit
+
+// Fixed region (Decision 8)
+term.Input.Prompt = promptLine;  term.Input.ReadOnly = isThinking;
+term.Status.Set(modelLine, modeLine);
+term.Autocomplete.Show(items);   term.Autocomplete.Hide();
+
+// Dialogs — async modals
+enum DialogOutcome { Submitted, Back, Cancelled }            // ↔ dmon's WizardStepOutcome
+readonly record struct DialogResult<T>(DialogOutcome Outcome, T Value);
+await term.SelectAsync(req, ct);       // → DialogResult<int>
+await term.MultiSelectAsync(req, ct);  // → DialogResult<int[]>   (was a dmon TODO)
+await term.InputAsync(req, ct);        // → DialogResult<string>  (secret / default / validate)
+await term.ChoiceAsync(req, ct);       // → DialogResult<int>     (yes/no, tool-confirm)
+
+// Outbound events (Decision 10; dmon drains on its own thread)
+ChannelReader<TerminalEvent> Events { get; }
+//   InputSubmitted(string) · InputChanged(string) · KeyPressed(KeyEvent) · Resized(c,r)
+```
+
+**Decisions within:**
+- **Dialogs awaitable** (TCS-bundled command) — *not* fire-and-forget + a later `DialogSubmitted` event, which would shred every wizard step into a state-machine fragment.
+- **Autocomplete via event round-trip:** `InputChanged` → dmon computes candidates → `Autocomplete.Show`. Rejected a registered `Func<string,items>` provider (it would run consumer code on the loop thread — banned by Decision 10; round-trip latency is negligible at typing speed).
+- **Lifecycle = background loop + `StartAsync`/`DisposeAsync` + `Events` stream.** Rejected a blocking `RunAsync(handlers)` — background lets dmon's RPC reader and input-event consumer run independently.
+- **`Style.Format` is a `[Flags]` enum,** not a row of `bool` fields — composable and idiomatic.
+- **Markdown stays in dmon:** dcli ships `Segment`/`Line`/`Style`/`Format`/`LineBuilder`; `Markdown → Line[]` is dmon's (content/semantics, Decision 3). An optional `Dcli.Markdown` companion is possible later.
+- **One overlay at a time** (Decision 8 invariant): opening a dialog while one is active is rejected/queued by the loop; a dialog's `CancellationToken` closes the overlay and returns `Cancelled`.
+
+**Absorbs the current `Dmon.Terminal` layer** (validates the surface): `AppendToken`/`SettleTurn` → `BeginLive`/`SetContent`/`Commit`; `ConsolePicker` & `InlinePrompt.ChooseAsync` → `SelectAsync`; `ReadLineAsync(secret)` → `InputAsync`; `WizardRenderer` step → dmon maps a `WizardStep` to a dialog call and reads `DialogResult.Outcome`; `ToolConfirmPrompt` → `ChoiceAsync`; `SlashCommandParser` autocomplete → `InputChanged` + `Autocomplete.Show`; `MarkdownRenderer` → `Markdown → Line[]` in dmon; the `ConsoleEventHandler` god-object splits into an RPC→`Scrollback` translator and an `Events` consumer. Every changed call site is a net improvement — real scrollback, no blocking reads, no Spectre coupling, first-class multi-select.
+
+### 12. Testability: a fakeable façade (tier A) + a public headless harness (tier B)
+
+The public surface (Decision 11) must be **substitutable**, and dcli must ship a **terminal-free** way to drive and observe itself. This is a first-class constraint on the API, not an afterthought — because a renderer that can only be exercised against a real TTY is a renderer the consumer can't test.
+
+**The motivating evidence (dmon's `Dmon.Terminal`):** exactly one source file there is unit-tested — `WizardEngine` — and *only* because its render boundary was hand-quarantined behind an injected `Func<WizardStep, Outcome>` delegate. Every console-coupled file (`TerminalRenderer`, `InlinePrompt`, `ConsolePicker`, `ToolConfirmPrompt`, …) is untested, for one reason: **Spectre.Console's static `AnsiConsole` cannot be substituted.** Even the wizard leaks — its success message goes straight to `AnsiConsole.MarkupLine`, invisible to the test. The rule we draw from this: *testability is the inverse of static coupling; never repeat the static-singleton sin.*
+
+Two tiers, serving the two rungs of a consumer's test pyramid.
+
+**Tier A — the façade is fakeable (lean, in the main package).** The `Terminal` of Decision 11 implements an interface `ITerminal` (with sub-surfaces `IScrollback` / `IInput` / `IStatus` / `IAutocomplete`); consumers depend on the interface and substitute their own fake. Requirements:
+
+- **No static singletons** anywhere a consumer touches — the entire anti-Spectre lesson, encoded.
+- The **event and dialog-result types are public and constructible in tests** — `KeyEvent`, `PasteEvent`, `ResizeEvent`, the `TerminalEvent.*` cases, `DialogResult<T>`, `DialogOutcome`. A test must be able to synthesize "the user pressed ↓ then Enter" or "the dialog returned `Submitted(2)`" with no real parser or loop. (Watch for internal-only constructors added for safety — they'd silently break this.)
+- A fake **records the command side** (scrollback appends, status sets, which dialog was shown with what content), so those calls become assertable — closing the `AnsiConsole.MarkupLine` class of blind spot.
+
+This makes the `WizardEngine` test pattern the **default** posture for every dmon controller, not a per-feature hand-roll: `await terminal.SelectAsync(req)` replaces the injected `renderStep`; the fake returns a scripted `DialogResult`. dmon already prefers writing its own three-line fakes — dcli's obligation is simply to *not prevent* them.
+
+**Tier B — a public, documented headless harness, shipped as `Dcli.Testing`.** Run the *real* engine terminal-free and assert on rendered output. Guiding principle: **abstract the OS edges, share the core** — one render loop + layout (Decisions 9–10), swap only the I/O endpoints:
+
+```
+   PRODUCTION edge              TEST edge (Dcli.Testing)        the seam*
+   ──────────────────────       ────────────────────────       ───────────────
+   Posix/Win RawModeSession     no-op session                  raw-mode session
+   tty timed byte reader        scripted feeder (bytes/keys)   input source
+   ioctl size + SIGWINCH        injected size + Resize()       size / resize source
+   stdout writer                in-memory capture              output sink
+   system clock / cadence       virtual clock + Settle()       clock / cadence
+              └────────── SAME loop · layout · overlays · scrollback ──────────┘
+   (* exact interface names are spec/impl detail — they are the testable boundary,
+      not new architecture; the load-bearing core is never mocked.)
+```
+
+- **`HeadlessTerminal`** — a real `ITerminal` wired to the test-double edges; driven by `Feed(bytes)` / `SendKey(KeyEvent)` / `Type(string)` / `Paste(string)` / `Resize(cols, rows)`.
+- **Determinism — `await harness.SettleAsync()`** drains all pending inbound work and produces exactly **one** coalesced frame with no wall-clock time passing. A **virtual clock** backs the handful of cadence tests that *must* assert timing (the coalesce / throttle / idle properties of task 7.6) — which `Settle` deliberately bypasses.
+- **Observation = a structured frame snapshot, not VT bytes.** `harness.Snapshot` exposes the *logical* frame — live-window visual rows as `{text, style}` runs, fixed-region layout, caret position, active overlay — which is stable and human-diffable (golden frames). Raw byte output is asserted only by a thin separate fidelity layer. This snapshot is exactly the artifact task 8.5 already requires for dcli's own golden-frame tests.
+
+- **Why public, and why a separate package:** dcli must build this substrate to test *itself* (5.6 / 7.6 / 8.5 are all terminal-free). Promoting it to a supported, documented contract hands dmon integration-level confidence for free. A separate `Dcli.Testing` package keeps the production NuGet lean and lets the harness version independently — and it honours Decision 3: the snapshot is a **testing** contract that production code and dcli's own semantics never depend on.
+- **Alternatives considered:** baking the harness into the main package (bloats the prod surface, couples release cadence — rejected); a second standalone "headless" engine (drifts from the real one — rejected; we swap edges, never the core); making raw VT bytes the primary assertion surface (escape-soup brittleness — kept only as the fidelity layer).
+- **Trade-off:** the edge-interfaces and the snapshot type become a maintained surface (`Dcli.Testing`'s public contract). Accepted — it's the surface dcli's own tests need regardless, and the production API (Decision 11) is untouched.
+
+In short: **tier A** = test your controller against a *faked* dcli (cheap, what dmon reaches for today); **tier B** = test the integrated render against a *real, terminal-free* dcli (the confidence layer, there when a rendering bug needs catching).
+
+## Risks / Trade-offs
+
+- **Collapsibles freeze at the horizon** → a block that scrolls out of the live window before expansion is frozen as its summary; `Ctrl+O` on history does nothing. *Mitigation:* β reprint-into-flow covers "I really want to see it"; the consumer's per-Turn policy expands proactively while content is still live. Accepted as a deliberate property of inline rendering.
+- **Opening tall bottom UI permanently commits scrollback** → a large paste / open dropdown shrinks the live window, committing top lines that can't be un-committed when it closes. *Mitigation:* document as expected; on-brand for Decision 1.
+- **Cursor accounting + display-width bugs** (smearing) → off-by-one visual-row math wrecks the frame. *Mitigation:* contain all width/wrap/`wcwidth` math inside `Render()`; enforce content-model ≠ visual-model; the bounded live window limits blast radius.
+- **Interleaved writes corrupt the display** (async RPC + keystrokes) → *Mitigation:* a single render loop that is the **sole owner of stdout** and all UI state, fed by an inbound `Channel` (Decision 10).
+- **Unbounded inbound channel under sustained flood** → if dmoncore streams faster than the loop drains, queued commands grow memory. *Mitigation:* `apply` is O(1)-cheap and paints are coalesced, so the queue drains far faster than it fills in practice; bounded channel + backpressure to dmoncore is the planned refinement if it bites.
+- **Raw-mode keyboard input is a .NET soft spot** → now designed in Decision 9 (own VT-stream driver, WT-only floor). Residual risk: per-platform `termios` struct marshalling, and **guaranteed terminal restore** on crash/signal — if missed, a crash leaves the user's shell with no echo. *Mitigation:* `RawModeSession` as `IDisposable` **plus** a last-resort net — `PosixSignalRegistration` (SIGINT/SIGTERM/SIGQUIT/SIGCONT) and `AppDomain.ProcessExit`; the pure `VtInputParser` is unit-tested against byte fixtures.
+- **Lossy key encoding under VT (accepted)** → Ctrl-letter collisions, Shift implicit in runes, Shift lost under Ctrl. *Mitigation:* none possible without the Kitty protocol (deferred); documented as terminal physics, not bugs (Decision 9).
+- **Live window squeezed to zero** on tiny terminals → *Mitigation:* clamp and degrade gracefully; live window may be 0.
+- **Headless harness drift / snapshot brittleness** (Decision 12) → a `Dcli.Testing` that diverges from the real loop, or golden snapshots that churn on incidental detail, erode trust in the tests. *Mitigation:* swap only the OS edges over the shared core (never a second engine); keep the snapshot minimal and logical (`{text, style}` runs + caret + overlay), with a pretty-printer for diffs; dcli's own tests (5.6 / 7.6 / 8.5) run on the same harness, so drift surfaces immediately.
+
+## Open Questions
+
+These are **mechanism, not architecture** — none can contradict the decisions above. Raw-mode input is settled in Decision 9; the render-loop topology and cadence in Decision 10; the public API surface in Decision 11; testability (fakeable façade + headless harness) in Decision 12. Remaining:
+
+1. **Frame strategy — `renderFrame()` internals** — cadence/coalescing/throttle is decided (Decision 10); what's open is the *paint mechanism*: leaning synchronized-output fence (`ESC [ ? 2026 h … l`) + clear/repaint the bounded live window for v1, with a per-line diff reconciler as a later optimization for flicker on terminals that ignore sync-output.
+2. **Resize + capability detection** — SIGWINCH via `PosixSignalRegistration` (POSIX) / buffer-size events or polling (Windows); plus truecolor, synchronized-output, and unicode-width detection and fallbacks. Resize is delivered as an inbound message (Decision 10); what's open is detection + the reflow/repaint mechanics (intersects with #1).
+
+(Over-budget priority resolved in Decision 8; render-loop shape in Decision 10; public API surface in Decision 11.)
diff --git a/openspec/changes/archive/2026-05-28-core-rendering-architecture/proposal.md b/openspec/changes/archive/2026-05-28-core-rendering-architecture/proposal.md
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+## Why
+
+`dcli` needs a foundational rendering architecture before any feature work can begin. The library targets a specific, deliberately-constrained UX — Claude-Code-style **inline** rendering: marked-up output that scrolls into the terminal's *real* scrollback, with a small interactive region (input + status + dropdowns) pinned at the bottom. This is distinct from a full-screen TUI, and the constraints it imposes shape every later decision, so they must be settled and recorded up front.
+
+## What Changes
+
+- Establish **inline rendering** (no alternate screen) as the core model: output flows into native terminal scrollback; only a bounded region near the bottom is ever re-rendered.
+- Define the **commit horizon**: a live, re-renderable window (`≤ rows − fixedRegionHeight`) above which content is frozen, write-once, and owned by the terminal.
+- Define the **scrollback data model**: a flat list of line-objects (`Segment → Line → {TextBlock | Collapsible}`), with collapsibles as **one-way** (monotonic-height) expandable blocks.
+- Define the **fixed region** as a component stack (`Input`, `ScrollableList`, `StatusLine`) with two mutually-exclusive overlays — a **Dialog** slot above the input and **Autocomplete** below — intercept-chain key routing, and a `MaxHeight` budget.
+- Establish the **dcli ↔ consumer boundary**: dcli owns rendering and widget *mechanics*; the consumer (dmon) owns *data and semantics* (RPC/JSONL, completion candidates, the notion of a "Turn").
+- Define a single **programmatic styling** primitive (`Segment`/`Style`) shared by both zones — no markup parser.
+- Establish the **raw-mode input layer**: one VT byte-stream driver (own `termios`/Win32-console shims, no `Console.ReadKey`), one `VtInputParser`, a School-A `KeyEvent` contract (`Char(Rune) | Named` + modifier flags), bracketed paste, eaten Ctrl+C, and guaranteed terminal restore. Modern VT terminals only.
+- Establish the **render loop**: an actor-model single thread owning all UI state, stdout, and the raw-mode session; producers post to an inbound `Channel`, dcli→dmon events flow out on a separate `Channel`; event-driven drain-coalesce-throttle cadence; fire-and-forget async API.
+- Establish the **public API surface**: a background `Terminal` façade with fire-and-forget scrollback/status commands, an outbound `Events` stream, and `await`-able modal **dialogs** (select / multi-select / input / choice → `DialogResult<T>`); styling via `Segment`/`Line`/`Style` with a `[Flags] Format` enum. Validated against — and a near-drop-in for — dmon's existing `Dmon.Terminal` layer.
+- Make the public surface **testable without a terminal**: the `Terminal` façade is an injectable interface (`ITerminal`) with publicly-constructible event/result types so consumers can fake it (tier A), and a companion **`Dcli.Testing`** package ships a **headless harness** — `HeadlessTerminal` + scripted input + deterministic `SettleAsync` + a structured frame `Snapshot` — for terminal-free integration tests (tier B). This is also the substrate dcli uses to test itself.
+
+## Capabilities
+
+### New Capabilities
+- `inline-scrollback`: the append-mostly scrollback buffer — line-objects, the commit horizon, one-way collapsibles, and the content-model-to-visual-rows render contract.
+- `fixed-region`: the pinned bottom component stack — owned input editor, two mutually-exclusive overlays (a **Dialog** slot above the input, **Autocomplete** below), the reusable scrollable selection list, status lines, height budgeting, and intercept-chain key routing.
+- `styled-text`: the programmatic `Segment`/`Line`/`Style` primitive (with a `[Flags] Format` enum and a `LineBuilder`) shared across both zones.
+- `terminal-input`: the raw-mode VT input driver — OS mode shims (`RawModeSession`), the `VtInputParser` state machine, the `KeyEvent`/`PasteEvent`/`ResizeEvent` contract, and lifecycle/restore safety.
+- `render-loop`: the actor-model render loop and `Terminal` lifecycle — single-writer event loop, fire-and-forget command intake, the outbound event stream, coalesced/throttled frames, and guaranteed terminal restore.
+- `test-harness`: the terminal-free testing surface — a substitutable `ITerminal` façade with publicly-constructible event/result types (tier A) and the public `Dcli.Testing` headless harness (tier B: `HeadlessTerminal`, scripted input drivers, deterministic `SettleAsync` / virtual clock, and a structured frame `Snapshot`). The same substrate dcli uses to test its own loop and frames.
+
+### Modified Capabilities
+<!-- None — this is the first change; openspec/specs/ is empty. -->
+
+## Impact
+
+- **New library surface** (NuGet, C#/.NET): **two packages** — `dcli` (public render model + interactive components + input driver, behind a substitutable `ITerminal` façade) and **`Dcli.Testing`** (the public headless harness).
+- **Terminal requirement**: a modern VT-capable terminal (Windows Terminal / Win10 1809+ / xterm-class). Legacy conhost is unsupported.
+- **Out of scope here, but unlocked next**: the `renderFrame()` **paint mechanism** (synchronized output vs. per-line diff) and **capability detection / resize plumbing**. These are mechanism, tracked as Open Questions in `design.md`. (Raw-mode input and the render-loop topology, originally grouped here, are now specified — see Decisions 9 and 10.)
+- **Deferred to a follow-up change — cross-platform manual smoke (§14.1)**: the §14 implementer has access only to macOS, so §14.1 covers macOS only. Windows Terminal and Linux real-terminal smoke are deferred until a tester with those OSes can drive the §14.2 demo. Cross-platform `dotnet build` / `dotnet test` continues to run in CI on Linux / macOS / Windows (§1.2), so cross-platform *build* parity is gated; only *visual-rendering* smoke is deferred. NuGet publication to nuget.org is held until that smoke clears — §14.3 stops at `dotnet pack`.
+- **Consumer contract**: dmon must own the RPC/JSONL protocol and feed dcli through the render model; the protocol never leaks into the library.
+- **Consumer testing** (Decision 12): consumers unit-test controllers against a faked `ITerminal`, and write terminal-free integration tests against `Dcli.Testing`'s `HeadlessTerminal` + frame `Snapshot` — the same harness dcli uses for its own deterministic loop/frame tests.
diff --git a/openspec/changes/archive/2026-05-28-core-rendering-architecture/specs/fixed-region/spec.md b/openspec/changes/archive/2026-05-28-core-rendering-architecture/specs/fixed-region/spec.md
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+## ADDED Requirements
+
+### Requirement: Bottom-pinned component stack
+The fixed region SHALL be a contiguous, bottom-pinned stack of components — input, status, and overlays — with the live window rendered above it.
+
+#### Scenario: Stays pinned while content streams
+- **WHEN** scrollback content streams into the live window
+- **THEN** the fixed region remains pinned at the bottom and is redrawn in place
+
+### Requirement: Owned input editor
+The library SHALL own an input editor supporting caret movement, multiline text, display-width-aware wrapping, history recall, and internal scrolling when its content exceeds its allotted height.
+
+#### Scenario: Wrapping tracks the caret
+- **WHEN** input text exceeds the available width
+- **THEN** the text wraps and the caret is positioned at the correct visual row and column
+
+#### Scenario: History recall
+- **WHEN** the user navigates input history
+- **THEN** the buffer is replaced with the recalled entry
+
+### Requirement: Fixed-region height budget
+The fixed region SHALL have a `MaxHeight` equal to `clamp(appSet ?? 50% of rows, 8, rows)`, SHALL consume only the rows its content needs up to that cap, and SHALL scroll components internally beyond the cap.
+
+#### Scenario: Default cap
+- **WHEN** no `MaxHeight` is configured on a 24-row terminal
+- **THEN** the cap is 12 rows
+
+#### Scenario: Minimum floor
+- **WHEN** 50% of the terminal height is below 8 rows and the terminal has at least 8 rows
+- **THEN** the cap is 8 rows
+
+#### Scenario: Tiny terminal
+- **WHEN** the terminal has fewer than 8 rows
+- **THEN** the cap is the terminal height
+
+### Requirement: Mutually-exclusive overlays
+At most one overlay SHALL be active at a time — a Dialog above the input or Autocomplete below it; opening a Dialog SHALL suppress any active Autocomplete, and Autocomplete SHALL NOT open while a Dialog is active.
+
+#### Scenario: Dialog suppresses autocomplete
+- **WHEN** a Dialog opens while Autocomplete is showing
+- **THEN** the Autocomplete overlay is hidden
+
+#### Scenario: Single active overlay
+- **WHEN** a Dialog is active and an Autocomplete show is requested
+- **THEN** the Autocomplete does not appear
+
+### Requirement: Intercept-chain key routing
+Keys SHALL be routed through an ordered chain with the active overlay first, then the input editor; each component either consumes a key or passes it to the next.
+
+#### Scenario: Overlay consumes navigation keys
+- **WHEN** Autocomplete is active and the up or down arrow is pressed
+- **THEN** Autocomplete consumes the key and the input caret does not move
+
+#### Scenario: Unhandled keys fall through
+- **WHEN** Autocomplete is active and a printable key is pressed
+- **THEN** the key falls through to the input editor and is inserted
+
+#### Scenario: Modal dialog captures all keys
+- **WHEN** a modal Dialog is active
+- **THEN** it consumes every key and the input editor receives none
+
+### Requirement: Single cursor placement
+The hardware cursor SHALL park at the input caret normally and during Autocomplete, SHALL be hidden while a modal Dialog is active, and overlay selection SHALL be shown by styling rather than the cursor.
+
+#### Scenario: Cursor stays in input during autocomplete
+- **WHEN** the user arrows through Autocomplete candidates
+- **THEN** the hardware cursor remains at the input caret and the selected candidate is shown highlighted
+
+#### Scenario: Modal dialog hides the cursor
+- **WHEN** a modal Dialog is active
+- **THEN** the hardware cursor is hidden
+
+### Requirement: Awaitable modal dialogs
+The library SHALL expose select, multi-select, input, and choice dialogs as awaitable operations that return a `DialogResult` whose outcome is `Submitted`, `Back`, or `Cancelled`.
+
+#### Scenario: Select submitted
+- **WHEN** the user highlights an item in a select dialog and presses Enter
+- **THEN** the awaited result is `Submitted` carrying the chosen index
+
+#### Scenario: Cancelled by escape
+- **WHEN** the user presses Escape in a dialog
+- **THEN** the awaited result is `Cancelled`
+
+#### Scenario: Multi-select toggling
+- **WHEN** the user presses space on items in a multi-select dialog
+- **THEN** those items toggle in the returned selection set
+
+#### Scenario: Cancellation token closes the dialog
+- **WHEN** the `CancellationToken` passed to a dialog is cancelled
+- **THEN** the overlay closes and the awaited result is `Cancelled`
+
+### Requirement: Consumer-driven autocomplete
+Autocomplete candidates SHALL be supplied by the consumer in response to input-change notifications; the library SHALL render and navigate them within the overlay's row cap and apply the accepted candidate's insert text to the input buffer.
+
+#### Scenario: Render supplied candidates
+- **WHEN** the consumer supplies candidates
+- **THEN** they render below the input up to the overlay's maximum rows with internal scrolling
+
+#### Scenario: Accept a candidate
+- **WHEN** the user accepts a highlighted candidate
+- **THEN** the candidate's insert text is applied to the input buffer
diff --git a/openspec/changes/archive/2026-05-28-core-rendering-architecture/specs/inline-scrollback/spec.md b/openspec/changes/archive/2026-05-28-core-rendering-architecture/specs/inline-scrollback/spec.md
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+## ADDED Requirements
+
+### Requirement: Inline rendering preserves native scrollback
+The library SHALL render inline and SHALL NOT use the terminal's alternate screen buffer; committed output SHALL remain in the terminal's native scrollback.
+
+#### Scenario: Committed output stays in terminal history
+- **WHEN** content has been committed and the user scrolls the terminal back
+- **THEN** the prior output is visible in the terminal's native scrollback
+
+#### Scenario: Alternate screen is not used
+- **WHEN** a rendering session starts
+- **THEN** the alternate screen buffer is never entered
+
+### Requirement: Bounded live window and commit horizon
+The library SHALL maintain a live, re-renderable window no taller than `terminalRows − fixedRegionHeight`; content above that window SHALL be committed (frozen, write-once) into native scrollback and never rewritten.
+
+#### Scenario: Overflow commits the top
+- **WHEN** appended content would make the live window exceed its maximum height
+- **THEN** the topmost live lines are emitted into native scrollback and frozen
+
+#### Scenario: Frozen content is never rewritten
+- **WHEN** a line has been committed past the horizon
+- **THEN** the library issues no further writes that target that line
+
+### Requirement: Flat line-object model
+Scrollback SHALL be a flat, ordered list of line-objects, each being either a text block or a collapsible, and each able to render itself to visual rows for a given display width.
+
+#### Scenario: Width-aware wrapping
+- **WHEN** a line-object renders at a width narrower than its content
+- **THEN** it wraps to multiple visual rows
+
+#### Scenario: Display-width counting
+- **WHEN** content contains wide (CJK) or zero-width characters
+- **THEN** the visual row count is computed from display width, not code-unit length
+
+### Requirement: Live streaming block
+The library SHALL provide a live block that accepts appended text and whose content may be replaced wholesale before the block is committed.
+
+#### Scenario: Streaming append
+- **WHEN** text tokens are appended to a live block
+- **THEN** they appear in the live window as they arrive
+
+#### Scenario: Replace then commit
+- **WHEN** a live block's content is replaced and the block is then committed
+- **THEN** the replaced content is what freezes into native scrollback
+
+### Requirement: One-way collapsible
+A collapsible line-object SHALL begin collapsed (showing a summary) and SHALL expand at most once; it SHALL NOT re-collapse.
+
+#### Scenario: Expand reveals hidden lines
+- **WHEN** a collapsed collapsible is expanded
+- **THEN** its hidden lines become visible and remain visible
+
+#### Scenario: Cannot re-collapse
+- **WHEN** a collapse is requested on a collapsible
+- **THEN** the request has no effect and the collapsible stays expanded once expanded
+
+#### Scenario: Freezes collapsed at the horizon
+- **WHEN** a still-collapsed collapsible commits past the horizon before being expanded
+- **THEN** it freezes in the collapsed state and can no longer be expanded
+
+### Requirement: Oversized expansion reprints into flow
+WHEN expanding a collapsible would make it taller than the live window, the library SHALL reprint the expanded content as normal flowing output rather than keeping it re-renderable.
+
+#### Scenario: Expansion larger than the live window
+- **WHEN** an expanded collapsible would exceed the live window height
+- **THEN** its content is emitted as flowing output and scrolls into native scrollback
+
+### Requirement: Scrollback command surface
+The library SHALL expose commands to append a line, append a rule/separator, begin a live block, and begin a collapsible.
+
+#### Scenario: Append a line
+- **WHEN** a caller appends a styled line
+- **THEN** the line is enqueued for rendering into the live window
+
+#### Scenario: Begin a collapsible
+- **WHEN** a caller begins a collapsible with a summary line
+- **THEN** a collapsible line-object is created in the collapsed state showing that summary
diff --git a/openspec/changes/archive/2026-05-28-core-rendering-architecture/specs/render-loop/spec.md b/openspec/changes/archive/2026-05-28-core-rendering-architecture/specs/render-loop/spec.md
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+## ADDED Requirements
+
+### Requirement: Single-writer actor loop
+All mutable UI state, stdout, and the raw-mode session SHALL be owned by a single render loop running on one dedicated thread; all state mutations SHALL be applied on that thread via an inbound channel, requiring no locks on the render model.
+
+#### Scenario: No interleaved output
+- **WHEN** content commands and keystroke-driven updates arrive concurrently from different threads
+- **THEN** output is never interleaved or corrupted because a single thread owns all writes to stdout
+
+#### Scenario: Mutations applied on the loop thread
+- **WHEN** any producer posts a mutation
+- **THEN** the mutation is applied on the render-loop thread and the model needs no lock
+
+### Requirement: Fire-and-forget command API
+Public command methods SHALL enqueue a message and return before the corresponding frame is painted; value reads SHALL be served from a snapshot rather than a round-trip to the loop.
+
+#### Scenario: Command returns before paint
+- **WHEN** a caller invokes a scrollback or status command
+- **THEN** the method enqueues the work and returns, and the effect appears on a subsequent frame
+
+#### Scenario: Snapshot read
+- **WHEN** a caller reads a value such as the terminal size
+- **THEN** the value is returned from a snapshot without blocking on the loop
+
+### Requirement: Event-driven coalesced frames
+The loop SHALL be idle when no messages are pending, SHALL drain all currently-available messages before painting, and SHALL cap painting frequency with a minimum frame interval.
+
+#### Scenario: Burst coalescing
+- **WHEN** a burst of many append commands arrives
+- **THEN** they are drained and reflected in a single painted frame
+
+#### Scenario: Idle when quiet
+- **WHEN** no messages are pending
+- **THEN** the loop performs no work and issues no repaint
+
+#### Scenario: Frame-rate cap
+- **WHEN** messages arrive continuously faster than the minimum frame interval
+- **THEN** paints occur no more often than once per minimum frame interval
+
+### Requirement: Decoupled outbound event stream
+Events from the library to the consumer SHALL be delivered on a separate channel that the consumer drains on its own thread, and the loop SHALL NOT execute consumer code.
+
+#### Scenario: Slow consumer does not stall rendering
+- **WHEN** the consumer's handling of an outbound event is slow
+- **THEN** the render loop continues painting unaffected
+
+#### Scenario: Input submission delivered
+- **WHEN** the user submits the input line
+- **THEN** an `InputSubmitted` event becomes available on the outbound stream
+
+### Requirement: Lifecycle and guaranteed restore
+Starting SHALL enter raw mode and run the loop in the background; disposal SHALL stop the loop and restore the terminal; restoration SHALL also occur on an unhandled exception and on termination signals.
+
+#### Scenario: Dispose restores the terminal
+- **WHEN** the `Terminal` is disposed
+- **THEN** raw mode is undone and the cursor is shown
+
+#### Scenario: Restore on crash or signal
+- **WHEN** the process terminates via an unhandled exception or a termination signal (e.g., SIGINT, SIGTERM)
+- **THEN** the terminal is restored so the user's shell is not left without echo
+
+### Requirement: Substitutable façade for consumer testing
+The public terminal façade SHALL be exposed as an interface (`ITerminal`, together with its `Scrollback` / `Input` / `Status` / `Autocomplete` sub-surfaces) that a consumer can replace with its own test double, and the library SHALL NOT require static or singleton state on any consumer-facing path. All consumer-facing event and dialog-result types (`KeyEvent`, `PasteEvent`, `ResizeEvent`, the `TerminalEvent` cases, `DialogResult<T>`, `DialogOutcome`) SHALL be publicly constructible so a test can synthesize them without a real terminal.
+
+#### Scenario: Consumer fakes the façade
+- **WHEN** a consumer depends on the terminal interface and supplies its own fake implementation in a test
+- **THEN** the consumer's code compiles and runs against the fake with no real terminal and no static state to reset
+
+#### Scenario: Synthesizing events and results in a test
+- **WHEN** a test needs to simulate an input event or a dialog result (e.g., an arrow key press or a `Submitted` selection)
+- **THEN** it can construct the corresponding public event/result type directly, without driving the parser or the render loop
+
+#### Scenario: Command-side calls are observable
+- **WHEN** consumer code issues scrollback, status, or dialog commands against a fake façade
+- **THEN** the fake can record those calls for assertion
diff --git a/openspec/changes/archive/2026-05-28-core-rendering-architecture/specs/styled-text/spec.md b/openspec/changes/archive/2026-05-28-core-rendering-architecture/specs/styled-text/spec.md
new file mode 100644
index 0000000..345f302
--- /dev/null
+++ b/openspec/changes/archive/2026-05-28-core-rendering-architecture/specs/styled-text/spec.md
@@ -0,0 +1,48 @@
+## ADDED Requirements
+
+### Requirement: Programmatic styled-text model
+The library SHALL represent styled text as `Segment` values (text plus a `Style`) composed into `Line` values, and SHALL NOT parse any markup string syntax.
+
+#### Scenario: Segments retain their styles
+- **WHEN** a caller composes a `Line` from multiple `Segment`s each carrying a distinct `Style`
+- **THEN** each segment renders with its own style and no parsing step is performed
+
+#### Scenario: Markup-like text is literal
+- **WHEN** a segment's text contains markup-like characters such as `[bold]`
+- **THEN** those characters render literally and are not interpreted as formatting
+
+### Requirement: Formatting via a flags enum
+Text attributes SHALL be expressed through a `[Flags]` `Format` enum (at minimum `None`, `Bold`, `Italic`, `Underline`, `Dim`, `Reverse`, `Strikethrough`) combinable with bitwise OR, rather than separate boolean fields.
+
+#### Scenario: Combining attributes
+- **WHEN** a `Style` specifies `Format.Bold | Format.Italic`
+- **THEN** the segment renders with both bold and italic attributes
+
+#### Scenario: No formatting
+- **WHEN** a `Style` specifies `Format.None`
+- **THEN** the segment renders with no attributes applied
+
+### Requirement: Color model
+`Style` SHALL support optional foreground and background colors expressible as named, 256-indexed, and 24-bit truecolor values.
+
+#### Scenario: Truecolor foreground
+- **WHEN** a `Style` sets a 24-bit RGB foreground
+- **THEN** the rendered output carries that 24-bit color on a truecolor-capable terminal
+
+#### Scenario: No color
+- **WHEN** a `Style` leaves foreground and background unset
+- **THEN** the segment renders using the terminal's default colors
+
+### Requirement: Fluent line builder
+The library SHALL provide a `LineBuilder` that composes a `Line` incrementally from styled fragments in append order.
+
+#### Scenario: Building a line
+- **WHEN** a caller chains builder calls adding styled fragments and then calls `Build()`
+- **THEN** the returned `Line` contains the fragments as segments in the order they were added
+
+### Requirement: Shared across both zones
+The same styled-text primitives SHALL be usable both for scrollback content and for fixed-region components.
+
+#### Scenario: Reuse in both zones
+- **WHEN** a caller builds a status line and a scrollback line
+- **THEN** both are expressed with the same `Segment`/`Line`/`Style` types
diff --git a/openspec/changes/archive/2026-05-28-core-rendering-architecture/specs/terminal-input/spec.md b/openspec/changes/archive/2026-05-28-core-rendering-architecture/specs/terminal-input/spec.md
new file mode 100644
index 0000000..315b871
--- /dev/null
+++ b/openspec/changes/archive/2026-05-28-core-rendering-architecture/specs/terminal-input/spec.md
@@ -0,0 +1,63 @@
+## ADDED Requirements
+
+### Requirement: Raw-mode VT byte stream
+The library SHALL place the terminal into raw mode and read a raw VT byte stream directly, rather than relying on `Console.ReadKey`. It SHALL target modern VT-capable terminals and SHALL NOT support the legacy non-VT console input path.
+
+#### Scenario: Raw mode disables cooked behavior
+- **WHEN** a session starts
+- **THEN** canonical line buffering, local echo, and signal generation are disabled and input is delivered byte-wise
+
+#### Scenario: Non-VT terminal is rejected
+- **WHEN** the terminal is not VT-capable
+- **THEN** the library fails with a clear error rather than rendering incorrectly
+
+### Requirement: VT input parser and event model
+The library SHALL parse the byte stream into input events: `KeyEvent` (with `KeyCode` being either `Char(Rune)` or a `Named` key, plus modifier flags), `PasteEvent` (a single string), and `ResizeEvent`. Each key SHALL carry a single `Rune`; grapheme clustering SHALL be handled at the editing layer, not in `KeyEvent`.
+
+#### Scenario: Arrow key decoded
+- **WHEN** the bytes `ESC [ A` are received
+- **THEN** a `KeyEvent` with a `Named` code of `Up` is produced
+
+#### Scenario: Unicode key decoded
+- **WHEN** a multi-byte UTF-8 character is typed
+- **THEN** a `KeyEvent` carrying `Char(Rune)` for that Unicode scalar is produced
+
+#### Scenario: Paste delivered as one event
+- **WHEN** pasted text arrives wrapped in bracketed-paste markers
+- **THEN** a single `PasteEvent` carries the entire pasted text
+
+### Requirement: Escape disambiguation by timed read
+The parser SHALL distinguish a lone Escape key from the start of an escape sequence using a read timeout.
+
+#### Scenario: Lone escape
+- **WHEN** `ESC` is received and no further bytes arrive within the timeout
+- **THEN** a `KeyEvent` with `Named` code `Escape` is produced
+
+#### Scenario: Escape sequence
+- **WHEN** `ESC` is immediately followed by `[ A`
+- **THEN** the input is parsed as `Up` and not as `Escape`
+
+### Requirement: Terminal-truth key reporting
+Under VT encoding the library SHALL report the named key for control-character collisions (Tab, Enter, Backspace, Escape), SHALL treat Shift as implicit in printable runes, and SHALL NOT report Shift on Ctrl-letter combinations.
+
+#### Scenario: Tab not Ctrl+I
+- **WHEN** byte `0x09` is received
+- **THEN** a `Tab` `KeyEvent` is produced rather than `Ctrl+I`
+
+#### Scenario: Shift implicit in printable
+- **WHEN** the character `A` is typed
+- **THEN** the `KeyEvent` is `Char('A')` with no Shift modifier set
+
+### Requirement: Ctrl+C surfaced, not acted upon
+The library SHALL deliver Ctrl+C as a `KeyEvent` and SHALL NOT itself terminate or interrupt the process; interpreting Ctrl+C is the consumer's responsibility.
+
+#### Scenario: Ctrl+C as a key event
+- **WHEN** the user presses Ctrl+C
+- **THEN** a `KeyEvent(Char('c'), Ctrl)` is delivered and the library does not terminate the process
+
+### Requirement: Deferred input features
+Mouse input, the Kitty keyboard protocol, and focus events SHALL be out of scope for v1, and the parser SHALL be structured so they can be added without redesign.
+
+#### Scenario: Mouse reporting off by default
+- **WHEN** a session starts
+- **THEN** mouse reporting is not enabled
diff --git a/openspec/changes/archive/2026-05-28-core-rendering-architecture/specs/test-harness/spec.md b/openspec/changes/archive/2026-05-28-core-rendering-architecture/specs/test-harness/spec.md
new file mode 100644
index 0000000..0060c2f
--- /dev/null
+++ b/openspec/changes/archive/2026-05-28-core-rendering-architecture/specs/test-harness/spec.md
@@ -0,0 +1,51 @@
+## ADDED Requirements
+
+### Requirement: Headless terminal over the real engine
+The testing surface SHALL provide a `HeadlessTerminal` that implements the same public terminal interface as the production façade and runs the real render loop, layout, overlay routing, and scrollback model with no terminal, no raw-mode session, and no real stdout. It SHALL be constructed by substituting only the OS-facing edges (raw-mode session, input source, terminal-size source, output sink, clock); the load-bearing engine SHALL NOT be reimplemented or mocked.
+
+#### Scenario: Drives the real engine without a terminal
+- **WHEN** a test starts a `HeadlessTerminal`
+- **THEN** the real render loop, layout, and scrollback run against in-memory edges, requiring no TTY, no raw mode, and no real stdout
+
+#### Scenario: One engine, swapped edges
+- **WHEN** the production terminal and the headless terminal are compared
+- **THEN** they share a single render-loop / layout / scrollback implementation and differ only in the OS-facing edge implementations
+
+### Requirement: Scripted input
+The harness SHALL let a test inject input at both the byte level and the event level: raw bytes (exercising the real `VtInputParser`), synthesized key events, literal typed text, pasted text, and resize events.
+
+#### Scenario: Feed raw bytes through the parser
+- **WHEN** a test feeds a raw byte sequence (e.g., the escape sequence for an arrow key)
+- **THEN** the real parser decodes it and the resulting event flows through the loop as if it had been read from a terminal
+
+#### Scenario: Inject high-level input
+- **WHEN** a test types text, sends a named key, pastes text, or triggers a resize
+- **THEN** the corresponding input is delivered to the loop without going through the OS input path
+
+### Requirement: Deterministic settling
+The harness SHALL provide an awaitable settle operation that drains all currently-pending inbound work and produces exactly one coalesced frame while advancing no wall-clock time, so assertions are deterministic and require no sleeps.
+
+#### Scenario: Settle produces a single frame
+- **WHEN** a test posts several commands and/or input events and then awaits settle
+- **THEN** all pending work is applied and exactly one frame is produced, with no real time elapsed
+
+### Requirement: Virtual clock for cadence tests
+The harness SHALL expose a controllable virtual clock so that time-dependent behaviour (frame-interval throttling, burst-coalescing windows, idle) can be tested by advancing time explicitly rather than by sleeping.
+
+#### Scenario: Advance time to trigger a throttled paint
+- **WHEN** a test advances the virtual clock past the minimum frame interval while a paint is pending
+- **THEN** the throttled paint becomes due, with no real time elapsed
+
+### Requirement: Structured frame snapshot
+The harness SHALL expose the rendered result as a structured snapshot — the live-window visual rows as styled text runs, the fixed-region layout, the caret position, and the active overlay — suitable for golden-frame assertions. Assertions over raw output bytes SHALL be confined to a separate fidelity check and SHALL NOT be the primary observation surface.
+
+#### Scenario: Assert a rendered frame logically
+- **WHEN** a test reads the snapshot after settling
+- **THEN** it can assert on the visual rows, their styles, the caret position, and the active overlay without parsing escape sequences
+
+### Requirement: Independent, documented testing package
+The harness SHALL ship as a separate package (`Dcli.Testing`) with a documented public API, and production code SHALL NOT depend on it. The structured snapshot SHALL be a testing-only contract that production behaviour and consumer semantics do not depend on.
+
+#### Scenario: Production takes no dependency on the harness
+- **WHEN** the production `dcli` package is built and consumed
+- **THEN** it has no dependency on `Dcli.Testing`, and the harness can be versioned independently
diff --git a/openspec/changes/archive/2026-05-28-core-rendering-architecture/tasks.md b/openspec/changes/archive/2026-05-28-core-rendering-architecture/tasks.md
new file mode 100644
index 0000000..3e54cf1
--- /dev/null
+++ b/openspec/changes/archive/2026-05-28-core-rendering-architecture/tasks.md
@@ -0,0 +1,118 @@
+## 1. Project setup
+
+- [x] 1.1 Create the `dcli` class-library project (target `net10.0`; nullable + analyzers enabled) and the test project
+- [x] 1.2 Add CI building/testing on Linux, macOS, and Windows; wire NuGet packaging metadata (id, license, README placeholder)
+- [x] 1.3 Define the public namespace/assembly layout and `InternalsVisibleTo` for tests
+
+## 2. Styled text (styled-text)
+
+- [x] 2.1 Implement `[Flags] Format` enum (None, Bold, Italic, Underline, Dim, Reverse, Strikethrough)
+- [x] 2.2 Implement `Color` (named / 256-indexed / 24-bit truecolor) and `Style` (optional fg/bg + `Format`)
+- [x] 2.3 Implement `Segment` and `Line` records and `LineBuilder` (append-order composition)
+- [x] 2.4 Tests: segments retain styles, markup-like text is literal, `Format` combination, builder ordering
+
+## 3. Display width & wrapping (shared)
+
+- [x] 3.1 Implement display-width measurement (wcwidth: wide CJK = 2, combining/zero-width = 0, control/tab handling)
+- [x] 3.2 Implement width-aware wrapping of a `Line` into visual rows at a given width
+- [x] 3.3 Tests: CJK/emoji/combining/zero-width width counts; wrap boundaries
+
+## 4. Terminal driver: raw mode & capability detection (terminal-input)
+
+- [x] 4.1 Implement `RawModeSession` for POSIX (P/Invoke `termios`: clear ICANON/ECHO/ISIG/IEXTEN, IXON/ICRNL; VMIN=0/VTIME=1) with platform-specific struct layout
+- [x] 4.2 Implement `RawModeSession` for Windows (`SetConsoleMode`: +VIRTUAL_TERMINAL_INPUT, −LINE/ECHO/PROCESSED_INPUT; +VIRTUAL_TERMINAL_PROCESSING on output)
+- [x] 4.3 Capability detection: require a modern VT terminal; fail with a clear error otherwise
+- [x] 4.4 Guaranteed restore: `IDisposable` + `PosixSignalRegistration` (SIGINT/SIGTERM/SIGQUIT/SIGCONT) + `AppDomain.ProcessExit`
+- [x] 4.5 Tests/manual harness: raw mode entered; restore on normal exit, exception, and signal
+
+## 5. VT input parser (terminal-input)
+
+- [x] 5.1 Implement the parser state machine (GROUND → ESC → CSI → emit) with UTF-8 accumulation, emitting `KeyEvent`/`PasteEvent`/`ResizeEvent`
+- [x] 5.2 Implement `KeyCode = Char(Rune) | Named(...)` + `[Flags]` modifiers; named-key table (arrows, Home/End, PgUp/Dn, F-keys, BackTab, etc.)
+- [x] 5.3 ESC disambiguation via read timeout; Alt-as-ESC-prefix
+- [x] 5.4 Bracketed paste (`ESC[?2004h`) → single `PasteEvent`
+- [x] 5.5 Terminal-truth rules: named key for Ctrl-letter collisions; Shift implicit in runes; no Shift under Ctrl
+- [x] 5.6 Tests: byte-fixture suite covering arrows, unicode scalars, paste, lone-ESC vs sequence, Tab≡0x09, 'A' has no Shift
+
+## 6. Input reader thread (terminal-input)
+
+- [x] 6.1 Implement `InputReader` on a dedicated thread doing timed byte reads, feeding the parser, posting events to the inbound channel
+- [x] 6.2 Surface Ctrl+C as a `KeyEvent` (no process termination by the library)
+
+## 7. Render loop core (render-loop)
+
+- [x] 7.1 Define the inbound message union (input events + API commands) and outbound `TerminalEvent` types
+- [x] 7.2 Implement the single-writer loop on a dedicated thread: `await WaitToReadAsync(orUntil: nextPaintDeadline)`, drain-all, apply, throttled paint
+- [x] 7.3 Implement fire-and-forget command intake (unbounded inbound `Channel`) and snapshot reads (terminal size)
+- [x] 7.4 Implement the outbound `Channel<TerminalEvent>` (consumer drains on its own thread; loop never runs consumer code)
+- [x] 7.5 Implement `Terminal.StartAsync` / `DisposeAsync` lifecycle, wiring raw mode + restore into the loop's `finally`
+- [x] 7.6 Tests: no interleaving under concurrent producers; burst coalescing → one frame; idle = no repaint; slow consumer doesn't stall the loop (deterministic, via the headless harness — §15)
+
+## 8. Frame painting (render-loop — Open Question #1)
+
+- [x] 8.1 Implement `renderFrame()` cursor accounting for the bounded live window + fixed region (move-to-anchor, clear, repaint)
+- [x] 8.2 Wrap each frame in a synchronized-output fence (`ESC[?2026h … l`); no-op safely where unsupported
+- [x] 8.3 Park/hide the hardware cursor per fixed-region rules at end of frame
+- [x] 8.4 (Deferred) per-line diff reconciler as a flicker optimization — leave a seam, do not implement in v1
+- [x] 8.5 Tests: golden-frame output for representative live-window + fixed-region states (snapshot via the headless harness — §15)
+
+## 9. Scrollback model (inline-scrollback)
+
+- [x] 9.1 Implement the `ILineObject` contract `Render(width) → rows` for `TextBlock`
+- [x] 9.2 Implement the commit horizon + bounded live window; commit/freeze topmost live lines on overflow; drop committed objects from memory
+- [x] 9.3 Implement the live (streaming) block: `AppendText` / `SetContent` / `Commit`
+- [x] 9.4 Implement one-way `Collapsible`: collapsed summary, single `Expand`, freeze-collapsed at horizon
+- [x] 9.5 Implement oversized-expansion reprint-into-flow (β)
+- [x] 9.6 Tests: overflow commits & never rewrites frozen rows; live append/settle/commit; expand-once & no-recollapse; oversized reprint
+
+## 10. Fixed region: input editor & status (fixed-region)
+
+- [x] 10.1 Implement the owned input `TextBuffer`: caret, multiline, width-aware wrap, internal scroll, history recall
+- [x] 10.2 Implement grapheme-cluster handling at the editing layer (navigate/measure by visual character)
+- [x] 10.3 Implement the `StatusLine` component and the bottom-pinned component stack
+- [x] 10.4 Implement the `MaxHeight` budget `clamp(appSet ?? 50%, 8, rows)` + internal-scroll priority (status sacred / caret visible / overlay absorbs squeeze)
+- [x] 10.5 Tests: wrap tracks caret; history recall; default/min-floor/tiny-terminal cap values
+
+## 11. Fixed region: overlays — autocomplete & dialog (fixed-region)
+
+- [x] 11.1 Implement the reusable `ScrollableList` (bounded viewport, selection, auto-scroll, optional multi-select via space)
+- [x] 11.2 Implement the `OverlayState = None | Dialog | Autocomplete` invariant + intercept-chain routing (active overlay first)
+- [x] 11.3 Implement Autocomplete (below input): show/hide, consumer-supplied candidates, apply accepted insert-text to the buffer
+- [x] 11.4 Implement the Dialog slot (above input): modal-by-default with opt-in type-to-filter; hide hardware cursor while modal
+- [x] 11.5 Tests: dialog suppresses autocomplete; nav keys consumed vs printable fall-through; modal captures all keys; cursor placement
+
+## 12. Public API: dialogs, events, façade (render-loop + fixed-region)
+
+- [x] 12.1 Implement `DialogResult<T>` / `DialogOutcome` and the awaitable dialog methods (`SelectAsync`, `MultiSelectAsync`, `InputAsync`, `ChoiceAsync`) via TCS-bundled open-dialog commands
+- [x] 12.2 Implement dialog cancellation via `CancellationToken` (closes overlay → `Cancelled`) and reject/queue a second concurrent dialog
+- [x] 12.3 Implement the `Scrollback`, `Input`, `Status`, `Autocomplete` façade surfaces over the inbound channel
+- [x] 12.4 Implement the `Events` stream emission (`InputSubmitted`, `InputChanged`, `KeyPressed`, `Resized`)
+- [x] 12.5 Tests: select submit/cancel; multi-select toggle; cancellation token; input-change → candidates round-trip
+- [x] 12.6 Expose the façade as `ITerminal` (+ `IScrollback`/`IInput`/`IStatus`/`IAutocomplete`); make event/result types (`KeyEvent`/`PasteEvent`/`ResizeEvent`/`TerminalEvent.*`/`DialogResult<T>`/`DialogOutcome`) public and constructible; no static/singleton state on any consumer-facing path (tier A — `test-harness`)
+- [x] 12.7 Tests: a hand-written fake `ITerminal` substitutes for the real façade; synthesized events/results drive consumer-style code; command-side calls (scrollback/status/dialog) are recorded and asserted
+
+## 13. Resize & reflow (Open Question #2)
+
+- [x] 13.1 Wire resize delivery: `PosixSignalRegistration(SIGWINCH)` on POSIX; buffer-size event / polling on Windows → inbound `ResizeEvent`
+- [x] 13.2 On resize, recompute width and reflow + repaint the live window and fixed region (frozen scrollback left to the terminal)
+- [x] 13.3 Detect truecolor / synchronized-output / unicode-width support; record fallbacks
+- [x] 13.4 Tests: live window reflows on width change; cap recomputes; no rewrite of frozen content
+
+## 14. Cross-platform validation & packaging
+
+- [x] 14.1 Manual/smoke validation on macOS (xterm-class). Windows Terminal and Linux smoke are **deferred to a follow-up change** — the implementer has no access to those OSes; cross-platform `dotnet build`/`dotnet test` continues to run in CI on all three. The follow-up should reuse 14.2's demo as the smoke driver.
+- [x] 14.2 Build a sample/demo app exercising streaming, collapsible thinking, autocomplete, and a wizard-style dialog sequence
+- [x] 14.3 Finalize NuGet package (README, XML docs on the public surface) and **produce** a pre-release build (`dotnet pack`) — actual `nuget.org` publication is out of scope until the deferred 14.1 Win/Linux smoke clears
+- [x] 14.4 Port a vertical slice of dmon's `Dmon.Terminal` (the `WizardEngine` + `WizardRenderer` flow, source at `dmon-core/src/Dmon.Terminal/`) onto dcli to validate the API ergonomics end-to-end (the §15 headless-harness tests for this slice land alongside §15, not here)
+
+## 15. Headless test harness — `Dcli.Testing` (test-harness)
+
+> The OS-facing edges are introduced behind interfaces as the driver/loop/painting are built (§4, §6–§8); this section generalizes that substrate into the public, documented `Dcli.Testing` package and retargets dcli's own terminal-free tests onto it.
+
+- [x] 15.1 Factor the OS-facing edges behind interfaces — raw-mode session, input byte source, terminal-size source, output sink, clock — so the real loop/driver run against swappable edges (the load-bearing core is never mocked)
+- [x] 15.2 Implement `HeadlessTerminal` (a real `ITerminal` over in-memory edges) with scripted input drivers: `Feed(bytes)`, `SendKey`, `Type`, `Paste`, `Resize`
+- [x] 15.3 Implement deterministic `SettleAsync` (drain all pending work → exactly one coalesced frame, no wall-clock time) and a controllable virtual clock for cadence tests
+- [x] 15.4 Implement the structured frame `Snapshot` (live-window rows as styled runs, fixed-region layout, caret position, active overlay) + a human-diffable pretty-printer for golden frames
+- [x] 15.5 Ship `Dcli.Testing` as a separate package with an XML-documented public API; the production `dcli` package takes no dependency on it
+- [x] 15.6 Retarget the terminal-free tests onto the harness (parser fixtures §5.6, loop coalescing §7.6, golden frames §8.5) so dcli and its consumers share one substrate
+- [x] 15.7 Tests: byte-feed → parsed event; `SettleAsync` → a single frame; virtual-clock advance triggers a throttled paint; snapshot reflects scrollback content + overlay state
diff --git a/openspec/config.yaml b/openspec/config.yaml
new file mode 100644
index 0000000..392946c
--- /dev/null
+++ b/openspec/config.yaml
@@ -0,0 +1,20 @@
+schema: spec-driven
+
+# Project context (optional)
+# This is shown to AI when creating artifacts.
+# Add your tech stack, conventions, style guides, domain knowledge, etc.
+# Example:
+#   context: |
+#     Tech stack: TypeScript, React, Node.js
+#     We use conventional commits
+#     Domain: e-commerce platform
+
+# Per-artifact rules (optional)
+# Add custom rules for specific artifacts.
+# Example:
+#   rules:
+#     proposal:
+#       - Keep proposals under 500 words
+#       - Always include a "Non-goals" section
+#     tasks:
+#       - Break tasks into chunks of max 2 hours
diff --git a/openspec/specs/fixed-region/spec.md b/openspec/specs/fixed-region/spec.md
new file mode 100644
index 0000000..681548a
--- /dev/null
+++ b/openspec/specs/fixed-region/spec.md
@@ -0,0 +1,105 @@
+## Purpose
+
+The `fixed-region` capability defines the pinned bottom component stack: an owned input editor, two mutually-exclusive overlays (a Dialog slot above the input, Autocomplete below), the reusable scrollable selection list, status lines, height budgeting, and intercept-chain key routing.
+
+## Requirements
+
+### Requirement: Bottom-pinned component stack
+The fixed region SHALL be a contiguous, bottom-pinned stack of components — input, status, and overlays — with the live window rendered above it.
+
+#### Scenario: Stays pinned while content streams
+- **WHEN** scrollback content streams into the live window
+- **THEN** the fixed region remains pinned at the bottom and is redrawn in place
+
+### Requirement: Owned input editor
+The library SHALL own an input editor supporting caret movement, multiline text, display-width-aware wrapping, history recall, and internal scrolling when its content exceeds its allotted height.
+
+#### Scenario: Wrapping tracks the caret
+- **WHEN** input text exceeds the available width
+- **THEN** the text wraps and the caret is positioned at the correct visual row and column
+
+#### Scenario: History recall
+- **WHEN** the user navigates input history
+- **THEN** the buffer is replaced with the recalled entry
+
+### Requirement: Fixed-region height budget
+The fixed region SHALL have a `MaxHeight` equal to `clamp(appSet ?? 50% of rows, 8, rows)`, SHALL consume only the rows its content needs up to that cap, and SHALL scroll components internally beyond the cap.
+
+#### Scenario: Default cap
+- **WHEN** no `MaxHeight` is configured on a 24-row terminal
+- **THEN** the cap is 12 rows
+
+#### Scenario: Minimum floor
+- **WHEN** 50% of the terminal height is below 8 rows and the terminal has at least 8 rows
+- **THEN** the cap is 8 rows
+
+#### Scenario: Tiny terminal
+- **WHEN** the terminal has fewer than 8 rows
+- **THEN** the cap is the terminal height
+
+### Requirement: Mutually-exclusive overlays
+At most one overlay SHALL be active at a time — a Dialog above the input or Autocomplete below it; opening a Dialog SHALL suppress any active Autocomplete, and Autocomplete SHALL NOT open while a Dialog is active.
+
+#### Scenario: Dialog suppresses autocomplete
+- **WHEN** a Dialog opens while Autocomplete is showing
+- **THEN** the Autocomplete overlay is hidden
+
+#### Scenario: Single active overlay
+- **WHEN** a Dialog is active and an Autocomplete show is requested
+- **THEN** the Autocomplete does not appear
+
+### Requirement: Intercept-chain key routing
+Keys SHALL be routed through an ordered chain with the active overlay first, then the input editor; each component either consumes a key or passes it to the next.
+
+#### Scenario: Overlay consumes navigation keys
+- **WHEN** Autocomplete is active and the up or down arrow is pressed
+- **THEN** Autocomplete consumes the key and the input caret does not move
+
+#### Scenario: Unhandled keys fall through
+- **WHEN** Autocomplete is active and a printable key is pressed
+- **THEN** the key falls through to the input editor and is inserted
+
+#### Scenario: Modal dialog captures all keys
+- **WHEN** a modal Dialog is active
+- **THEN** it consumes every key and the input editor receives none
+
+### Requirement: Single cursor placement
+The hardware cursor SHALL park at the input caret normally and during Autocomplete, SHALL be hidden while a modal Dialog is active, and overlay selection SHALL be shown by styling rather than the cursor.
+
+#### Scenario: Cursor stays in input during autocomplete
+- **WHEN** the user arrows through Autocomplete candidates
+- **THEN** the hardware cursor remains at the input caret and the selected candidate is shown highlighted
+
+#### Scenario: Modal dialog hides the cursor
+- **WHEN** a modal Dialog is active
+- **THEN** the hardware cursor is hidden
+
+### Requirement: Awaitable modal dialogs
+The library SHALL expose select, multi-select, input, and choice dialogs as awaitable operations that return a `DialogResult` whose outcome is `Submitted`, `Back`, or `Cancelled`.
+
+#### Scenario: Select submitted
+- **WHEN** the user highlights an item in a select dialog and presses Enter
+- **THEN** the awaited result is `Submitted` carrying the chosen index
+
+#### Scenario: Cancelled by escape
+- **WHEN** the user presses Escape in a dialog
+- **THEN** the awaited result is `Cancelled`
+
+#### Scenario: Multi-select toggling
+- **WHEN** the user presses space on items in a multi-select dialog
+- **THEN** those items toggle in the returned selection set
+
+#### Scenario: Cancellation token closes the dialog
+- **WHEN** the `CancellationToken` passed to a dialog is cancelled
+- **THEN** the overlay closes and the awaited result is `Cancelled`
+
+### Requirement: Consumer-driven autocomplete
+Autocomplete candidates SHALL be supplied by the consumer in response to input-change notifications; the library SHALL render and navigate them within the overlay's row cap and apply the accepted candidate's insert text to the input buffer.
+
+#### Scenario: Render supplied candidates
+- **WHEN** the consumer supplies candidates
+- **THEN** they render below the input up to the overlay's maximum rows with internal scrolling
+
+#### Scenario: Accept a candidate
+- **WHEN** the user accepts a highlighted candidate
+- **THEN** the candidate's insert text is applied to the input buffer
diff --git a/openspec/specs/inline-scrollback/spec.md b/openspec/specs/inline-scrollback/spec.md
new file mode 100644
index 0000000..4d64aef
--- /dev/null
+++ b/openspec/specs/inline-scrollback/spec.md
@@ -0,0 +1,82 @@
+## Purpose
+
+The `inline-scrollback` capability defines the append-mostly scrollback buffer — line-objects, the commit horizon, one-way collapsibles, and the content-model-to-visual-rows render contract — so styled output flows into the terminal's native scrollback while a bounded live window remains re-renderable.
+
+## Requirements
+
+### Requirement: Inline rendering preserves native scrollback
+The library SHALL render inline and SHALL NOT use the terminal's alternate screen buffer; committed output SHALL remain in the terminal's native scrollback.
+
+#### Scenario: Committed output stays in terminal history
+- **WHEN** content has been committed and the user scrolls the terminal back
+- **THEN** the prior output is visible in the terminal's native scrollback
+
+#### Scenario: Alternate screen is not used
+- **WHEN** a rendering session starts
+- **THEN** the alternate screen buffer is never entered
+
+### Requirement: Bounded live window and commit horizon
+The library SHALL maintain a live, re-renderable window no taller than `terminalRows − fixedRegionHeight`; content above that window SHALL be committed (frozen, write-once) into native scrollback and never rewritten.
+
+#### Scenario: Overflow commits the top
+- **WHEN** appended content would make the live window exceed its maximum height
+- **THEN** the topmost live lines are emitted into native scrollback and frozen
+
+#### Scenario: Frozen content is never rewritten
+- **WHEN** a line has been committed past the horizon
+- **THEN** the library issues no further writes that target that line
+
+### Requirement: Flat line-object model
+Scrollback SHALL be a flat, ordered list of line-objects, each being either a text block or a collapsible, and each able to render itself to visual rows for a given display width.
+
+#### Scenario: Width-aware wrapping
+- **WHEN** a line-object renders at a width narrower than its content
+- **THEN** it wraps to multiple visual rows
+
+#### Scenario: Display-width counting
+- **WHEN** content contains wide (CJK) or zero-width characters
+- **THEN** the visual row count is computed from display width, not code-unit length
+
+### Requirement: Live streaming block
+The library SHALL provide a live block that accepts appended text and whose content may be replaced wholesale before the block is committed.
+
+#### Scenario: Streaming append
+- **WHEN** text tokens are appended to a live block
+- **THEN** they appear in the live window as they arrive
+
+#### Scenario: Replace then commit
+- **WHEN** a live block's content is replaced and the block is then committed
+- **THEN** the replaced content is what freezes into native scrollback
+
+### Requirement: One-way collapsible
+A collapsible line-object SHALL begin collapsed (showing a summary) and SHALL expand at most once; it SHALL NOT re-collapse.
+
+#### Scenario: Expand reveals hidden lines
+- **WHEN** a collapsed collapsible is expanded
+- **THEN** its hidden lines become visible and remain visible
+
+#### Scenario: Cannot re-collapse
+- **WHEN** a collapse is requested on a collapsible
+- **THEN** the request has no effect and the collapsible stays expanded once expanded
+
+#### Scenario: Freezes collapsed at the horizon
+- **WHEN** a still-collapsed collapsible commits past the horizon before being expanded
+- **THEN** it freezes in the collapsed state and can no longer be expanded
+
+### Requirement: Oversized expansion reprints into flow
+WHEN expanding a collapsible would make it taller than the live window, the library SHALL reprint the expanded content as normal flowing output rather than keeping it re-renderable.
+
+#### Scenario: Expansion larger than the live window
+- **WHEN** an expanded collapsible would exceed the live window height
+- **THEN** its content is emitted as flowing output and scrolls into native scrollback
+
+### Requirement: Scrollback command surface
+The library SHALL expose commands to append a line, append a rule/separator, begin a live block, and begin a collapsible.
+
+#### Scenario: Append a line
+- **WHEN** a caller appends a styled line
+- **THEN** the line is enqueued for rendering into the live window
+
+#### Scenario: Begin a collapsible
+- **WHEN** a caller begins a collapsible with a summary line
+- **THEN** a collapsible line-object is created in the collapsed state showing that summary
diff --git a/openspec/specs/render-loop/spec.md b/openspec/specs/render-loop/spec.md
new file mode 100644
index 0000000..008eb57
--- /dev/null
+++ b/openspec/specs/render-loop/spec.md
@@ -0,0 +1,79 @@
+## Purpose
+
+The `render-loop` capability defines the actor-model render loop and `Terminal` lifecycle — a single-writer event loop owning all UI state, stdout and the raw-mode session; fire-and-forget command intake; an outbound event stream; coalesced/throttled frames; and guaranteed terminal restore on dispose, crash, or signal.
+
+## Requirements
+
+### Requirement: Single-writer actor loop
+All mutable UI state, stdout, and the raw-mode session SHALL be owned by a single render loop running on one dedicated thread; all state mutations SHALL be applied on that thread via an inbound channel, requiring no locks on the render model.
+
+#### Scenario: No interleaved output
+- **WHEN** content commands and keystroke-driven updates arrive concurrently from different threads
+- **THEN** output is never interleaved or corrupted because a single thread owns all writes to stdout
+
+#### Scenario: Mutations applied on the loop thread
+- **WHEN** any producer posts a mutation
+- **THEN** the mutation is applied on the render-loop thread and the model needs no lock
+
+### Requirement: Fire-and-forget command API
+Public command methods SHALL enqueue a message and return before the corresponding frame is painted; value reads SHALL be served from a snapshot rather than a round-trip to the loop.
+
+#### Scenario: Command returns before paint
+- **WHEN** a caller invokes a scrollback or status command
+- **THEN** the method enqueues the work and returns, and the effect appears on a subsequent frame
+
+#### Scenario: Snapshot read
+- **WHEN** a caller reads a value such as the terminal size
+- **THEN** the value is returned from a snapshot without blocking on the loop
+
+### Requirement: Event-driven coalesced frames
+The loop SHALL be idle when no messages are pending, SHALL drain all currently-available messages before painting, and SHALL cap painting frequency with a minimum frame interval.
+
+#### Scenario: Burst coalescing
+- **WHEN** a burst of many append commands arrives
+- **THEN** they are drained and reflected in a single painted frame
+
+#### Scenario: Idle when quiet
+- **WHEN** no messages are pending
+- **THEN** the loop performs no work and issues no repaint
+
+#### Scenario: Frame-rate cap
+- **WHEN** messages arrive continuously faster than the minimum frame interval
+- **THEN** paints occur no more often than once per minimum frame interval
+
+### Requirement: Decoupled outbound event stream
+Events from the library to the consumer SHALL be delivered on a separate channel that the consumer drains on its own thread, and the loop SHALL NOT execute consumer code.
+
+#### Scenario: Slow consumer does not stall rendering
+- **WHEN** the consumer's handling of an outbound event is slow
+- **THEN** the render loop continues painting unaffected
+
+#### Scenario: Input submission delivered
+- **WHEN** the user submits the input line
+- **THEN** an `InputSubmitted` event becomes available on the outbound stream
+
+### Requirement: Lifecycle and guaranteed restore
+Starting SHALL enter raw mode and run the loop in the background; disposal SHALL stop the loop and restore the terminal; restoration SHALL also occur on an unhandled exception and on termination signals.
+
+#### Scenario: Dispose restores the terminal
+- **WHEN** the `Terminal` is disposed
+- **THEN** raw mode is undone and the cursor is shown
+
+#### Scenario: Restore on crash or signal
+- **WHEN** the process terminates via an unhandled exception or a termination signal (e.g., SIGINT, SIGTERM)
+- **THEN** the terminal is restored so the user's shell is not left without echo
+
+### Requirement: Substitutable façade for consumer testing
+The public terminal façade SHALL be exposed as an interface (`ITerminal`, together with its `Scrollback` / `Input` / `Status` / `Autocomplete` sub-surfaces) that a consumer can replace with its own test double, and the library SHALL NOT require static or singleton state on any consumer-facing path. All consumer-facing event and dialog-result types (`KeyEvent`, `PasteEvent`, `ResizeEvent`, the `TerminalEvent` cases, `DialogResult<T>`, `DialogOutcome`) SHALL be publicly constructible so a test can synthesize them without a real terminal.
+
+#### Scenario: Consumer fakes the façade
+- **WHEN** a consumer depends on the terminal interface and supplies its own fake implementation in a test
+- **THEN** the consumer's code compiles and runs against the fake with no real terminal and no static state to reset
+
+#### Scenario: Synthesizing events and results in a test
+- **WHEN** a test needs to simulate an input event or a dialog result (e.g., an arrow key press or a `Submitted` selection)
+- **THEN** it can construct the corresponding public event/result type directly, without driving the parser or the render loop
+
+#### Scenario: Command-side calls are observable
+- **WHEN** consumer code issues scrollback, status, or dialog commands against a fake façade
+- **THEN** the fake can record those calls for assertion
diff --git a/openspec/specs/styled-text/spec.md b/openspec/specs/styled-text/spec.md
new file mode 100644
index 0000000..c11100f
--- /dev/null
+++ b/openspec/specs/styled-text/spec.md
@@ -0,0 +1,52 @@
+## Purpose
+
+The `styled-text` capability defines the programmatic `Segment`/`Line`/`Style` primitive — with a `[Flags] Format` enum and a `LineBuilder` — shared across both the scrollback and fixed-region zones. There is no markup parser: styled text is built programmatically.
+
+## Requirements
+
+### Requirement: Programmatic styled-text model
+The library SHALL represent styled text as `Segment` values (text plus a `Style`) composed into `Line` values, and SHALL NOT parse any markup string syntax.
+
+#### Scenario: Segments retain their styles
+- **WHEN** a caller composes a `Line` from multiple `Segment`s each carrying a distinct `Style`
+- **THEN** each segment renders with its own style and no parsing step is performed
+
+#### Scenario: Markup-like text is literal
+- **WHEN** a segment's text contains markup-like characters such as `[bold]`
+- **THEN** those characters render literally and are not interpreted as formatting
+
+### Requirement: Formatting via a flags enum
+Text attributes SHALL be expressed through a `[Flags]` `Format` enum (at minimum `None`, `Bold`, `Italic`, `Underline`, `Dim`, `Reverse`, `Strikethrough`) combinable with bitwise OR, rather than separate boolean fields.
+
+#### Scenario: Combining attributes
+- **WHEN** a `Style` specifies `Format.Bold | Format.Italic`
+- **THEN** the segment renders with both bold and italic attributes
+
+#### Scenario: No formatting
+- **WHEN** a `Style` specifies `Format.None`
+- **THEN** the segment renders with no attributes applied
+
+### Requirement: Color model
+`Style` SHALL support optional foreground and background colors expressible as named, 256-indexed, and 24-bit truecolor values.
+
+#### Scenario: Truecolor foreground
+- **WHEN** a `Style` sets a 24-bit RGB foreground
+- **THEN** the rendered output carries that 24-bit color on a truecolor-capable terminal
+
+#### Scenario: No color
+- **WHEN** a `Style` leaves foreground and background unset
+- **THEN** the segment renders using the terminal's default colors
+
+### Requirement: Fluent line builder
+The library SHALL provide a `LineBuilder` that composes a `Line` incrementally from styled fragments in append order.
+
+#### Scenario: Building a line
+- **WHEN** a caller chains builder calls adding styled fragments and then calls `Build()`
+- **THEN** the returned `Line` contains the fragments as segments in the order they were added
+
+### Requirement: Shared across both zones
+The same styled-text primitives SHALL be usable both for scrollback content and for fixed-region components.
+
+#### Scenario: Reuse in both zones
+- **WHEN** a caller builds a status line and a scrollback line
+- **THEN** both are expressed with the same `Segment`/`Line`/`Style` types
diff --git a/openspec/specs/terminal-input/spec.md b/openspec/specs/terminal-input/spec.md
new file mode 100644
index 0000000..a6a1ccc
--- /dev/null
+++ b/openspec/specs/terminal-input/spec.md
@@ -0,0 +1,67 @@
+## Purpose
+
+The `terminal-input` capability defines the raw-mode VT input driver — OS mode shims (`RawModeSession`), the `VtInputParser` state machine, the `KeyEvent`/`PasteEvent`/`ResizeEvent` contract, and lifecycle/restore safety. Modern VT-capable terminals only; the legacy non-VT console input path is not supported.
+
+## Requirements
+
+### Requirement: Raw-mode VT byte stream
+The library SHALL place the terminal into raw mode and read a raw VT byte stream directly, rather than relying on `Console.ReadKey`. It SHALL target modern VT-capable terminals and SHALL NOT support the legacy non-VT console input path.
+
+#### Scenario: Raw mode disables cooked behavior
+- **WHEN** a session starts
+- **THEN** canonical line buffering, local echo, and signal generation are disabled and input is delivered byte-wise
+
+#### Scenario: Non-VT terminal is rejected
+- **WHEN** the terminal is not VT-capable
+- **THEN** the library fails with a clear error rather than rendering incorrectly
+
+### Requirement: VT input parser and event model
+The library SHALL parse the byte stream into input events: `KeyEvent` (with `KeyCode` being either `Char(Rune)` or a `Named` key, plus modifier flags), `PasteEvent` (a single string), and `ResizeEvent`. Each key SHALL carry a single `Rune`; grapheme clustering SHALL be handled at the editing layer, not in `KeyEvent`.
+
+#### Scenario: Arrow key decoded
+- **WHEN** the bytes `ESC [ A` are received
+- **THEN** a `KeyEvent` with a `Named` code of `Up` is produced
+
+#### Scenario: Unicode key decoded
+- **WHEN** a multi-byte UTF-8 character is typed
+- **THEN** a `KeyEvent` carrying `Char(Rune)` for that Unicode scalar is produced
+
+#### Scenario: Paste delivered as one event
+- **WHEN** pasted text arrives wrapped in bracketed-paste markers
+- **THEN** a single `PasteEvent` carries the entire pasted text
+
+### Requirement: Escape disambiguation by timed read
+The parser SHALL distinguish a lone Escape key from the start of an escape sequence using a read timeout.
+
+#### Scenario: Lone escape
+- **WHEN** `ESC` is received and no further bytes arrive within the timeout
+- **THEN** a `KeyEvent` with `Named` code `Escape` is produced
+
+#### Scenario: Escape sequence
+- **WHEN** `ESC` is immediately followed by `[ A`
+- **THEN** the input is parsed as `Up` and not as `Escape`
+
+### Requirement: Terminal-truth key reporting
+Under VT encoding the library SHALL report the named key for control-character collisions (Tab, Enter, Backspace, Escape), SHALL treat Shift as implicit in printable runes, and SHALL NOT report Shift on Ctrl-letter combinations.
+
+#### Scenario: Tab not Ctrl+I
+- **WHEN** byte `0x09` is received
+- **THEN** a `Tab` `KeyEvent` is produced rather than `Ctrl+I`
+
+#### Scenario: Shift implicit in printable
+- **WHEN** the character `A` is typed
+- **THEN** the `KeyEvent` is `Char('A')` with no Shift modifier set
+
+### Requirement: Ctrl+C surfaced, not acted upon
+The library SHALL deliver Ctrl+C as a `KeyEvent` and SHALL NOT itself terminate or interrupt the process; interpreting Ctrl+C is the consumer's responsibility.
+
+#### Scenario: Ctrl+C as a key event
+- **WHEN** the user presses Ctrl+C
+- **THEN** a `KeyEvent(Char('c'), Ctrl)` is delivered and the library does not terminate the process
+
+### Requirement: Deferred input features
+Mouse input, the Kitty keyboard protocol, and focus events SHALL be out of scope for v1, and the parser SHALL be structured so they can be added without redesign.
+
+#### Scenario: Mouse reporting off by default
+- **WHEN** a session starts
+- **THEN** mouse reporting is not enabled
diff --git a/openspec/specs/test-harness/spec.md b/openspec/specs/test-harness/spec.md
new file mode 100644
index 0000000..72c17b9
--- /dev/null
+++ b/openspec/specs/test-harness/spec.md
@@ -0,0 +1,55 @@
+## Purpose
+
+The `test-harness` capability defines the terminal-free testing surface: a substitutable `ITerminal` façade with publicly-constructible event/result types (tier A), and the public `Dcli.Testing` headless harness (tier B) — `HeadlessTerminal`, scripted input drivers, deterministic `SettleAsync` / virtual clock, and a structured frame `Snapshot`. The same substrate dcli uses to test its own loop and frames.
+
+## Requirements
+
+### Requirement: Headless terminal over the real engine
+The testing surface SHALL provide a `HeadlessTerminal` that implements the same public terminal interface as the production façade and runs the real render loop, layout, overlay routing, and scrollback model with no terminal, no raw-mode session, and no real stdout. It SHALL be constructed by substituting only the OS-facing edges (raw-mode session, input source, terminal-size source, output sink, clock); the load-bearing engine SHALL NOT be reimplemented or mocked.
+
+#### Scenario: Drives the real engine without a terminal
+- **WHEN** a test starts a `HeadlessTerminal`
+- **THEN** the real render loop, layout, and scrollback run against in-memory edges, requiring no TTY, no raw mode, and no real stdout
+
+#### Scenario: One engine, swapped edges
+- **WHEN** the production terminal and the headless terminal are compared
+- **THEN** they share a single render-loop / layout / scrollback implementation and differ only in the OS-facing edge implementations
+
+### Requirement: Scripted input
+The harness SHALL let a test inject input at both the byte level and the event level: raw bytes (exercising the real `VtInputParser`), synthesized key events, literal typed text, pasted text, and resize events.
+
+#### Scenario: Feed raw bytes through the parser
+- **WHEN** a test feeds a raw byte sequence (e.g., the escape sequence for an arrow key)
+- **THEN** the real parser decodes it and the resulting event flows through the loop as if it had been read from a terminal
+
+#### Scenario: Inject high-level input
+- **WHEN** a test types text, sends a named key, pastes text, or triggers a resize
+- **THEN** the corresponding input is delivered to the loop without going through the OS input path
+
+### Requirement: Deterministic settling
+The harness SHALL provide an awaitable settle operation that drains all currently-pending inbound work and produces exactly one coalesced frame while advancing no wall-clock time, so assertions are deterministic and require no sleeps.
+
+#### Scenario: Settle produces a single frame
+- **WHEN** a test posts several commands and/or input events and then awaits settle
+- **THEN** all pending work is applied and exactly one frame is produced, with no real time elapsed
+
+### Requirement: Virtual clock for cadence tests
+The harness SHALL expose a controllable virtual clock so that time-dependent behaviour (frame-interval throttling, burst-coalescing windows, idle) can be tested by advancing time explicitly rather than by sleeping.
+
+#### Scenario: Advance time to trigger a throttled paint
+- **WHEN** a test advances the virtual clock past the minimum frame interval while a paint is pending
+- **THEN** the throttled paint becomes due, with no real time elapsed
+
+### Requirement: Structured frame snapshot
+The harness SHALL expose the rendered result as a structured snapshot — the live-window visual rows as styled text runs, the fixed-region layout, the caret position, and the active overlay — suitable for golden-frame assertions. Assertions over raw output bytes SHALL be confined to a separate fidelity check and SHALL NOT be the primary observation surface.
+
+#### Scenario: Assert a rendered frame logically
+- **WHEN** a test reads the snapshot after settling
+- **THEN** it can assert on the visual rows, their styles, the caret position, and the active overlay without parsing escape sequences
+
+### Requirement: Independent, documented testing package
+The harness SHALL ship as a separate package (`Dcli.Testing`) with a documented public API, and production code SHALL NOT depend on it. The structured snapshot SHALL be a testing-only contract that production behaviour and consumer semantics do not depend on.
+
+#### Scenario: Production takes no dependency on the harness
+- **WHEN** the production `dcli` package is built and consumed
+- **THEN** it has no dependency on `Dcli.Testing`, and the harness can be versioned independently
diff --git a/samples/Dcli.Demo.DmonWizard/Dcli.Demo.DmonWizard.csproj b/samples/Dcli.Demo.DmonWizard/Dcli.Demo.DmonWizard.csproj
new file mode 100644
index 0000000..b8b6cf2
--- /dev/null
+++ b/samples/Dcli.Demo.DmonWizard/Dcli.Demo.DmonWizard.csproj
@@ -0,0 +1,20 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <AssemblyName>Dcli.Demo.DmonWizard</AssemblyName>
+    <RootNamespace>Dcli.Demo.DmonWizard</RootNamespace>
+
+    <!-- Not a NuGet package. -->
+    <IsPackable>false</IsPackable>
+
+    <!-- CS1591: demo code does not need XML doc comments. -->
+    <!-- CA1303: demo is a smoke driver; hardcoded strings are intentional. -->
+    <NoWarn>CS1591;CA1303</NoWarn>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\src\Dcli\Dcli.csproj" />
+  </ItemGroup>
+
+</Project>
diff --git a/samples/Dcli.Demo.DmonWizard/Engine/WizardEngine.cs b/samples/Dcli.Demo.DmonWizard/Engine/WizardEngine.cs
new file mode 100644
index 0000000..3f812de
--- /dev/null
+++ b/samples/Dcli.Demo.DmonWizard/Engine/WizardEngine.cs
@@ -0,0 +1,120 @@
+using Dcli.Demo.DmonWizard.Providers;
+using Dcli.Demo.DmonWizard.Steps;
+
+namespace Dcli.Demo.DmonWizard.Engine;
+
+/// <summary>
+/// Drives the provider setup wizard: presents provider selection, loops on
+/// IProviderFactory.GetNextStepAsync, and returns the result on success.
+///
+/// The back-stack logic and step-routing are byte-for-byte identical to Dmon.Terminal.WizardEngine.
+/// Only the rendering dependency changes: the Spectre.Console + InlinePrompt renderer is replaced
+/// by WizardRenderer.RenderAsync(ITerminal, step, ct), and AnsiConsole.MarkupLine is replaced
+/// by IScrollback.Append.
+/// </summary>
+internal sealed class WizardEngine
+{
+    private readonly ITerminal _terminal;
+    private readonly IReadOnlyList<IProviderFactory> _factories;
+    private readonly Func<WizardStep, CancellationToken, Task<WizardStepOutcome>> _renderStep;
+
+    public WizardEngine(ITerminal terminal, IReadOnlyList<IProviderFactory> factories)
+    {
+        _terminal = terminal;
+        _factories = factories;
+        _renderStep = (step, ct) => WizardRenderer.RenderAsync(terminal, step, ct);
+    }
+
+    /// <summary>
+    /// Runs the wizard. Returns a WizardResult on success, or null if the user cancelled.
+    /// </summary>
+    public async Task<WizardResult?> RunAsync(CancellationToken cancellationToken)
+    {
+        IProviderFactory? factory = null;
+        WizardState state = WizardState.Empty;
+
+        while (true)
+        {
+            cancellationToken.ThrowIfCancellationRequested();
+
+            if (factory is null)
+            {
+                ChooseOneStep selectStep = BuildProviderSelectionStep();
+                WizardStepOutcome selectOutcome = await _renderStep(selectStep, cancellationToken)
+                    .ConfigureAwait(false);
+
+                if (selectOutcome is WizardStepOutcome.Cancel or WizardStepOutcome.Back)
+                    return null;
+
+                string adapterName = selectStep.Options[selectStep.SelectedIndex!.Value].Value;
+                factory = _factories.First(
+                    f => string.Equals(f.AdapterName, adapterName, StringComparison.OrdinalIgnoreCase));
+                state = new WizardState([selectStep]);
+                continue;
+            }
+
+            WizardStep step = await factory.GetNextStepAsync(state, cancellationToken)
+                .ConfigureAwait(false);
+
+            if (step is WizardCompletedStep completed)
+            {
+                // Replaces: AnsiConsole.MarkupLine($"[green]{Markup.Escape(completed.Message)}[/]");
+                _terminal.Scrollback.Append(new LineBuilder()
+                    .Fg(completed.Message, Color.Named(Color.AnsiColor.Green))
+                    .Build());
+                return BuildResult(factory, state);
+            }
+
+            WizardStepOutcome outcome = await _renderStep(step, cancellationToken)
+                .ConfigureAwait(false);
+
+            switch (outcome)
+            {
+                case WizardStepOutcome.Answered:
+                    List<WizardStep> newSteps = [.. state.Steps, step];
+                    state = state with { Steps = newSteps };
+                    break;
+
+                case WizardStepOutcome.Back:
+                    if (state.Steps.Count <= 1)
+                    {
+                        // Back from first factory step → return to provider selection.
+                        factory = null;
+                        state = WizardState.Empty;
+                    }
+                    else
+                    {
+                        List<WizardStep> trimmed = state.Steps.Take(state.Steps.Count - 1).ToList();
+                        state = state with { Steps = trimmed };
+                    }
+                    break;
+
+                case WizardStepOutcome.Cancel:
+                    return null;
+            }
+        }
+    }
+
+    private ChooseOneStep BuildProviderSelectionStep() =>
+        new()
+        {
+            Id = "adapter",
+            Prompt = "Select a provider:",
+            Options = _factories
+                .Select(f => new WizardOption(f.DisplayName, f.AdapterName))
+                .ToList(),
+        };
+
+    private static WizardResult BuildResult(IProviderFactory factory, WizardState state)
+    {
+        ChooseOneStep? modelStep = state.Steps
+            .OfType<ChooseOneStep>()
+            .FirstOrDefault(s => s.Id == "model");
+
+        string modelId = modelStep is not null
+            ? modelStep.Options[modelStep.SelectedIndex!.Value].Value
+            : factory.DefaultModelId;
+
+        return new WizardResult(factory.AdapterName, modelId, factory.DefaultEnvVar);
+    }
+}
diff --git a/samples/Dcli.Demo.DmonWizard/Engine/WizardRenderer.cs b/samples/Dcli.Demo.DmonWizard/Engine/WizardRenderer.cs
new file mode 100644
index 0000000..e79b307
--- /dev/null
+++ b/samples/Dcli.Demo.DmonWizard/Engine/WizardRenderer.cs
@@ -0,0 +1,138 @@
+using Dcli.Demo.DmonWizard.Steps;
+
+namespace Dcli.Demo.DmonWizard.Engine;
+
+/// <summary>
+/// Renders wizard steps using dcli's public dialog API.
+/// Replaces Spectre.Console + InlinePrompt from Dmon.Terminal.WizardRenderer.
+/// </summary>
+internal static class WizardRenderer
+{
+    public static Task<WizardStepOutcome> RenderAsync(
+        ITerminal terminal, WizardStep step, CancellationToken ct)
+    {
+        return step switch
+        {
+            ChooseOneStep s => RenderChooseOneAsync(terminal, s, ct),
+            ChooseManyStep s => RenderChooseManyAsync(terminal, s, ct),
+            TextInputStep s => RenderTextInputAsync(terminal, s, ct),
+            YesNoStep s => RenderYesNoAsync(terminal, s, ct),
+            InfoStep s => RenderInfoAsync(terminal, s, ct),
+            _ => Task.FromResult(WizardStepOutcome.Cancel),
+        };
+    }
+
+    private static async Task<WizardStepOutcome> RenderChooseOneAsync(
+        ITerminal terminal, ChooseOneStep step, CancellationToken ct)
+    {
+        List<Line> items = step.Options
+            .Select(o => new LineBuilder().Text(o.Label).Build())
+            .ToList();
+
+        DialogResult<int> result = await terminal.SelectAsync(
+            new SelectRequest(
+                Items: items,
+                Title: new LineBuilder().Bold(step.Prompt).Build()),
+            ct).ConfigureAwait(false);
+
+        if (result.Outcome == DialogOutcome.Cancelled)
+            return WizardStepOutcome.Cancel;
+
+        // API ergonomics gap: DialogOutcome.Back is never produced by v1 dialogs.
+        // Esc maps to Cancelled, not Back. We treat Cancelled as Cancel here.
+        step.SelectedIndex = result.Value;
+        return WizardStepOutcome.Answered;
+    }
+
+    private static async Task<WizardStepOutcome> RenderChooseManyAsync(
+        ITerminal terminal, ChooseManyStep step, CancellationToken ct)
+    {
+        // Ergonomics WIN: dcli MultiSelectAsync supports real multi-select.
+        // Dmon fell back to single-pick because Spectre.Console couldn't multi-select cleanly.
+        List<Line> items = step.Options
+            .Select(o => new LineBuilder().Text(o.Label).Build())
+            .ToList();
+
+        DialogResult<int[]> result = await terminal.MultiSelectAsync(
+            new MultiSelectRequest(
+                Items: items,
+                Title: new LineBuilder().Bold(step.Prompt).Build()),
+            ct).ConfigureAwait(false);
+
+        if (result.Outcome == DialogOutcome.Cancelled)
+            return WizardStepOutcome.Cancel;
+
+        step.SelectedIndices = result.Value.ToList();
+        return WizardStepOutcome.Answered;
+    }
+
+    private static async Task<WizardStepOutcome> RenderTextInputAsync(
+        ITerminal terminal, TextInputStep step, CancellationToken ct)
+    {
+        if (step.Default is not null)
+        {
+            string shown = step.Secret ? new string('*', 8) : step.Default;
+            terminal.Scrollback.Append(new LineBuilder()
+                .Dim($"Default: {shown}")
+                .Build());
+        }
+
+        DialogResult<string> result = await terminal.InputAsync(
+            new InputRequest(
+                Prompt: new LineBuilder().Bold(step.Prompt).Build(),
+                Default: step.Default,
+                IsSecret: step.Secret),
+            ct).ConfigureAwait(false);
+
+        if (result.Outcome == DialogOutcome.Cancelled)
+            return WizardStepOutcome.Cancel;
+
+        string value = result.Value ?? string.Empty;
+
+        if (value.Length == 0 && step.Default is not null)
+            value = step.Default;
+
+        if (value.Length == 0 && step.Required)
+            return WizardStepOutcome.Cancel;
+
+        step.Value = value.Length > 0 ? value : null;
+        return WizardStepOutcome.Answered;
+    }
+
+    private static async Task<WizardStepOutcome> RenderYesNoAsync(
+        ITerminal terminal, YesNoStep step, CancellationToken ct)
+    {
+        // Use ChoiceAsync for a clean Yes/No prompt.
+        // Honour step.Default by placing the default option first.
+        List<Line> options = step.Default
+            ? [new LineBuilder().Fg("Yes", Color.Named(Color.AnsiColor.Green)).Build(),
+               new LineBuilder().Fg("No",  Color.Named(Color.AnsiColor.Red)).Build()]
+            : [new LineBuilder().Fg("No",  Color.Named(Color.AnsiColor.Red)).Build(),
+               new LineBuilder().Fg("Yes", Color.Named(Color.AnsiColor.Green)).Build()];
+
+        string hint = step.Default ? "[Y/n]" : "[y/N]";
+
+        DialogResult<int> result = await terminal.ChoiceAsync(
+            new ChoiceRequest(
+                Options: options,
+                Prompt: new LineBuilder().Bold($"{step.Prompt} {hint}").Build()),
+            ct).ConfigureAwait(false);
+
+        if (result.Outcome == DialogOutcome.Cancelled)
+            return WizardStepOutcome.Cancel;
+
+        // When default=true, index 0 = Yes; when default=false, index 0 = No.
+        bool selectedYes = step.Default ? result.Value == 0 : result.Value == 1;
+        step.Answer = selectedYes;
+        return WizardStepOutcome.Answered;
+    }
+
+    private static Task<WizardStepOutcome> RenderInfoAsync(
+        ITerminal terminal, InfoStep step, CancellationToken ct)
+    {
+        terminal.Scrollback.Append(new LineBuilder()
+            .Dim(step.Prompt)
+            .Build());
+        return Task.FromResult(WizardStepOutcome.Answered);
+    }
+}
diff --git a/samples/Dcli.Demo.DmonWizard/Program.cs b/samples/Dcli.Demo.DmonWizard/Program.cs
new file mode 100644
index 0000000..5e598be
--- /dev/null
+++ b/samples/Dcli.Demo.DmonWizard/Program.cs
@@ -0,0 +1,89 @@
+// dcli API ergonomics validator — dmon WizardEngine/WizardRenderer port.
+// Exercises the multi-step wizard flow (provider-select → model-select → API-key → confirm)
+// against dcli's SelectAsync / MultiSelectAsync / InputAsync / ChoiceAsync / Scrollback.Append.
+//
+// To run interactively, replace `cts.Token` with `CancellationToken.None` in the engine.RunAsync call.
+// Run: dotnet run --project samples/Dcli.Demo.DmonWizard
+
+using Dcli;
+using Dcli.Demo.DmonWizard.Engine;
+using Dcli.Demo.DmonWizard.Providers;
+
+await using Terminal t = await Terminal.StartAsync(new TerminalOptions
+{
+    MaxFixedHeight = 14,
+    MinFrameIntervalMs = 16,
+});
+
+t.Status.SetRows(new LineBuilder()
+    .Bold("dcli wizard port")
+    .Dim(" -- dmon WizardEngine ergonomics validator")
+    .Build());
+
+t.Scrollback.Append(new LineBuilder()
+    .Bold("dmon WizardEngine port")
+    .Dim(" -- dcli API ergonomics validator")
+    .Build());
+
+t.Scrollback.Append(new LineBuilder()
+    .Dim("Exercises: SelectAsync / MultiSelectAsync / InputAsync / ChoiceAsync / Scrollback.Append")
+    .Build());
+
+t.Scrollback.Append(new LineBuilder().Text(string.Empty).Build());
+
+// Stub provider factories — representative of the real dmon provider graph.
+IReadOnlyList<IProviderFactory> factories =
+[
+    new AnthropicStub(),
+    new OpenAIStub(),
+    new MockStub(),
+];
+
+WizardEngine engine = new(t, factories);
+
+// Auto-cancel after 10s so the binary self-exits without input in CI / demo mode.
+// To run interactively, replace `cts.Token` with `CancellationToken.None`.
+using CancellationTokenSource cts = new(TimeSpan.FromSeconds(10));
+
+WizardResult? result;
+try
+{
+    result = await engine.RunAsync(cts.Token).ConfigureAwait(false);
+}
+catch (OperationCanceledException)
+{
+    result = null;
+}
+
+t.Scrollback.Append(new LineBuilder().Text(string.Empty).Build());
+
+if (result is not null)
+{
+    t.Scrollback.Append(new LineBuilder()
+        .Fg("Wizard completed.", Color.Named(Color.AnsiColor.BrightGreen))
+        .Build());
+    t.Scrollback.Append(new LineBuilder()
+        .Text("  Adapter: ")
+        .Bold(result.Adapter)
+        .Build());
+    t.Scrollback.Append(new LineBuilder()
+        .Text("  Model: ")
+        .Bold(result.ModelId)
+        .Build());
+    t.Scrollback.Append(new LineBuilder()
+        .Text("  Env var: ")
+        .Bold(result.EnvVar)
+        .Build());
+}
+else
+{
+    t.Scrollback.Append(new LineBuilder()
+        .Fg("Wizard cancelled.", Color.Named(Color.AnsiColor.Yellow))
+        .Build());
+}
+
+t.Status.SetRows(new LineBuilder()
+    .Fg("Done — exiting in 2s.", Color.Named(Color.AnsiColor.BrightGreen))
+    .Build());
+
+await Task.Delay(TimeSpan.FromSeconds(2)).ConfigureAwait(false);
diff --git a/samples/Dcli.Demo.DmonWizard/Providers/AnthropicStub.cs b/samples/Dcli.Demo.DmonWizard/Providers/AnthropicStub.cs
new file mode 100644
index 0000000..41ca702
--- /dev/null
+++ b/samples/Dcli.Demo.DmonWizard/Providers/AnthropicStub.cs
@@ -0,0 +1,53 @@
+using Dcli.Demo.DmonWizard.Steps;
+
+namespace Dcli.Demo.DmonWizard.Providers;
+
+/// <summary>
+/// Stub Anthropic provider: exercises provider-select → model-select → API-key-input → confirm flow.
+/// </summary>
+internal sealed class AnthropicStub : IProviderFactory
+{
+    public string AdapterName => "anthropic";
+    public string DisplayName => "Anthropic (Claude)";
+    public string DefaultModelId => "claude-opus-4-5";
+    public string DefaultEnvVar => "ANTHROPIC_API_KEY";
+
+    public ValueTask<WizardStep> GetNextStepAsync(WizardState state, CancellationToken cancellationToken = default)
+    {
+        WizardStep step = state.Steps.Count switch
+        {
+            // Step 0 is the adapter-selection step (already answered). Steps >= 1 are factory steps.
+            1 => new ChooseOneStep
+            {
+                Id = "model",
+                Prompt = "Select a Claude model:",
+                Options =
+                [
+                    new WizardOption("Claude Opus 4.5", "claude-opus-4-5"),
+                    new WizardOption("Claude Sonnet 4.5", "claude-sonnet-4-5"),
+                    new WizardOption("Claude Haiku 3.5", "claude-haiku-3-5"),
+                ],
+            },
+            2 => new TextInputStep
+            {
+                Id = "api-key",
+                Prompt = "Anthropic API key",
+                Secret = true,
+                Required = true,
+                Default = null,
+            },
+            3 => new InfoStep
+            {
+                Id = "info",
+                Prompt = "Anthropic configured. Validating credentials...",
+            },
+            _ => new WizardCompletedStep
+            {
+                Id = "done",
+                Prompt = string.Empty,
+                Message = "Anthropic provider configured successfully.",
+            },
+        };
+        return ValueTask.FromResult(step);
+    }
+}
diff --git a/samples/Dcli.Demo.DmonWizard/Providers/IProviderFactory.cs b/samples/Dcli.Demo.DmonWizard/Providers/IProviderFactory.cs
new file mode 100644
index 0000000..8051802
--- /dev/null
+++ b/samples/Dcli.Demo.DmonWizard/Providers/IProviderFactory.cs
@@ -0,0 +1,21 @@
+using Dcli.Demo.DmonWizard.Steps;
+
+namespace Dcli.Demo.DmonWizard.Providers;
+
+/// <summary>
+/// Minimal provider-factory contract — shape matches Dmon.Abstractions.Providers.IProviderFactory
+/// minus the Microsoft.Extensions.AI types (not available in this standalone port).
+/// </summary>
+internal interface IProviderFactory
+{
+    string AdapterName { get; }
+    string DisplayName { get; }
+    string DefaultModelId { get; }
+    string DefaultEnvVar { get; }
+
+    /// <summary>
+    /// Returns the next wizard step to present given the steps answered so far, or a
+    /// WizardCompletedStep when setup is complete.
+    /// </summary>
+    ValueTask<WizardStep> GetNextStepAsync(WizardState state, CancellationToken cancellationToken = default);
+}
diff --git a/samples/Dcli.Demo.DmonWizard/Providers/MockStub.cs b/samples/Dcli.Demo.DmonWizard/Providers/MockStub.cs
new file mode 100644
index 0000000..b7387ec
--- /dev/null
+++ b/samples/Dcli.Demo.DmonWizard/Providers/MockStub.cs
@@ -0,0 +1,27 @@
+using Dcli.Demo.DmonWizard.Steps;
+
+namespace Dcli.Demo.DmonWizard.Providers;
+
+/// <summary>
+/// Stub mock provider: immediately completes (no factory steps), used to verify the
+/// WizardCompletedStep short-circuit in the engine.
+/// </summary>
+internal sealed class MockStub : IProviderFactory
+{
+    public string AdapterName => "mock";
+    public string DisplayName => "Mock (no credentials)";
+    public string DefaultModelId => "mock-v1";
+    public string DefaultEnvVar => string.Empty;
+
+    public ValueTask<WizardStep> GetNextStepAsync(WizardState state, CancellationToken cancellationToken = default)
+    {
+        // Immediately complete — exercises the WizardCompletedStep short-circuit path.
+        WizardStep step = new WizardCompletedStep
+        {
+            Id = "done",
+            Prompt = string.Empty,
+            Message = "Mock provider ready — no credentials required.",
+        };
+        return ValueTask.FromResult(step);
+    }
+}
diff --git a/samples/Dcli.Demo.DmonWizard/Providers/OpenAIStub.cs b/samples/Dcli.Demo.DmonWizard/Providers/OpenAIStub.cs
new file mode 100644
index 0000000..cdd63de
--- /dev/null
+++ b/samples/Dcli.Demo.DmonWizard/Providers/OpenAIStub.cs
@@ -0,0 +1,55 @@
+using Dcli.Demo.DmonWizard.Steps;
+
+namespace Dcli.Demo.DmonWizard.Providers;
+
+/// <summary>
+/// Stub OpenAI provider: exercises model multi-select (demonstrates dcli's real multi-select win)
+/// and a YesNo confirmation step.
+/// </summary>
+internal sealed class OpenAIStub : IProviderFactory
+{
+    public string AdapterName => "openai";
+    public string DisplayName => "OpenAI (GPT)";
+    public string DefaultModelId => "gpt-4o";
+    public string DefaultEnvVar => "OPENAI_API_KEY";
+
+    public ValueTask<WizardStep> GetNextStepAsync(WizardState state, CancellationToken cancellationToken = default)
+    {
+        WizardStep step = state.Steps.Count switch
+        {
+            1 => new ChooseManyStep
+            {
+                Id = "models",
+                Prompt = "Select models to enable (Space to toggle, Enter to confirm):",
+                MinSelections = 1,
+                Options =
+                [
+                    new WizardOption("gpt-4o", "gpt-4o"),
+                    new WizardOption("gpt-4o-mini", "gpt-4o-mini"),
+                    new WizardOption("o1-preview", "o1-preview"),
+                    new WizardOption("o1-mini", "o1-mini"),
+                ],
+            },
+            2 => new TextInputStep
+            {
+                Id = "api-key",
+                Prompt = "OpenAI API key",
+                Secret = true,
+                Required = true,
+            },
+            3 => new YesNoStep
+            {
+                Id = "confirm",
+                Prompt = "Enable streaming responses?",
+                Default = true,
+            },
+            _ => new WizardCompletedStep
+            {
+                Id = "done",
+                Prompt = string.Empty,
+                Message = "OpenAI provider configured successfully.",
+            },
+        };
+        return ValueTask.FromResult(step);
+    }
+}
diff --git a/samples/Dcli.Demo.DmonWizard/Providers/WizardResult.cs b/samples/Dcli.Demo.DmonWizard/Providers/WizardResult.cs
new file mode 100644
index 0000000..6f0aac2
--- /dev/null
+++ b/samples/Dcli.Demo.DmonWizard/Providers/WizardResult.cs
@@ -0,0 +1,7 @@
+namespace Dcli.Demo.DmonWizard.Providers;
+
+/// <summary>
+/// The result produced by a completed wizard run.
+/// Shape matches Dmon.Terminal.WizardResult.
+/// </summary>
+internal sealed record WizardResult(string Adapter, string ModelId, string EnvVar);
diff --git a/samples/Dcli.Demo.DmonWizard/Steps/ChooseManyStep.cs b/samples/Dcli.Demo.DmonWizard/Steps/ChooseManyStep.cs
new file mode 100644
index 0000000..4569c37
--- /dev/null
+++ b/samples/Dcli.Demo.DmonWizard/Steps/ChooseManyStep.cs
@@ -0,0 +1,23 @@
+namespace Dcli.Demo.DmonWizard.Steps;
+
+/// <summary>
+/// A step asking the user to select one or more options.
+/// Shape is identical to Dmon.Abstractions.Wizard.ChooseManyStep.
+/// </summary>
+internal sealed class ChooseManyStep : WizardStep
+{
+    public required IReadOnlyList<WizardOption> Options { get; init; }
+    public int MinSelections { get; init; }
+
+    private IReadOnlyList<int>? _selectedIndices;
+
+    public IReadOnlyList<int>? SelectedIndices
+    {
+        get => _selectedIndices;
+        set
+        {
+            _selectedIndices = value;
+            IsAnswered = value is not null && value.Count >= MinSelections;
+        }
+    }
+}
diff --git a/samples/Dcli.Demo.DmonWizard/Steps/ChooseOneStep.cs b/samples/Dcli.Demo.DmonWizard/Steps/ChooseOneStep.cs
new file mode 100644
index 0000000..54cb21c
--- /dev/null
+++ b/samples/Dcli.Demo.DmonWizard/Steps/ChooseOneStep.cs
@@ -0,0 +1,22 @@
+namespace Dcli.Demo.DmonWizard.Steps;
+
+/// <summary>
+/// A step asking the user to pick exactly one option from a list.
+/// Shape is identical to Dmon.Abstractions.Wizard.ChooseOneStep.
+/// </summary>
+internal sealed class ChooseOneStep : WizardStep
+{
+    public required IReadOnlyList<WizardOption> Options { get; init; }
+
+    private int? _selectedIndex;
+
+    public int? SelectedIndex
+    {
+        get => _selectedIndex;
+        set
+        {
+            _selectedIndex = value;
+            IsAnswered = value.HasValue;
+        }
+    }
+}
diff --git a/samples/Dcli.Demo.DmonWizard/Steps/InfoStep.cs b/samples/Dcli.Demo.DmonWizard/Steps/InfoStep.cs
new file mode 100644
index 0000000..302b5f1
--- /dev/null
+++ b/samples/Dcli.Demo.DmonWizard/Steps/InfoStep.cs
@@ -0,0 +1,9 @@
+namespace Dcli.Demo.DmonWizard.Steps;
+
+/// <summary>
+/// An informational step with no answer. The renderer displays the Prompt and advances automatically.
+/// Shape is identical to Dmon.Abstractions.Wizard.InfoStep.
+/// </summary>
+internal sealed class InfoStep : WizardStep
+{
+}
diff --git a/samples/Dcli.Demo.DmonWizard/Steps/TextInputStep.cs b/samples/Dcli.Demo.DmonWizard/Steps/TextInputStep.cs
new file mode 100644
index 0000000..d266bc1
--- /dev/null
+++ b/samples/Dcli.Demo.DmonWizard/Steps/TextInputStep.cs
@@ -0,0 +1,24 @@
+namespace Dcli.Demo.DmonWizard.Steps;
+
+/// <summary>
+/// A step asking for free-form text. Secret input is masked in the terminal.
+/// Shape is identical to Dmon.Abstractions.Wizard.TextInputStep.
+/// </summary>
+internal sealed class TextInputStep : WizardStep
+{
+    public string? Default { get; init; }
+    public bool Secret { get; init; }
+    public bool Required { get; init; }
+
+    private string? _value;
+
+    public string? Value
+    {
+        get => _value;
+        set
+        {
+            _value = value;
+            IsAnswered = value is not null;
+        }
+    }
+}
diff --git a/samples/Dcli.Demo.DmonWizard/Steps/WizardCompletedStep.cs b/samples/Dcli.Demo.DmonWizard/Steps/WizardCompletedStep.cs
new file mode 100644
index 0000000..77a50d0
--- /dev/null
+++ b/samples/Dcli.Demo.DmonWizard/Steps/WizardCompletedStep.cs
@@ -0,0 +1,10 @@
+namespace Dcli.Demo.DmonWizard.Steps;
+
+/// <summary>
+/// Terminal marker returned by a factory when setup is complete.
+/// Shape is identical to Dmon.Abstractions.Wizard.WizardCompletedStep.
+/// </summary>
+internal sealed class WizardCompletedStep : WizardStep
+{
+    public required string Message { get; init; }
+}
diff --git a/samples/Dcli.Demo.DmonWizard/Steps/WizardOption.cs b/samples/Dcli.Demo.DmonWizard/Steps/WizardOption.cs
new file mode 100644
index 0000000..0a7c43f
--- /dev/null
+++ b/samples/Dcli.Demo.DmonWizard/Steps/WizardOption.cs
@@ -0,0 +1,7 @@
+namespace Dcli.Demo.DmonWizard.Steps;
+
+/// <summary>
+/// An option presented in a selection step. Label is shown in the UI; Value is stored as the answer.
+/// Shape is identical to Dmon.Abstractions.Wizard.WizardOption.
+/// </summary>
+internal sealed record WizardOption(string Label, string Value, string? Description = null);
diff --git a/samples/Dcli.Demo.DmonWizard/Steps/WizardState.cs b/samples/Dcli.Demo.DmonWizard/Steps/WizardState.cs
new file mode 100644
index 0000000..beb2242
--- /dev/null
+++ b/samples/Dcli.Demo.DmonWizard/Steps/WizardState.cs
@@ -0,0 +1,11 @@
+namespace Dcli.Demo.DmonWizard.Steps;
+
+/// <summary>
+/// Accumulates the ordered list of answered WizardStep objects.
+/// Index 0 is the adapter-selection step; subsequent entries are factory-produced.
+/// Shape is identical to Dmon.Abstractions.Wizard.WizardState.
+/// </summary>
+internal sealed record WizardState(IReadOnlyList<WizardStep> Steps)
+{
+    public static readonly WizardState Empty = new([]);
+}
diff --git a/samples/Dcli.Demo.DmonWizard/Steps/WizardStep.cs b/samples/Dcli.Demo.DmonWizard/Steps/WizardStep.cs
new file mode 100644
index 0000000..8cba29e
--- /dev/null
+++ b/samples/Dcli.Demo.DmonWizard/Steps/WizardStep.cs
@@ -0,0 +1,11 @@
+namespace Dcli.Demo.DmonWizard.Steps;
+
+/// <summary>
+/// Abstract base for all wizard steps. Shape is identical to Dmon.Abstractions.Wizard.WizardStep.
+/// </summary>
+internal abstract class WizardStep
+{
+    public required string Id { get; init; }
+    public required string Prompt { get; init; }
+    public bool IsAnswered { get; protected set; }
+}
diff --git a/samples/Dcli.Demo.DmonWizard/Steps/WizardStepOutcome.cs b/samples/Dcli.Demo.DmonWizard/Steps/WizardStepOutcome.cs
new file mode 100644
index 0000000..03ce20d
--- /dev/null
+++ b/samples/Dcli.Demo.DmonWizard/Steps/WizardStepOutcome.cs
@@ -0,0 +1,12 @@
+namespace Dcli.Demo.DmonWizard.Steps;
+
+/// <summary>
+/// The outcome of rendering a single wizard step.
+/// Shape is identical to Dmon.Terminal.WizardStepOutcome.
+/// </summary>
+internal enum WizardStepOutcome
+{
+    Answered,
+    Back,
+    Cancel,
+}
diff --git a/samples/Dcli.Demo.DmonWizard/Steps/YesNoStep.cs b/samples/Dcli.Demo.DmonWizard/Steps/YesNoStep.cs
new file mode 100644
index 0000000..6fc5d4b
--- /dev/null
+++ b/samples/Dcli.Demo.DmonWizard/Steps/YesNoStep.cs
@@ -0,0 +1,22 @@
+namespace Dcli.Demo.DmonWizard.Steps;
+
+/// <summary>
+/// A yes/no confirmation step.
+/// Shape is identical to Dmon.Abstractions.Wizard.YesNoStep.
+/// </summary>
+internal sealed class YesNoStep : WizardStep
+{
+    public bool Default { get; init; }
+
+    private bool? _answer;
+
+    public bool? Answer
+    {
+        get => _answer;
+        set
+        {
+            _answer = value;
+            IsAnswered = value.HasValue;
+        }
+    }
+}
diff --git a/samples/Dcli.Demo/Dcli.Demo.csproj b/samples/Dcli.Demo/Dcli.Demo.csproj
new file mode 100644
index 0000000..9ee657e
--- /dev/null
+++ b/samples/Dcli.Demo/Dcli.Demo.csproj
@@ -0,0 +1,20 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <AssemblyName>Dcli.Demo</AssemblyName>
+    <RootNamespace>Dcli.Demo</RootNamespace>
+
+    <!-- Not a NuGet package. -->
+    <IsPackable>false</IsPackable>
+
+    <!-- CS1591: demo code does not need XML doc comments. -->
+    <!-- CA1303: demo is a smoke driver; hardcoded strings are intentional. -->
+    <NoWarn>CS1591;CA1303</NoWarn>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\src\Dcli\Dcli.csproj" />
+  </ItemGroup>
+
+</Project>
diff --git a/samples/Dcli.Demo/Program.cs b/samples/Dcli.Demo/Program.cs
new file mode 100644
index 0000000..e4423f4
--- /dev/null
+++ b/samples/Dcli.Demo/Program.cs
@@ -0,0 +1,254 @@
+// dcli Demo — full-surface smoke driver.
+// Run: dotnet run --project samples/Dcli.Demo
+// The tour is self-driving. Each wizard dialog auto-cancels after a short timeout so
+// the binary finishes without keyboard input.
+
+using Dcli;
+
+// ── Start ───────────────────────────────────────────────────────────────────
+
+await using Terminal t = await Terminal.StartAsync(new TerminalOptions
+{
+    MaxFixedHeight = 12,
+    MinFrameIntervalMs = 16,
+});
+
+// ── Phase 1: Banner (~2s) ────────────────────────────────────────────────────
+
+t.Status.SetRows(new LineBuilder()
+    .Bold("dcli demo")
+    .Text(" -- full surface tour")
+    .Build());
+
+t.Scrollback.Append(new LineBuilder()
+    .Bold("dcli ")
+    .Italic("inline terminal rendering library")
+    .Text(" -- smoke tour")
+    .Build());
+
+t.Scrollback.Append(new LineBuilder()
+    .Fg("  Styled output flows into the real terminal scrollback.", Color.Named(Color.AnsiColor.Cyan))
+    .Build());
+
+t.Scrollback.Append(new LineBuilder()
+    .Text("  A small interactive region is pinned at the bottom.")
+    .Build());
+
+t.Scrollback.Append(new LineBuilder()
+    .Dim("  Content above the commit horizon is frozen and terminal-owned.")
+    .Build());
+
+await Task.Delay(TimeSpan.FromMilliseconds(800));
+
+// ── Phase 2: Streaming live block (~3s) ──────────────────────────────────────
+
+t.Status.SetRows(new LineBuilder().Dim("Phase 2/6 -- streaming live block").Build());
+t.Scrollback.Append(new LineBuilder().Bold("--- Streaming live block ---").Build());
+
+ILiveBlock live = t.Scrollback.BeginLive();
+
+string[] streamTokens =
+[
+    "The ", "model ", "is ", "thinking", "...\n",
+    "Generating ", "a ", "structured ", "response", "...\n",
+    "Done.",
+];
+
+foreach (string token in streamTokens)
+{
+    live.AppendText(token);
+    await Task.Delay(TimeSpan.FromMilliseconds(180));
+}
+
+// Demonstrate SetContent: replace the accumulated buffer wholesale.
+await Task.Delay(TimeSpan.FromMilliseconds(300));
+live.SetContent(
+[
+    new LineBuilder().Bold("Response (final): ").Text("Hello from dcli!").Build(),
+    new LineBuilder().Dim("  (SetContent replaced the streamed buffer)").Build(),
+]);
+
+await Task.Delay(TimeSpan.FromMilliseconds(400));
+live.Commit();
+
+t.Scrollback.Append(new LineBuilder().Dim("  Live block committed.").Build());
+await Task.Delay(TimeSpan.FromMilliseconds(300));
+
+// ── Phase 3: Collapsible "thinking" block (~2s) ───────────────────────────────
+
+t.Status.SetRows(new LineBuilder().Dim("Phase 3/6 -- collapsible block").Build());
+t.Scrollback.Append(new LineBuilder().Bold("--- Collapsible block ---").Build());
+
+// Build 24 hidden lines so the expand is visually obvious.
+List<Line> hiddenLines = [];
+for (int i = 1; i <= 24; i++)
+{
+    hiddenLines.Add(new LineBuilder()
+        .Dim($"  thinking line {i,2}: reasoning about token {i * 7}...")
+        .Build());
+}
+
+ICollapsible collapsed = t.Scrollback.BeginCollapsible(
+    summary: new LineBuilder().Dim("> thinking (24 lines hidden)").Build(),
+    hiddenLines: hiddenLines);
+
+await Task.Delay(TimeSpan.FromMilliseconds(1000));
+
+collapsed.Expand();
+
+t.Scrollback.Append(new LineBuilder()
+    .Text("  Subsequent content lands below the expanded block.")
+    .Build());
+
+await Task.Delay(TimeSpan.FromMilliseconds(500));
+
+// ── Phase 4: Autocomplete overlay (~3s) ─────────────────────────────────────
+
+t.Status.SetRows(new LineBuilder()
+    .Text("Phase 4/6 -- autocomplete: type ")
+    .Bold("/")
+    .Text(" and you would see suggestions; Esc to dismiss")
+    .Build());
+
+t.Scrollback.Append(new LineBuilder().Bold("--- Autocomplete overlay ---").Build());
+
+AutocompleteCandidate[] candidates =
+[
+    new AutocompleteCandidate(
+        "/help",
+        new LineBuilder().Bold("/help").Dim("  -- show available commands").Build()),
+    new AutocompleteCandidate(
+        "/quit",
+        new LineBuilder().Bold("/quit").Dim("  -- exit the session").Build()),
+    new AutocompleteCandidate(
+        "/clear",
+        new LineBuilder().Bold("/clear").Dim("  -- clear scrollback").Build()),
+];
+
+t.Autocomplete.Show(candidates);
+await Task.Delay(TimeSpan.FromMilliseconds(1500));
+t.Autocomplete.Hide();
+
+t.Scrollback.Append(new LineBuilder().Dim("  Autocomplete dismissed.").Build());
+await Task.Delay(TimeSpan.FromMilliseconds(300));
+
+// ── Phase 5: Wizard chain (Select → Input → MultiSelect → Choice) ────────────
+// Dialogs require keyboard input to submit. The demo auto-cancels each via a
+// CancellationTokenSource timeout so the tour is fully self-driving.
+
+t.Status.SetRows(new LineBuilder().Dim("Phase 5/6 -- wizard chain (auto-cancels after 1.5s each)").Build());
+t.Scrollback.Append(new LineBuilder().Bold("--- Wizard chain ---").Build());
+
+// 5a: Select
+{
+    using CancellationTokenSource cts = new(TimeSpan.FromMilliseconds(1500));
+
+    DialogResult<int> lang = await t.SelectAsync(
+        new SelectRequest(
+            Items:
+            [
+                new LineBuilder().Fg("C#", Color.Named(Color.AnsiColor.Cyan)).Build(),
+                new LineBuilder().Fg("Go", Color.Named(Color.AnsiColor.Yellow)).Build(),
+                new LineBuilder().Fg("Rust", Color.Named(Color.AnsiColor.Red)).Build(),
+            ],
+            Title: new LineBuilder().Bold("Pick your favourite language").Build()),
+        cts.Token);
+
+    string langText = lang.Outcome == DialogOutcome.Submitted
+        ? $"index {lang.Value}"
+        : "Cancelled";
+    t.Scrollback.Append(new LineBuilder()
+        .Text("Select result: ")
+        .Bold(langText)
+        .Build());
+}
+
+await Task.Delay(TimeSpan.FromMilliseconds(200));
+
+// 5b: Input
+{
+    using CancellationTokenSource cts = new(TimeSpan.FromMilliseconds(1500));
+
+    DialogResult<string> name = await t.InputAsync(
+        new InputRequest(
+            Prompt: new LineBuilder().Bold("What's your name?").Build(),
+            Default: "ada"),
+        cts.Token);
+
+    string nameText = name.Outcome == DialogOutcome.Submitted
+        ? name.Value
+        : "Cancelled";
+    t.Scrollback.Append(new LineBuilder()
+        .Text("Input result: ")
+        .Bold(nameText)
+        .Build());
+}
+
+await Task.Delay(TimeSpan.FromMilliseconds(200));
+
+// 5c: MultiSelect
+{
+    using CancellationTokenSource cts = new(TimeSpan.FromMilliseconds(1500));
+
+    DialogResult<int[]> features = await t.MultiSelectAsync(
+        new MultiSelectRequest(
+            Items:
+            [
+                new LineBuilder().Text("Inline scrollback rendering").Build(),
+                new LineBuilder().Text("Collapsible blocks").Build(),
+                new LineBuilder().Text("Autocomplete overlay").Build(),
+                new LineBuilder().Text("Dialog wizard chain").Build(),
+            ],
+            Title: new LineBuilder().Bold("Which features interest you?").Build()),
+        cts.Token);
+
+    string featText = features.Outcome == DialogOutcome.Submitted
+        ? $"[{string.Join(", ", features.Value)}]"
+        : "Cancelled";
+    t.Scrollback.Append(new LineBuilder()
+        .Text("MultiSelect result: ")
+        .Bold(featText)
+        .Build());
+}
+
+await Task.Delay(TimeSpan.FromMilliseconds(200));
+
+// 5d: Choice
+{
+    using CancellationTokenSource cts = new(TimeSpan.FromMilliseconds(1500));
+
+    DialogResult<int> confirm = await t.ChoiceAsync(
+        new ChoiceRequest(
+            Options:
+            [
+                new LineBuilder().Fg("Yes", Color.Named(Color.AnsiColor.Green)).Build(),
+                new LineBuilder().Fg("No", Color.Named(Color.AnsiColor.Red)).Build(),
+            ],
+            Prompt: new LineBuilder().Bold("Run the tour again?").Build()),
+        cts.Token);
+
+    string choiceText = confirm.Outcome == DialogOutcome.Submitted
+        ? (confirm.Value == 0 ? "Yes" : "No")
+        : "Cancelled";
+    t.Scrollback.Append(new LineBuilder()
+        .Text("Choice result: ")
+        .Bold(choiceText)
+        .Build());
+}
+
+await Task.Delay(TimeSpan.FromMilliseconds(300));
+
+// ── Phase 6: Finale (~3s) ────────────────────────────────────────────────────
+
+t.Status.SetRows(new LineBuilder()
+    .Fg("DONE - Tour complete - press Ctrl+C to exit, or wait 3s.", Color.Named(Color.AnsiColor.Green))
+    .Build());
+
+t.Scrollback.Append(new LineBuilder().Bold("--- Tour complete ---").Build());
+t.Scrollback.Append(new LineBuilder()
+    .Fg("All dcli public surfaces exercised successfully.", Color.Named(Color.AnsiColor.BrightGreen))
+    .Build());
+
+await Task.Delay(TimeSpan.FromSeconds(3));
+
+// await using disposes the terminal, restoring raw mode on exit.
diff --git a/samples/Dcli.RawModeHarness/Dcli.RawModeHarness.csproj b/samples/Dcli.RawModeHarness/Dcli.RawModeHarness.csproj
new file mode 100644
index 0000000..bd1ebf3
--- /dev/null
+++ b/samples/Dcli.RawModeHarness/Dcli.RawModeHarness.csproj
@@ -0,0 +1,23 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <OutputType>Exe</OutputType>
+    <AssemblyName>Dcli.RawModeHarness</AssemblyName>
+    <RootNamespace>Dcli.RawModeHarness</RootNamespace>
+
+    <!-- Not a NuGet package; not part of the library distribution. -->
+    <IsPackable>false</IsPackable>
+
+    <!-- CS1591: harness code does not need XML doc comments. -->
+    <!-- CA1303: harness is a developer tool; hardcoded strings are intentional and do not need localisation. -->
+    <NoWarn>CS1591;CA1303</NoWarn>
+
+    <!-- Required for the unsafe read(2) P/Invoke pointer loop that bypasses managed stream EOF misreporting. -->
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\src\Dcli\Dcli.csproj" />
+  </ItemGroup>
+
+</Project>
diff --git a/samples/Dcli.RawModeHarness/Program.cs b/samples/Dcli.RawModeHarness/Program.cs
new file mode 100644
index 0000000..3ddec75
--- /dev/null
+++ b/samples/Dcli.RawModeHarness/Program.cs
@@ -0,0 +1,162 @@
+// Dcli.RawModeHarness — manual verification of §4 (raw-mode session and guaranteed restore).
+//
+// Run this in a real terminal (macOS/Linux). It is NOT an automated test — it exists so a human
+// can confirm the behaviours that cannot be unit-tested without a real tty:
+//
+//   CHECK 1 — Entry:           after starting, local echo is OFF and input is byte-wise.
+//   CHECK 2 — Normal exit:     pressing 'q' exits; the shell is back to normal (echo on, cooked).
+//   CHECK 3 — Exception exit:  pressing 'x' throws; the shell is still restored.
+//   CHECK 4 — Signal restore:  sending SIGTERM/SIGINT restores the terminal.
+//   CHECK 5 — Resize delivery: resizing the terminal window prints the new dimensions.
+//
+// Windows: the Windows path compiles but runtime verification of raw-mode entry is deferred to
+// §14.1 cross-platform validation (the Windows CI runner has no interactive console). The harness
+// will detect that stdout is not a tty and exit gracefully.
+
+using System.Runtime.InteropServices;
+using System.Runtime.Versioning;
+using Dcli;
+using Dcli.Internal;
+using Dcli.Internal.Posix;
+
+Console.OutputEncoding = System.Text.Encoding.UTF8;
+
+Console.WriteLine("=== Dcli.RawModeHarness --- Section 4/13 manual check ===");
+Console.WriteLine();
+Console.WriteLine("This harness puts the terminal in raw mode and lets you verify:");
+Console.WriteLine("  CHECK 1  Echo is OFF: type a few chars -- they should NOT appear.");
+Console.WriteLine("           Ctrl+C should NOT kill the process (arrives as bytes 0x03).");
+Console.WriteLine("           Arrow keys should NOT move a cursor (arrive as ESC sequences).");
+Console.WriteLine();
+Console.WriteLine("  CHECK 2  Press 'q' -> normal exit. Verify: shell echo works after exit.");
+Console.WriteLine("  CHECK 3  Press 'x' -> simulated exception. Shell should still be restored.");
+Console.WriteLine("  CHECK 4  In another terminal, run:");
+Console.WriteLine($"             kill -TERM {Environment.ProcessId}");
+Console.WriteLine("           Then verify the original shell is back to normal.");
+Console.WriteLine("  CHECK 5  Resize the terminal window -- new dimensions should appear below.");
+Console.WriteLine();
+Console.WriteLine("Press ENTER to enter raw mode...");
+Console.ReadLine();
+
+// Verify the terminal is capable before entering raw mode.
+try
+{
+    TerminalCapabilityDetector.EnsureVtCapable();
+}
+catch (TerminalNotSupportedException ex)
+{
+    Console.WriteLine($"[SKIP] Terminal not supported: {ex.Message}");
+    Console.WriteLine("       This is expected when stdout is not a tty (e.g. CI, pipe).");
+    return;
+}
+
+Console.WriteLine("[OK] Terminal is VT-capable. Entering raw mode NOW...");
+Console.WriteLine();
+
+using var session = RawModeSession.Enter();
+using var coord = RestoreCoordinator.Wire(session);
+
+Console.Write("[RAW MODE ACTIVE] Bytes received: ");
+
+int col = "[RAW MODE ACTIVE] Bytes received: ".Length;
+bool running = true;
+
+if (OperatingSystem.IsLinux() || OperatingSystem.IsMacOS())
+{
+    RunPosixSession();
+}
+else
+{
+    Console.Write("\r\n[SKIP] Non-POSIX platform: stdin read loop deferred to §14.1.\r\n");
+}
+
+Console.Write("\r\n[DONE] Harness exited normally. Shell should be back to cooked mode.\r\n");
+
+// ── POSIX session: resize watcher + read loop ───────────────────────────────
+
+[SupportedOSPlatform("linux")]
+[SupportedOSPlatform("macos")]
+void RunPosixSession()
+{
+    PosixTerminalSizeSource sizeSource = new();
+    (int initCols, int initRows) = sizeSource.GetSize();
+    Console.Write($"\r\n[CHECK 5] Initial size: {initCols}x{initRows}. Resize the window to see new dimensions.\r\n");
+
+    using PosixResizeWatcher resizeWatcher = new(sizeSource);
+    resizeWatcher.Start((c, r) =>
+    {
+        Console.Write($"\r\n[CHECK 5] Resized to: {c}x{r}\r\n");
+    });
+
+    RunPosixLoop();
+}
+
+[SupportedOSPlatform("linux")]
+[SupportedOSPlatform("macos")]
+void RunPosixLoop()
+{
+    // Use a direct POSIX read(2) syscall instead of the managed Console stream.
+    //
+    // With VMIN=0 / VTIME=1 (set by RawModeSession), the kernel returns 0 from read() when the
+    // 100ms timeout expires with no bytes available. The managed Stream.ReadByte() path treats
+    // that 0-byte return as end-of-stream (returns -1), causing the loop to exit immediately on
+    // the first timeout tick. The direct syscall lets us distinguish:
+    //   n > 0  → bytes available; process them.
+    //   n == 0 → timeout, no data; keep waiting (the entire fix).
+    //   n < 0  → error; EINTR means retry, anything else is fatal.
+    unsafe
+    {
+        byte buf = 0;
+        while (running)
+        {
+            nint n = PosixReadInterop.read(0, &buf, 1);
+
+            if (n == 0)
+            {
+                // Timed out (VMIN=0 / VTIME=1): no data this 100ms window — keep polling.
+                continue;
+            }
+
+            if (n < 0)
+            {
+                int err = Marshal.GetLastPInvokeError();
+                if (err == PosixReadInterop.EINTR)
+                    continue;  // interrupted by a signal; retry
+
+                Console.Write($"\r\n[ERROR] read() returned {n}, errno={err}. Exiting.\r\n");
+                break;
+            }
+
+            // n == 1: got a byte.
+            string desc = buf switch
+            {
+                0x03 => "[Ctrl+C=0x03]",
+                0x1B => "[ESC=0x1B]",
+                0x0D => "[CR=0x0D]",
+                0x0A => "[LF=0x0A]",
+                _ => $"[0x{buf:X2}={(buf >= 32 && buf < 127 ? (char)buf : '?')}]",
+            };
+
+            Console.Write(desc);
+            col += desc.Length;
+            if (col > 70)
+            {
+                Console.Write("\r\n");
+                col = 0;
+            }
+
+            if (buf == (byte)'q')
+            {
+                Console.Write("\r\n[CHECK 2] 'q' pressed -- normal exit follows. Shell should restore.\r\n");
+                running = false;
+            }
+            else if (buf == (byte)'x')
+            {
+                Console.Write("\r\n[CHECK 3] 'x' pressed -- throwing exception now...\r\n");
+                // Dispose (via using) runs before the exception propagates, restoring the terminal.
+                throw new InvalidOperationException(
+                    "Simulated exception from harness (CHECK 3). Terminal should be restored.");
+            }
+        }
+    }
+}
diff --git a/src/Dcli.Testing/Dcli.Testing.csproj b/src/Dcli.Testing/Dcli.Testing.csproj
new file mode 100644
index 0000000..0a2e981
--- /dev/null
+++ b/src/Dcli.Testing/Dcli.Testing.csproj
@@ -0,0 +1,45 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <AssemblyName>Dcli.Testing</AssemblyName>
+    <RootNamespace>Dcli.Testing</RootNamespace>
+
+    <!-- NuGet packaging -->
+    <IsPackable>true</IsPackable>
+    <PackageId>dcli.testing</PackageId>
+    <Version>0.1.0-rc.1</Version>
+    <Authors>daemonicai</Authors>
+    <Company>daemonicai</Company>
+    <Description>Headless test harness for the dcli inline terminal-rendering library.</Description>
+    <PackageLicenseExpression>MPL-2.0</PackageLicenseExpression>
+    <PackageProjectUrl>https://github.com/daemonicai/dcli</PackageProjectUrl>
+    <RepositoryUrl>https://github.com/daemonicai/dcli</RepositoryUrl>
+    <RepositoryType>git</RepositoryType>
+    <PackageTags>terminal;cli;tui;rendering;testing;headless</PackageTags>
+    <PackageReadmeFile>README.md</PackageReadmeFile>
+    <PackageReleaseNotes>Headless test harness for dcli: HeadlessTerminal, scripted input drivers, deterministic SettleAsync, virtual clock, structured frame Snapshot.</PackageReleaseNotes>
+
+    <!-- Source Link / symbols -->
+    <PublishRepositoryUrl>true</PublishRepositoryUrl>
+    <EmbedUntrackedSources>true</EmbedUntrackedSources>
+    <IncludeSymbols>true</IncludeSymbols>
+    <SymbolPackageFormat>snupkg</SymbolPackageFormat>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <None Include="README.md" Pack="true" PackagePath="\" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <InternalsVisibleTo Include="Dcli.Tests" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\Dcli\Dcli.csproj" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.SourceLink.GitHub" Version="8.0.0" PrivateAssets="All" />
+  </ItemGroup>
+
+</Project>
diff --git a/src/Dcli.Testing/FrameSnapshot.cs b/src/Dcli.Testing/FrameSnapshot.cs
new file mode 100644
index 0000000..56382dd
--- /dev/null
+++ b/src/Dcli.Testing/FrameSnapshot.cs
@@ -0,0 +1,361 @@
+using System.Globalization;
+using System.Text;
+using Dcli.Internal.FixedRegion;
+using Dcli.Internal.RenderLoop;
+
+namespace Dcli.Testing;
+
+// ── Overlay descriptor ────────────────────────────────────────────────────────
+
+/// <summary>
+/// The kind of overlay active in a <see cref="FrameSnapshot"/>.
+/// </summary>
+public enum OverlayKind
+{
+    /// <summary>No overlay is active.</summary>
+    None,
+    /// <summary>An autocomplete dropdown is active.</summary>
+    Autocomplete,
+    /// <summary>A single-select or multi-select dialog is active.</summary>
+    Dialog,
+    /// <summary>An input-text dialog is active.</summary>
+    Input,
+}
+
+/// <summary>
+/// A lightweight descriptor for the active overlay in a <see cref="FrameSnapshot"/>.
+/// </summary>
+/// <remarks>
+/// For list-style overlays (<see cref="OverlayKind.Autocomplete"/>, <see cref="OverlayKind.Dialog"/>)
+/// <see cref="SelectedIndex"/> is the zero-based selected row, or <c>-1</c> when the list is empty.
+/// For <see cref="OverlayKind.Input"/> overlays <see cref="InputText"/> is the current buffer text
+/// when <see cref="IsSecret"/> is <see langword="false"/>; <see langword="null"/> when secret.
+/// All other fields are <see langword="null"/> / <c>0</c> when not applicable.
+/// </remarks>
+public sealed record OverlayDescriptor
+{
+    /// <summary>The kind of active overlay.</summary>
+    public required OverlayKind Kind { get; init; }
+
+    /// <summary>
+    /// Selected item index (zero-based) for <see cref="OverlayKind.Autocomplete"/> and
+    /// <see cref="OverlayKind.Dialog"/> overlays. <c>-1</c> when the list is empty or the overlay
+    /// kind does not use a list.
+    /// </summary>
+    public int SelectedIndex { get; init; } = -1;
+
+    /// <summary>
+    /// Number of visible list rows at snapshot time. <c>0</c> when not applicable.
+    /// </summary>
+    public int VisibleRowCount { get; init; }
+
+    /// <summary>
+    /// Current buffer text for <see cref="OverlayKind.Input"/> overlays when
+    /// <see cref="IsSecret"/> is <see langword="false"/>; otherwise <see langword="null"/>.
+    /// <see langword="null"/> for all other overlay kinds.
+    /// </summary>
+    public string? InputText { get; init; }
+
+    /// <summary>
+    /// <see langword="true"/> when the input overlay is in secret (password) mode.
+    /// Always <see langword="false"/> for non-input overlay kinds.
+    /// </summary>
+    public bool IsSecret { get; init; }
+
+    /// <summary>The singleton descriptor for <see cref="OverlayKind.None"/>.</summary>
+    internal static readonly OverlayDescriptor NoOverlay = new() { Kind = OverlayKind.None };
+}
+
+// ── FrameSnapshot ─────────────────────────────────────────────────────────────
+
+/// <summary>
+/// An immutable snapshot of the logical frame produced by the most recent paint.
+/// </summary>
+/// <remarks>
+/// <para>
+/// Obtain a snapshot via <see cref="HeadlessTerminal.Snapshot"/> after calling
+/// <see cref="HeadlessTerminal.SettleAsync"/>. The snapshot is captured at the moment of
+/// property access; subsequent paints do not mutate it.
+/// </para>
+/// <para>
+/// Style information is stored in the <see cref="Line"/> and <see cref="Segment"/> objects but
+/// stripped when producing the human-readable string via <see cref="FrameSnapshotPrinter.PrettyPrint"/>.
+/// Golden-frame assertions should use <see cref="FrameSnapshotPrinter.PrettyPrint"/> so that
+/// tests assert structure rather than raw ANSI codes.
+/// </para>
+/// <para>
+/// When no paint has occurred yet (pre-paint), <see cref="HeadlessTerminal.Snapshot"/> returns
+/// an empty snapshot: <see cref="Size"/> is <c>(0, 0)</c>, all row lists are empty, <see cref="Caret"/>
+/// is <see langword="null"/>, and <see cref="Overlay"/> is <see cref="OverlayKind.None"/>.
+/// </para>
+/// </remarks>
+public sealed record FrameSnapshot
+{
+    /// <summary>
+    /// The live-window visual rows at the time of the snapshot.
+    /// These are the rows above the fixed region that have not yet been committed to scrollback.
+    /// </summary>
+    public required IReadOnlyList<Line> LiveWindowRows { get; init; }
+
+    /// <summary>
+    /// The fixed-region rows (input editor, status line, overlays) at the time of the snapshot.
+    /// </summary>
+    public required IReadOnlyList<Line> FixedRegionRows { get; init; }
+
+    /// <summary>
+    /// Caret position in the rendered frame (zero-based row and column), or <see langword="null"/>
+    /// when no caret is placed (cursor hidden or modal dialog active).
+    /// </summary>
+    public required (int Row, int Col)? Caret { get; init; }
+
+    /// <summary>Whether the hardware cursor is visible at end of frame.</summary>
+    public required bool IsCursorVisible { get; init; }
+
+    /// <summary>Terminal size at the time of the snapshot.</summary>
+    public required (int Columns, int Rows) Size { get; init; }
+
+    /// <summary>
+    /// Rows that were committed to scrollback (crossed the commit horizon) on this frame.
+    /// Empty when no rows were committed.
+    /// </summary>
+    public required IReadOnlyList<Line> NewlyCommittedRows { get; init; }
+
+    /// <summary>Descriptor for the active overlay, or <see cref="OverlayKind.None"/> when none.</summary>
+    public required OverlayDescriptor Overlay { get; init; }
+
+    // ── Internal factory ──────────────────────────────────────────────────────
+
+    /// <summary>The empty snapshot returned before the first paint.</summary>
+    internal static readonly FrameSnapshot Empty = new()
+    {
+        LiveWindowRows = [],
+        FixedRegionRows = [],
+        Caret = null,
+        IsCursorVisible = true,
+        Size = (0, 0),
+        NewlyCommittedRows = [],
+        Overlay = OverlayDescriptor.NoOverlay,
+    };
+
+    /// <summary>
+    /// Captures an immutable snapshot from the render model produced by the most recent paint.
+    /// </summary>
+    /// <param name="model">The model to snapshot; must not be <see langword="null"/>.</param>
+    internal static FrameSnapshot Capture(RenderModel model)
+    {
+        // Defensive copies: future paints mutate the model's lists in-place. We snapshot by
+        // allocating new arrays so the snapshot's row lists are stable after capture.
+        Line[] liveRows = [.. model.LiveWindowRows];
+        Line[] fixedRows = [.. model.FixedRegionRows];
+        Line[] committed = [.. model.NewlyCommittedRows];
+
+        return new FrameSnapshot
+        {
+            LiveWindowRows = liveRows,
+            FixedRegionRows = fixedRows,
+            Caret = model.CaretPosition,
+            IsCursorVisible = model.IsCursorVisible,
+            Size = (model.Columns, model.Rows),
+            NewlyCommittedRows = committed,
+            Overlay = BuildOverlay(model.ActiveOverlay, model.Columns),
+        };
+    }
+
+    private static OverlayDescriptor BuildOverlay(IOverlay? overlay, int terminalWidth)
+    {
+        if (overlay is null)
+            return OverlayDescriptor.NoOverlay;
+
+        if (overlay is Autocomplete autocomplete)
+        {
+            IReadOnlyList<Line> rows = autocomplete.Render(terminalWidth);
+            return new OverlayDescriptor
+            {
+                Kind = OverlayKind.Autocomplete,
+                SelectedIndex = autocomplete.SelectedIndex,
+                VisibleRowCount = rows.Count,
+            };
+        }
+
+        if (overlay is Dialog dialog)
+        {
+            IReadOnlyList<Line> rows = dialog.Render(terminalWidth);
+            return new OverlayDescriptor
+            {
+                Kind = OverlayKind.Dialog,
+                SelectedIndex = dialog.List.SelectedIndex,
+                VisibleRowCount = rows.Count,
+            };
+        }
+
+        if (overlay is InputDialog inputDialog)
+        {
+            IReadOnlyList<Line> rows = inputDialog.Render(terminalWidth);
+            return new OverlayDescriptor
+            {
+                Kind = OverlayKind.Input,
+                VisibleRowCount = rows.Count,
+                IsSecret = inputDialog.IsSecret,
+                InputText = inputDialog.IsSecret ? null : inputDialog.Text,
+            };
+        }
+
+        // Unknown overlay type — expose as None rather than throwing.
+        return OverlayDescriptor.NoOverlay;
+    }
+}
+
+// ── FrameSnapshotPrinter ──────────────────────────────────────────────────────
+
+/// <summary>
+/// Produces a human-readable, style-stripped representation of a <see cref="FrameSnapshot"/>
+/// suitable for golden-frame assertions.
+/// </summary>
+/// <remarks>
+/// <para>
+/// The output format is stable and intentionally simple — ASCII borders, row indices, and plain
+/// text only. Style information (colours, bold, italic) is stripped so golden strings remain
+/// readable and diff-friendly. No ANSI codes appear in the output.
+/// </para>
+/// <para>
+/// Format overview:
+/// <list type="bullet">
+///   <item><description>Any newly-committed rows are printed first, one per line, with a
+///   <c>[committed N]</c> prefix.</description></item>
+///   <item><description>A bordered box follows. Live-window rows are numbered from 0 upward.
+///   A <c>--- horizon ---</c> line separates the live window from the fixed region.
+///   Fixed-region rows continue the numbering.</description></item>
+///   <item><description>A summary line at the end of the box records caret position and overlay
+///   state.</description></item>
+/// </list>
+/// </para>
+/// </remarks>
+public static class FrameSnapshotPrinter
+{
+    /// <summary>
+    /// Renders <paramref name="snapshot"/> as a bordered ASCII frame string.
+    /// The output format is stable and suitable for golden-frame test assertions.
+    /// </summary>
+    /// <param name="snapshot">The snapshot to render.</param>
+    /// <returns>A multi-line string representing the frame.</returns>
+    public static string PrettyPrint(FrameSnapshot snapshot)
+    {
+        ArgumentNullException.ThrowIfNull(snapshot);
+
+        // Determine box width from the terminal size recorded in the snapshot.
+        // Minimum 20 columns so the summary line is always readable.
+        int contentWidth = Math.Max(snapshot.Size.Columns, 20);
+        int innerWidth = contentWidth - 2; // box inner width (between '|' borders)
+
+        StringBuilder sb = new();
+
+        // ── Newly committed rows ──────────────────────────────────────────────
+        for (int i = 0; i < snapshot.NewlyCommittedRows.Count; i++)
+        {
+            sb.Append(CultureInfo.InvariantCulture, $"[committed {i}] ");
+            sb.AppendLine(ExtractText(snapshot.NewlyCommittedRows[i]));
+        }
+
+        // ── Box top border ────────────────────────────────────────────────────
+        sb.Append('+');
+        sb.Append('-', contentWidth);
+        sb.AppendLine("+");
+
+        // ── Live-window rows ──────────────────────────────────────────────────
+        int rowIdx = 0;
+        foreach (Line row in snapshot.LiveWindowRows)
+        {
+            AppendRow(sb, rowIdx, row, innerWidth);
+            rowIdx++;
+        }
+
+        // ── Horizon line ──────────────────────────────────────────────────────
+        if (snapshot.FixedRegionRows.Count > 0)
+        {
+            string horizonLabel = "--- horizon ---";
+            int horizonPad = contentWidth - horizonLabel.Length;
+            int leftPad = horizonPad / 2;
+            int rightPad = horizonPad - leftPad;
+            sb.Append('|');
+            sb.Append(' ', leftPad);
+            sb.Append(horizonLabel);
+            sb.Append(' ', rightPad);
+            sb.AppendLine("|");
+
+            // ── Fixed-region rows ─────────────────────────────────────────────
+            foreach (Line row in snapshot.FixedRegionRows)
+            {
+                AppendRow(sb, rowIdx, row, innerWidth);
+                rowIdx++;
+            }
+        }
+
+        // ── Summary lines (caret + overlay) — not width-constrained so they're always readable ──
+        string caretStr = snapshot.Caret is { } pos
+            ? string.Create(CultureInfo.InvariantCulture,
+                $"Caret: (r={pos.Row}, c={pos.Col}, visible={snapshot.IsCursorVisible})")
+            : "Caret: hidden";
+        string overlayStr = BuildOverlayStr(snapshot.Overlay);
+        // Each summary line: "| <label>|" — unbounded width for readability.
+        sb.Append("| ");
+        sb.Append(caretStr);
+        sb.AppendLine();
+        sb.Append("| ");
+        sb.Append(overlayStr);
+        sb.AppendLine();
+
+        // ── Box bottom border ─────────────────────────────────────────────────
+        sb.Append('+');
+        sb.Append('-', contentWidth);
+        sb.Append('+');
+
+        return sb.ToString();
+    }
+
+    private static void AppendRow(StringBuilder sb, int rowIdx, Line row, int innerWidth)
+    {
+        string text = ExtractText(row);
+        // Prefix: " NNN | " (7 chars)
+        string prefix = string.Create(CultureInfo.InvariantCulture, $" {rowIdx,3} | ");
+        int textWidth = innerWidth - prefix.Length;
+        string displayText = textWidth > 0 && text.Length > textWidth
+            ? text[..textWidth]
+            : text;
+        // Pad to fill inner width
+        int totalInner = prefix.Length + displayText.Length;
+        int pad = innerWidth - totalInner;
+
+        sb.Append('|');
+        sb.Append(prefix);
+        sb.Append(displayText);
+        if (pad > 0)
+            sb.Append(' ', pad);
+        sb.AppendLine("|");
+    }
+
+    private static string ExtractText(Line line)
+    {
+        if (line.Segments.Count == 0)
+            return string.Empty;
+
+        StringBuilder sb = new();
+        foreach (Segment seg in line.Segments)
+            sb.Append(seg.Text);
+        return sb.ToString();
+    }
+
+    private static string BuildOverlayStr(OverlayDescriptor overlay) =>
+        overlay.Kind switch
+        {
+            OverlayKind.None => "Overlay: None",
+            OverlayKind.Autocomplete => string.Create(CultureInfo.InvariantCulture,
+                $"Overlay: Autocomplete(selected={overlay.SelectedIndex}, rows={overlay.VisibleRowCount})"),
+            OverlayKind.Dialog => string.Create(CultureInfo.InvariantCulture,
+                $"Overlay: Dialog(selected={overlay.SelectedIndex}, rows={overlay.VisibleRowCount})"),
+            OverlayKind.Input when overlay.IsSecret => string.Create(CultureInfo.InvariantCulture,
+                $"Overlay: Input(secret, rows={overlay.VisibleRowCount})"),
+            OverlayKind.Input => string.Create(CultureInfo.InvariantCulture,
+                $"Overlay: Input(text=\"{overlay.InputText}\", rows={overlay.VisibleRowCount})"),
+            _ => "Overlay: None",
+        };
+}
diff --git a/src/Dcli.Testing/HeadlessTerminal.cs b/src/Dcli.Testing/HeadlessTerminal.cs
new file mode 100644
index 0000000..26cfc1d
--- /dev/null
+++ b/src/Dcli.Testing/HeadlessTerminal.cs
@@ -0,0 +1,242 @@
+using System.Text;
+using Dcli.Internal;
+using Dcli.Internal.RenderLoop;
+using Dcli.Testing.Internal;
+
+namespace Dcli.Testing;
+
+/// <summary>
+/// A <see cref="ITerminal"/> harness that runs the real render engine, input parser, and
+/// fixed-region composer against in-memory OS edges — no tty required.
+/// </summary>
+/// <remarks>
+/// <para>
+/// Obtain an instance via <see cref="StartAsync"/>. All scripting methods
+/// (<see cref="Feed"/>, <see cref="SendKey"/>, <see cref="Type"/>, <see cref="Paste"/>,
+/// <see cref="Resize"/>) post events through the same channels the production path uses;
+/// they never mutate model state directly.
+/// </para>
+/// <para>
+/// Call <see cref="SettleAsync"/> after posting input to drain all pending work and wait for
+/// exactly one coalesced frame before asserting state.
+/// </para>
+/// </remarks>
+public sealed class HeadlessTerminal : IAsyncDisposable
+{
+    private readonly Terminal _terminal;
+    private readonly ScriptedInputByteSource _byteSource;
+    private readonly ScriptedResizeWatcher _resizeWatcher;
+    private readonly FixedSizeSource _sizeSource;
+    // Held here so the CA2000 analyzer sees ownership; Terminal also disposes it via IRawModeSession.
+    private readonly HeadlessRawModeSession _session;
+
+    private HeadlessTerminal(
+        Terminal terminal,
+        HeadlessRawModeSession session,
+        ScriptedInputByteSource byteSource,
+        ScriptedResizeWatcher resizeWatcher,
+        FixedSizeSource sizeSource,
+        VirtualClock clock,
+        InMemoryOutputSink sink)
+    {
+        _terminal = terminal;
+        _session = session;
+        _byteSource = byteSource;
+        _resizeWatcher = resizeWatcher;
+        _sizeSource = sizeSource;
+        Clock = clock;
+        Sink = sink;
+    }
+
+    /// <summary>
+    /// The real <see cref="ITerminal"/> facade. Use this to drive scrollback, input, status,
+    /// autocomplete, and dialogs exactly as production consumers do.
+    /// </summary>
+    public ITerminal Terminal => _terminal;
+
+    /// <summary>
+    /// The virtual clock controlling the render loop's cadence.
+    /// Call <see cref="VirtualClock.Advance"/> to trigger throttled paints in cadence tests.
+    /// </summary>
+    public VirtualClock Clock { get; }
+
+    /// <summary>
+    /// The in-memory output sink. Exposes <see cref="InMemoryOutputSink.LastModel"/> and
+    /// <see cref="InMemoryOutputSink.PaintCount"/> for frame-level assertions.
+    /// </summary>
+    /// <remarks>
+    /// Internal: consumers assert through <see cref="SettleAsync"/>, <see cref="Snapshot"/>,
+    /// and the public <see cref="Terminal"/> surface. Direct sink access is for advanced
+    /// assertions only.
+    /// </remarks>
+    internal InMemoryOutputSink Sink { get; }
+
+    /// <summary>
+    /// Returns an immutable snapshot of the most recently painted frame.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// Always call <see cref="SettleAsync"/> before reading this property to ensure the loop
+    /// has painted the latest state.
+    /// </para>
+    /// <para>
+    /// If no paint has occurred yet (before the first <see cref="SettleAsync"/>), returns an
+    /// empty snapshot: size <c>(0, 0)</c>, empty row lists, no caret, no overlay. Does not throw.
+    /// </para>
+    /// <para>
+    /// The snapshot is immutable — captured at the moment of property access. Subsequent
+    /// paints do not mutate previously captured snapshots.
+    /// </para>
+    /// </remarks>
+    public FrameSnapshot Snapshot
+    {
+        get
+        {
+            RenderModel? model = Sink.LastModel;
+            return model is null ? FrameSnapshot.Empty : FrameSnapshot.Capture(model);
+        }
+    }
+
+    // ── Factory ─────────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Builds and starts a headless terminal with the given options.
+    /// </summary>
+    /// <param name="options">
+    /// Configuration; pass <see langword="null"/> or <c>new HeadlessTerminalOptions()</c> for defaults.
+    /// </param>
+    /// <param name="cancellationToken">Token that cancels the start-up sequence.</param>
+    /// <returns>A live <see cref="HeadlessTerminal"/> handle.</returns>
+    public static Task<HeadlessTerminal> StartAsync(
+        HeadlessTerminalOptions? options = null,
+        CancellationToken cancellationToken = default)
+    {
+        options ??= new HeadlessTerminalOptions();
+        cancellationToken.ThrowIfCancellationRequested();
+
+        HeadlessRawModeSession session = new();
+        RestoreCoordinator? coordinator = null;
+        bool success = false;
+        try
+        {
+            coordinator = RestoreCoordinator.Wire(session);
+
+            ScriptedInputByteSource byteSource = new();
+            ScriptedResizeWatcher resizeWatcher = new();
+            FixedSizeSource sizeSource = new(options.InitialColumns, options.InitialRows);
+            InMemoryOutputSink sink = new();
+            VirtualClock clock = options.Clock ?? new VirtualClock(TimeSpan.Zero);
+
+            Dcli.Terminal dcliTerminal = Dcli.Terminal.StartCore(
+                session,
+                coordinator,
+                resizeWatcher,
+                byteSource,
+                clock,
+                sink,
+                sizeSource,
+                options.MinFrameInterval,
+                options.MaxFixedHeight);
+
+            // session and coordinator ownership transfers to harness (and via harness to Terminal).
+            HeadlessTerminal harness = new(dcliTerminal, session, byteSource, resizeWatcher, sizeSource, clock, sink);
+            success = true;
+            return Task.FromResult(harness);
+        }
+        finally
+        {
+            // Dispose only on failure; on success, harness owns session and coordinator via Terminal.
+            if (!success)
+            {
+                coordinator?.Dispose();
+                session.Dispose();
+            }
+        }
+    }
+
+    // ── Scripting methods ────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Enqueues raw bytes into the scripted input source.
+    /// The real VT input parser will decode them on the input-reader thread.
+    /// Call <see cref="SettleAsync"/> afterwards to wait for the resulting events to reach the loop.
+    /// </summary>
+    /// <param name="bytes">The raw bytes to feed through the parser.</param>
+    public void Feed(ReadOnlySpan<byte> bytes) => _byteSource.Enqueue(bytes);
+
+    /// <summary>
+    /// Posts a synthesised key event directly to the render loop's inbound channel,
+    /// bypassing the VT parser. The event is processed on the loop thread.
+    /// </summary>
+    /// <param name="key">The key event to inject.</param>
+    public void SendKey(KeyEvent key) => _terminal.Loop.InputWriter.TryWrite(key);
+
+    /// <summary>
+    /// Posts one <see cref="KeyEvent"/> per rune in <paramref name="text"/> to the render loop.
+    /// Each character is synthesised as <c>KeyCode.FromRune(rune)</c> with
+    /// <see cref="Modifiers.None"/>, exactly as if the user typed the text character-by-character.
+    /// </summary>
+    /// <remarks>
+    /// For named keys (Enter, Tab, arrows, etc.) prefer <see cref="SendKey"/>.
+    /// <c>Type</c> emits <see cref="KeyCode.FromRune"/> (<c>KeyCodeKind.Char</c>)
+    /// events only and cannot produce named-key events.
+    /// </remarks>
+    /// <param name="text">The text to type.</param>
+    public void Type(string text)
+    {
+        ArgumentNullException.ThrowIfNull(text);
+        foreach (Rune rune in text.EnumerateRunes())
+            _terminal.Loop.InputWriter.TryWrite(new KeyEvent(KeyCode.FromRune(rune), Modifiers.None));
+    }
+
+    /// <summary>
+    /// Posts a single <see cref="PasteEvent"/> to the render loop as if the user had used
+    /// bracketed-paste mode to paste <paramref name="text"/>.
+    /// </summary>
+    /// <param name="text">The pasted text.</param>
+    public void Paste(string text) => _terminal.Loop.InputWriter.TryWrite(new PasteEvent(text));
+
+    /// <summary>
+    /// Updates the reported terminal size and fires the scripted resize watcher, which posts
+    /// a <see cref="ResizeEvent"/> to the render loop's inbound channel — the same path that
+    /// POSIX SIGWINCH uses in production.
+    /// </summary>
+    /// <param name="columns">New terminal width in columns.</param>
+    /// <param name="rows">New terminal height in rows.</param>
+    public void Resize(int columns, int rows)
+    {
+        _sizeSource.SetSize(columns, rows);
+        _resizeWatcher.Fire(columns, rows);
+    }
+
+    // ── Settle ───────────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Drains all currently-pending inbound work and waits for exactly one coalesced frame
+    /// (if the model is dirty). Completes deterministically without advancing wall-clock time.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// Settlement is fully deterministic for event-level scripting — <see cref="SendKey"/>,
+    /// <see cref="Type"/>, <see cref="Paste"/>, and <see cref="Resize"/> post directly to the
+    /// render loop's inbound channel and are visible after a single <c>SettleAsync</c>.
+    /// </para>
+    /// <para>
+    /// <see cref="Feed"/> enqueues raw bytes that the independent InputReader thread decodes
+    /// asynchronously. Callers may need to call <c>SettleAsync</c> twice after a large feed:
+    /// once to let the InputReader drain the byte queue, and once to let the resulting
+    /// <see cref="InputEvent"/>s reach the render loop.
+    /// </para>
+    /// <para>
+    /// Delegates to <see cref="LoopEngine.SettleAsync"/> on the underlying loop engine.
+    /// </para>
+    /// </remarks>
+    /// <param name="cancellationToken">Cancels the wait.</param>
+    public Task SettleAsync(CancellationToken cancellationToken = default) =>
+        _terminal.Loop.SettleAsync(cancellationToken);
+
+    // ── Dispose ──────────────────────────────────────────────────────────────
+
+    /// <inheritdoc/>
+    public ValueTask DisposeAsync() => _terminal.DisposeAsync();
+}
diff --git a/src/Dcli.Testing/HeadlessTerminalOptions.cs b/src/Dcli.Testing/HeadlessTerminalOptions.cs
new file mode 100644
index 0000000..4bb0758
--- /dev/null
+++ b/src/Dcli.Testing/HeadlessTerminalOptions.cs
@@ -0,0 +1,40 @@
+namespace Dcli.Testing;
+
+/// <summary>
+/// Configuration for <see cref="HeadlessTerminal.StartAsync"/>.
+/// All properties have defaults suitable for most tests.
+/// </summary>
+public sealed class HeadlessTerminalOptions
+{
+    /// <summary>
+    /// Initial terminal width in columns. Defaults to <c>80</c>.
+    /// </summary>
+    public int InitialColumns { get; init; } = 80;
+
+    /// <summary>
+    /// Initial terminal height in rows. Defaults to <c>24</c>.
+    /// </summary>
+    public int InitialRows { get; init; } = 24;
+
+    /// <summary>
+    /// Minimum interval between successive paints.
+    /// Defaults to <see cref="TimeSpan.Zero"/> so every <see cref="HeadlessTerminal.SettleAsync"/>
+    /// call produces a frame without virtual-time advancement.
+    /// Set to a non-zero value for cadence tests that use <see cref="VirtualClock.Advance"/>.
+    /// </summary>
+    public TimeSpan MinFrameInterval { get; init; } = TimeSpan.Zero;
+
+    /// <summary>
+    /// Optional cap on the fixed-region height in rows.
+    /// <see langword="null"/> (the default) lets the render model use its built-in 50 % heuristic.
+    /// </summary>
+    public int? MaxFixedHeight { get; init; }
+
+    /// <summary>
+    /// Optional pre-constructed <see cref="VirtualClock"/> to use.
+    /// When <see langword="null"/> (the default), <see cref="HeadlessTerminal.StartAsync"/> creates
+    /// a fresh clock starting at <see cref="TimeSpan.Zero"/>.
+    /// Providing a clock lets the caller share a single clock across multiple harness instances.
+    /// </summary>
+    public VirtualClock? Clock { get; init; }
+}
diff --git a/src/Dcli.Testing/Internal/FixedSizeSource.cs b/src/Dcli.Testing/Internal/FixedSizeSource.cs
new file mode 100644
index 0000000..09eea77
--- /dev/null
+++ b/src/Dcli.Testing/Internal/FixedSizeSource.cs
@@ -0,0 +1,29 @@
+using Dcli.Internal.RenderLoop;
+
+namespace Dcli.Testing.Internal;
+
+/// <summary>
+/// A <see cref="ITerminalSizeSource"/> that returns a configurable fixed size.
+/// Thread-safe: <see cref="SetSize"/> may be called concurrently with <see cref="GetSize"/>.
+/// </summary>
+internal sealed class FixedSizeSource : ITerminalSizeSource
+{
+    private volatile int _columns;
+    private volatile int _rows;
+
+    internal FixedSizeSource(int columns, int rows)
+    {
+        _columns = columns;
+        _rows = rows;
+    }
+
+    /// <inheritdoc/>
+    public (int Columns, int Rows) GetSize() => (_columns, _rows);
+
+    /// <summary>Updates the reported size. Safe to call from any thread.</summary>
+    internal void SetSize(int columns, int rows)
+    {
+        _columns = columns;
+        _rows = rows;
+    }
+}
diff --git a/src/Dcli.Testing/Internal/HeadlessRawModeSession.cs b/src/Dcli.Testing/Internal/HeadlessRawModeSession.cs
new file mode 100644
index 0000000..ab72af7
--- /dev/null
+++ b/src/Dcli.Testing/Internal/HeadlessRawModeSession.cs
@@ -0,0 +1,19 @@
+using Dcli.Internal;
+
+namespace Dcli.Testing.Internal;
+
+/// <summary>
+/// A no-op <see cref="IRawModeSession"/> for headless tests.
+/// Raw-mode entry and restoration are skipped; lifecycle methods are safe no-ops.
+/// </summary>
+internal sealed class HeadlessRawModeSession : IRawModeSession
+{
+    /// <inheritdoc/>
+    public void Restore() { }
+
+    /// <inheritdoc/>
+    public void Reapply() { }
+
+    /// <inheritdoc/>
+    public void Dispose() { }
+}
diff --git a/src/Dcli.Testing/Internal/InMemoryOutputSink.cs b/src/Dcli.Testing/Internal/InMemoryOutputSink.cs
new file mode 100644
index 0000000..d797d9f
--- /dev/null
+++ b/src/Dcli.Testing/Internal/InMemoryOutputSink.cs
@@ -0,0 +1,30 @@
+using Dcli.Internal.RenderLoop;
+
+namespace Dcli.Testing.Internal;
+
+/// <summary>
+/// An in-memory <see cref="IOutputSink"/> for headless tests.
+/// Captures the most-recently painted <see cref="RenderModel"/> and counts paint calls.
+/// </summary>
+internal sealed class InMemoryOutputSink : IOutputSink
+{
+    private volatile RenderModel? _lastModel;
+    private int _paintCount;
+
+    /// <summary>The most-recently painted model, or <see langword="null"/> before the first paint.</summary>
+    internal RenderModel? LastModel => _lastModel;
+
+    /// <summary>Total number of <see cref="Paint"/> calls received.</summary>
+    internal int PaintCount => Volatile.Read(ref _paintCount);
+
+    /// <inheritdoc/>
+    public void Paint(RenderModel model)
+    {
+        _lastModel = model;
+        Interlocked.Increment(ref _paintCount);
+    }
+
+    /// <inheritdoc/>
+    /// <remarks>No-op: headless tests do not write to a real terminal.</remarks>
+    public void EmitRestoreSequence() { }
+}
diff --git a/src/Dcli.Testing/Internal/ScriptedInputByteSource.cs b/src/Dcli.Testing/Internal/ScriptedInputByteSource.cs
new file mode 100644
index 0000000..4f2c9f1
--- /dev/null
+++ b/src/Dcli.Testing/Internal/ScriptedInputByteSource.cs
@@ -0,0 +1,42 @@
+using System.Collections.Concurrent;
+using Dcli.Internal.Input;
+
+namespace Dcli.Testing.Internal;
+
+/// <summary>
+/// A thread-safe byte queue that implements <see cref="IInputByteSource"/> for headless tests.
+/// </summary>
+/// <remarks>
+/// <para>
+/// Respects the VMIN=0/VTIME=1 contract: when the queue is empty <see cref="Read"/> returns 0
+/// (timed-out, no data) rather than blocking or returning -1.
+/// </para>
+/// <para>
+/// <see cref="Enqueue"/> may be called from the test thread while <see cref="Read"/> runs on
+/// the <c>dcli-input-reader</c> thread; <see cref="ConcurrentQueue{T}"/> provides the required
+/// thread-safety.
+/// </para>
+/// </remarks>
+internal sealed class ScriptedInputByteSource : IInputByteSource
+{
+    private readonly ConcurrentQueue<byte> _queue = new();
+
+    /// <summary>
+    /// Enqueues bytes to be delivered by the next <see cref="Read"/> call.
+    /// Safe to call from any thread.
+    /// </summary>
+    internal void Enqueue(ReadOnlySpan<byte> bytes)
+    {
+        foreach (byte b in bytes)
+            _queue.Enqueue(b);
+    }
+
+    /// <inheritdoc/>
+    public int Read(Span<byte> buffer)
+    {
+        int count = 0;
+        while (count < buffer.Length && _queue.TryDequeue(out byte b))
+            buffer[count++] = b;
+        return count; // 0 means "no data this poll" — not EOF
+    }
+}
diff --git a/src/Dcli.Testing/Internal/ScriptedResizeWatcher.cs b/src/Dcli.Testing/Internal/ScriptedResizeWatcher.cs
new file mode 100644
index 0000000..491731b
--- /dev/null
+++ b/src/Dcli.Testing/Internal/ScriptedResizeWatcher.cs
@@ -0,0 +1,31 @@
+using Dcli.Internal.RenderLoop;
+
+namespace Dcli.Testing.Internal;
+
+/// <summary>
+/// A controllable <see cref="IResizeWatcher"/> for headless tests.
+/// Stores the callback registered via <see cref="Start"/> and exposes <see cref="Fire"/>
+/// to simulate a terminal resize event.
+/// </summary>
+internal sealed class ScriptedResizeWatcher : IResizeWatcher
+{
+    private volatile Action<int, int>? _callback;
+
+    /// <inheritdoc/>
+    public void Start(Action<int, int> onResize) => _callback = onResize;
+
+    /// <summary>
+    /// Simulates a terminal resize by invoking the registered callback.
+    /// Safe to call from any thread; no-op before <see cref="Start"/> is called.
+    /// </summary>
+    internal void Fire(int columns, int rows) => _callback?.Invoke(columns, rows);
+
+    /// <inheritdoc/>
+    /// <remarks>
+    /// No-op. The callback is retained so <see cref="Fire"/> can still be invoked after the
+    /// <see cref="Terminal"/> disposes the watcher during its own disposal. Any <see cref="Fire"/>
+    /// call after dispose is a silent no-op in practice because the loop's inbound channel is
+    /// already closed and the posted resize event is silently dropped.
+    /// </remarks>
+    public void Dispose() { }
+}
diff --git a/src/Dcli.Testing/README.md b/src/Dcli.Testing/README.md
new file mode 100644
index 0000000..a56ff2a
--- /dev/null
+++ b/src/Dcli.Testing/README.md
@@ -0,0 +1,63 @@
+# dcli.testing
+
+**Headless test harness for the [dcli](https://www.nuget.org/packages/dcli) inline terminal-rendering library.**
+
+`dcli.testing` lets you drive the real dcli render engine — input parser, fixed-region composer,
+scrollback model, overlays, virtual clock — against in-memory fakes instead of a real TTY.
+No PTY, no raw mode, no ANSI pollution in your test output.
+
+## Quick start
+
+```csharp
+using Dcli;
+using Dcli.Testing;
+using Xunit;
+
+public class MyFeatureTests
+{
+    [Fact]
+    public async Task StatusLineAppearsInSnapshot()
+    {
+        await using HeadlessTerminal harness = await HeadlessTerminal.StartAsync();
+
+        harness.Terminal.Status.SetRows([new Line([new Segment("Ready")])]);
+        await harness.SettleAsync();
+
+        FrameSnapshot snap = harness.Snapshot;
+        string pretty = FrameSnapshotPrinter.PrettyPrint(snap);
+
+        Assert.Contains("Ready", pretty);
+    }
+}
+```
+
+## Key types
+
+| Type | Description |
+|------|-------------|
+| `HeadlessTerminal` | Entry point. `StartAsync()` builds a live harness; `SettleAsync()` drains pending work and waits for a coalesced frame. |
+| `HeadlessTerminalOptions` | Configures initial size, minimum frame interval, and virtual clock. |
+| `VirtualClock` | Deterministic clock; `Advance(span)` advances time without wall-clock delay, triggering throttled paints. |
+| `FrameSnapshot` | Immutable capture of `LiveWindowRows`, `FixedRegionRows`, caret, size, newly-committed rows, and overlay state. |
+| `FrameSnapshotPrinter` | `PrettyPrint(snapshot)` — ASCII-bordered, style-stripped frame string, stable for golden-frame assertions. |
+
+## Scripting methods
+
+| Method | What it does |
+|--------|--------------|
+| `SendKey(KeyEvent)` | Injects a named or character key directly into the render loop. |
+| `Type(string)` | Types each rune as a `Char` key event. For named keys (Enter, Tab, arrows) use `SendKey`. |
+| `Paste(string)` | Posts a bracketed-paste event. |
+| `Feed(ReadOnlySpan<byte>)` | Feeds raw bytes through the real VT parser on the InputReader thread. |
+| `Resize(cols, rows)` | Triggers a resize event via the same path as POSIX SIGWINCH. |
+
+## What's NOT here
+
+This package contains only the headless harness. The production dcli rendering engine,
+public API (`ITerminal`, `Scrollback`, `Input`, `Status`, `Autocomplete`), and event model
+live in the `dcli` package. Add both packages to test projects:
+
+```xml
+<PackageReference Include="dcli" Version="0.1.0-rc.1" />
+<PackageReference Include="dcli.testing" Version="0.1.0-rc.1" />
+```
diff --git a/src/Dcli.Testing/VirtualClock.cs b/src/Dcli.Testing/VirtualClock.cs
new file mode 100644
index 0000000..5885d2a
--- /dev/null
+++ b/src/Dcli.Testing/VirtualClock.cs
@@ -0,0 +1,87 @@
+using Dcli.Internal.RenderLoop;
+
+namespace Dcli.Testing;
+
+/// <summary>
+/// A controllable clock for cadence tests.
+/// Time never advances unless <see cref="Advance"/> is called explicitly, so tests exercise
+/// throttle and coalescing logic without wall-clock sleeps.
+/// </summary>
+/// <remarks>
+/// All public members are thread-safe. Waiters whose deadline has been passed are completed
+/// outside the lock to prevent re-entrant deadlock if a continuation calls back into the clock.
+/// </remarks>
+public sealed class VirtualClock : IClock
+{
+    private readonly object _lock = new();
+    private TimeSpan _now;
+    private readonly List<(TimeSpan Deadline, TaskCompletionSource Tcs, CancellationTokenSource LinkedCts)> _waiters = [];
+
+    /// <summary>
+    /// Initialises the clock at the specified instant (defaults to <see cref="TimeSpan.Zero"/>).
+    /// </summary>
+    /// <param name="initial">The starting time value.</param>
+    public VirtualClock(TimeSpan? initial = null) => _now = initial ?? TimeSpan.Zero;
+
+    /// <inheritdoc/>
+    public TimeSpan Now
+    {
+        get { lock (_lock) { return _now; } }
+    }
+
+    /// <summary>
+    /// Advances the clock by <paramref name="by"/> and completes all waiters whose deadline
+    /// is at or before the new time.
+    /// </summary>
+    /// <param name="by">The amount of time to advance. Must be non-negative.</param>
+    public void Advance(TimeSpan by)
+    {
+        List<(TaskCompletionSource Tcs, CancellationTokenSource LinkedCts)> toComplete = [];
+        lock (_lock)
+        {
+            _now += by;
+            for (int i = _waiters.Count - 1; i >= 0; i--)
+            {
+                if (_waiters[i].Deadline <= _now)
+                {
+                    toComplete.Add((_waiters[i].Tcs, _waiters[i].LinkedCts));
+                    _waiters.RemoveAt(i);
+                }
+            }
+        }
+        // Complete outside the lock: a continuation that re-enters the clock must not deadlock.
+        foreach ((TaskCompletionSource tcs, CancellationTokenSource linkedCts) in toComplete)
+        {
+            tcs.TrySetResult();
+            linkedCts.Dispose();
+        }
+    }
+
+    /// <inheritdoc/>
+    public Task WaitUntilAsync(TimeSpan deadline, CancellationToken cancellationToken)
+    {
+        lock (_lock)
+        {
+            if (deadline <= _now)
+                return Task.CompletedTask;
+        }
+
+        CancellationTokenSource linkedCts = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
+        TaskCompletionSource tcs = new(TaskCreationOptions.RunContinuationsAsynchronously);
+        linkedCts.Token.Register(
+            static state => ((TaskCompletionSource)state!).TrySetCanceled(),
+            tcs,
+            useSynchronizationContext: false);
+
+        lock (_lock)
+        {
+            if (deadline <= _now)
+            {
+                linkedCts.Dispose();
+                return Task.CompletedTask;
+            }
+            _waiters.Add((deadline, tcs, linkedCts));
+        }
+        return tcs.Task;
+    }
+}
diff --git a/src/Dcli/AutocompleteCandidate.cs b/src/Dcli/AutocompleteCandidate.cs
new file mode 100644
index 0000000..b202840
--- /dev/null
+++ b/src/Dcli/AutocompleteCandidate.cs
@@ -0,0 +1,16 @@
+namespace Dcli;
+
+/// <summary>
+/// A single completion candidate offered to the user by the autocomplete overlay.
+/// </summary>
+/// <param name="InsertText">
+/// The text applied to the input buffer when this candidate is accepted.
+/// Applied via whole-buffer replace: the entire input buffer is replaced with this value and
+/// the caret is moved to the end. Span-replace (replacing only the typed prefix) is not
+/// supported in this release.
+/// </param>
+/// <param name="Display">
+/// The styled row rendered in the dropdown list. This is the consumer's presentation of
+/// the candidate; <c>dcli</c> never interprets its text.
+/// </param>
+public sealed record AutocompleteCandidate(string InsertText, Line Display);
diff --git a/src/Dcli/AutocompleteSurface.cs b/src/Dcli/AutocompleteSurface.cs
new file mode 100644
index 0000000..31110b8
--- /dev/null
+++ b/src/Dcli/AutocompleteSurface.cs
@@ -0,0 +1,91 @@
+using Dcli.Internal.FixedRegion;
+using Dcli.Internal.RenderLoop;
+
+namespace Dcli;
+
+/// <summary>
+/// Consumer-facing surface for showing and hiding the autocomplete dropdown overlay.
+/// </summary>
+/// <remarks>
+/// <para>
+/// All methods post fire-and-forget <see cref="Internal.RenderLoop.ILoopCommand"/>s; they
+/// return before the command is applied or a frame is painted.
+/// </para>
+/// <para>
+/// <strong>Overlay priority:</strong> <see cref="Show"/> is a no-op when a modal overlay (dialog)
+/// is currently active — modal overlays suppress autocomplete.
+/// <see cref="Hide"/> is safe to call at any time; it clears the overlay only when the active
+/// overlay is the autocomplete (not a dialog).
+/// </para>
+/// <para>
+/// <strong>Thread safety:</strong> all methods are safe to call from any thread.
+/// </para>
+/// </remarks>
+public sealed class AutocompleteSurface : IAutocomplete
+{
+    private readonly LoopEngine _loop;
+
+    internal AutocompleteSurface(LoopEngine loop)
+    {
+        ArgumentNullException.ThrowIfNull(loop);
+        _loop = loop;
+    }
+
+    /// <summary>
+    /// Shows the autocomplete dropdown with the given candidates.
+    /// No-op when a modal overlay is active.
+    /// </summary>
+    /// <param name="candidates">
+    /// The candidates to display, ordered by preference. An empty list is valid (renders
+    /// nothing; accepting on an empty list is a safe no-op).
+    /// </param>
+    public void Show(IReadOnlyList<AutocompleteCandidate> candidates)
+    {
+        ArgumentNullException.ThrowIfNull(candidates);
+        _loop.Post(new ShowAutocompleteCommand(candidates));
+    }
+
+    /// <summary>
+    /// Hides the autocomplete dropdown without modifying the input buffer.
+    /// No-op when the active overlay is not an autocomplete.
+    /// </summary>
+    public void Hide()
+    {
+        _loop.Post(new HideAutocompleteCommand());
+    }
+
+    // ── Commands ───────────────────────────────────────────────────────────────
+
+    private sealed class ShowAutocompleteCommand : ILoopCommand
+    {
+        private readonly IReadOnlyList<AutocompleteCandidate> _candidates;
+
+        internal ShowAutocompleteCommand(IReadOnlyList<AutocompleteCandidate> candidates) =>
+            _candidates = candidates;
+
+        void ILoopCommand.Apply(RenderModel model)
+        {
+            // Construct a fresh Autocomplete overlay bound to the current editor instance.
+            // ShowAutocomplete is a no-op when a modal overlay is already active.
+            Autocomplete overlay = new(model.FixedRegion.Editor);
+            overlay.Show(_candidates);
+            model.ShowAutocomplete(overlay);
+            model.MarkDirty();
+        }
+    }
+
+    private sealed class HideAutocompleteCommand : ILoopCommand
+    {
+        void ILoopCommand.Apply(RenderModel model)
+        {
+            // Clear only when the active overlay is an Autocomplete — not a dialog.
+            // The overlay's own Hide()/IsDismissed sweeping happens on the next key event,
+            // which is too late for an explicit programmatic Hide(); clear directly here.
+            if (model.ActiveOverlay is Autocomplete)
+            {
+                model.ClearOverlay();
+                model.MarkDirty();
+            }
+        }
+    }
+}
diff --git a/src/Dcli/Color.cs b/src/Dcli/Color.cs
new file mode 100644
index 0000000..10fdcc6
--- /dev/null
+++ b/src/Dcli/Color.cs
@@ -0,0 +1,165 @@
+namespace Dcli;
+
+/// <summary>
+/// Discriminated representation of a terminal color.
+/// Three representations are supported: the 16 named ANSI colors, a 256-palette index, and 24-bit RGB.
+/// Construct via the static factory methods <see cref="Named"/>, <see cref="FromIndex"/>, and <see cref="FromRgb"/>.
+/// </summary>
+public readonly record struct Color
+{
+    /// <summary>
+    /// The 16 standard ANSI named colors (8 base + 8 bright).
+    /// </summary>
+    public enum AnsiColor
+    {
+        /// <summary>ANSI color 0 — black.</summary>
+        Black = 0,
+
+        /// <summary>ANSI color 1 — red.</summary>
+        Red = 1,
+
+        /// <summary>ANSI color 2 — green.</summary>
+        Green = 2,
+
+        /// <summary>ANSI color 3 — yellow.</summary>
+        Yellow = 3,
+
+        /// <summary>ANSI color 4 — blue.</summary>
+        Blue = 4,
+
+        /// <summary>ANSI color 5 — magenta.</summary>
+        Magenta = 5,
+
+        /// <summary>ANSI color 6 — cyan.</summary>
+        Cyan = 6,
+
+        /// <summary>ANSI color 7 — white (light gray).</summary>
+        White = 7,
+
+        /// <summary>ANSI color 8 — bright black (dark gray).</summary>
+        BrightBlack = 8,
+
+        /// <summary>ANSI color 9 — bright red.</summary>
+        BrightRed = 9,
+
+        /// <summary>ANSI color 10 — bright green.</summary>
+        BrightGreen = 10,
+
+        /// <summary>ANSI color 11 — bright yellow.</summary>
+        BrightYellow = 11,
+
+        /// <summary>ANSI color 12 — bright blue.</summary>
+        BrightBlue = 12,
+
+        /// <summary>ANSI color 13 — bright magenta.</summary>
+        BrightMagenta = 13,
+
+        /// <summary>ANSI color 14 — bright cyan.</summary>
+        BrightCyan = 14,
+
+        /// <summary>ANSI color 15 — bright white.</summary>
+        BrightWhite = 15,
+    }
+
+    /// <summary>Identifies which color representation is stored.</summary>
+    public enum ColorKind
+    {
+        /// <summary>One of the 16 named ANSI colors.</summary>
+        Named,
+
+        /// <summary>An index into the 256-color palette.</summary>
+        Indexed,
+
+        /// <summary>A 24-bit RGB truecolor value.</summary>
+        Rgb,
+    }
+
+    private readonly ColorKind _kind;
+    private readonly int _value; // Named→(int)AnsiColor, Indexed→byte, Rgb→(r<<16)|(g<<8)|b
+
+    private Color(ColorKind kind, int value)
+    {
+        _kind = kind;
+        _value = value;
+    }
+
+    /// <summary>Which representation this color uses.</summary>
+    public ColorKind Kind => _kind;
+
+    /// <summary>
+    /// Returns a color from the 16 named ANSI palette.
+    /// </summary>
+    /// <param name="color">The named ANSI color.</param>
+    /// <returns>A <see cref="Color"/> with <see cref="ColorKind.Named"/> kind.</returns>
+    public static Color Named(AnsiColor color) => new(ColorKind.Named, (int)color);
+
+    /// <summary>
+    /// Returns a color from the 256-color terminal palette (indices 0–255).
+    /// </summary>
+    /// <param name="index">The palette index (0–255).</param>
+    /// <returns>A <see cref="Color"/> with <see cref="ColorKind.Indexed"/> kind.</returns>
+    public static Color FromIndex(byte index) => new(ColorKind.Indexed, index);
+
+    /// <summary>
+    /// Returns a 24-bit truecolor value from red, green, and blue components.
+    /// </summary>
+    /// <param name="r">Red component (0–255).</param>
+    /// <param name="g">Green component (0–255).</param>
+    /// <param name="b">Blue component (0–255).</param>
+    /// <returns>A <see cref="Color"/> with <see cref="ColorKind.Rgb"/> kind.</returns>
+    public static Color FromRgb(byte r, byte g, byte b) => new(ColorKind.Rgb, (r << 16) | (g << 8) | b);
+
+    /// <summary>
+    /// For <see cref="ColorKind.Named"/> colors, returns the <see cref="AnsiColor"/> value.
+    /// Throws <see cref="InvalidOperationException"/> for other kinds.
+    /// </summary>
+    public AnsiColor NamedValue =>
+        _kind == ColorKind.Named
+            ? (AnsiColor)_value
+            : throw new InvalidOperationException($"Color kind is {_kind}, not Named.");
+
+    /// <summary>
+    /// For <see cref="ColorKind.Indexed"/> colors, returns the palette index (0–255).
+    /// Throws <see cref="InvalidOperationException"/> for other kinds.
+    /// </summary>
+    public byte IndexValue =>
+        _kind == ColorKind.Indexed
+            ? (byte)_value
+            : throw new InvalidOperationException($"Color kind is {_kind}, not Indexed.");
+
+    /// <summary>
+    /// For <see cref="ColorKind.Rgb"/> colors, returns the red component (0–255).
+    /// Throws <see cref="InvalidOperationException"/> for other kinds.
+    /// </summary>
+    public byte R =>
+        _kind == ColorKind.Rgb
+            ? (byte)((_value >> 16) & 0xFF)
+            : throw new InvalidOperationException($"Color kind is {_kind}, not Rgb.");
+
+    /// <summary>
+    /// For <see cref="ColorKind.Rgb"/> colors, returns the green component (0–255).
+    /// Throws <see cref="InvalidOperationException"/> for other kinds.
+    /// </summary>
+    public byte G =>
+        _kind == ColorKind.Rgb
+            ? (byte)((_value >> 8) & 0xFF)
+            : throw new InvalidOperationException($"Color kind is {_kind}, not Rgb.");
+
+    /// <summary>
+    /// For <see cref="ColorKind.Rgb"/> colors, returns the blue component (0–255).
+    /// Throws <see cref="InvalidOperationException"/> for other kinds.
+    /// </summary>
+    public byte B =>
+        _kind == ColorKind.Rgb
+            ? (byte)(_value & 0xFF)
+            : throw new InvalidOperationException($"Color kind is {_kind}, not Rgb.");
+
+    /// <inheritdoc/>
+    public override string ToString() => _kind switch
+    {
+        ColorKind.Named => $"Named({(AnsiColor)_value})",
+        ColorKind.Indexed => $"Index({_value})",
+        ColorKind.Rgb => $"Rgb({(_value >> 16) & 0xFF},{(_value >> 8) & 0xFF},{_value & 0xFF})",
+        _ => "Unknown",
+    };
+}
diff --git a/src/Dcli/Dcli.csproj b/src/Dcli/Dcli.csproj
new file mode 100644
index 0000000..575c385
--- /dev/null
+++ b/src/Dcli/Dcli.csproj
@@ -0,0 +1,51 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <AssemblyName>Dcli</AssemblyName>
+    <RootNamespace>Dcli</RootNamespace>
+    <!-- Required for [LibraryImport] source-generated P/Invoke (termios / Win32 console). -->
+    <AllowUnsafeBlocks>true</AllowUnsafeBlocks>
+
+    <!-- NuGet packaging -->
+    <IsPackable>true</IsPackable>
+    <PackageId>dcli</PackageId>
+    <Version>0.1.0-rc.1</Version>
+    <Authors>daemonicai</Authors>
+    <Company>daemonicai</Company>
+    <Description>Inline terminal-rendering library. Claude-Code-style styled output flows into the terminal's real scrollback, with a small interactive region pinned at the bottom.</Description>
+    <PackageLicenseExpression>MPL-2.0</PackageLicenseExpression>
+    <PackageProjectUrl>https://github.com/daemonicai/dcli</PackageProjectUrl>
+    <RepositoryUrl>https://github.com/daemonicai/dcli</RepositoryUrl>
+    <RepositoryType>git</RepositoryType>
+    <PackageReadmeFile>README.md</PackageReadmeFile>
+    <PackageTags>terminal;cli;tui;rendering;ansi;vt100</PackageTags>
+    <PackageReleaseNotes>
+- Inline scrollback rendering: styled output flows into native terminal scrollback with a pinned interactive region.
+- Awaitable modal dialogs: SelectAsync, MultiSelectAsync, ChoiceAsync, InputAsync.
+- Scrollback model: append lines, live mutable blocks, and one-way collapsibles.
+- Autocomplete dropdown overlay with whole-buffer-replace candidate acceptance.
+- Raw-mode VT input pipeline: own termios/Win32 shims, VtInputParser, bracketed-paste, SIGWINCH resize. Validated on macOS.
+    </PackageReleaseNotes>
+
+    <!-- Source Link: enables consumers to step into dcli source in the debugger. -->
+    <PublishRepositoryUrl>true</PublishRepositoryUrl>
+    <EmbedUntrackedSources>true</EmbedUntrackedSources>
+    <IncludeSymbols>true</IncludeSymbols>
+    <SymbolPackageFormat>snupkg</SymbolPackageFormat>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <None Include="README.md" Pack="true" PackagePath="\" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <InternalsVisibleTo Include="Dcli.Tests" />
+    <InternalsVisibleTo Include="Dcli.RawModeHarness" />
+    <InternalsVisibleTo Include="Dcli.Testing" />
+  </ItemGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.SourceLink.GitHub" Version="8.0.0" PrivateAssets="All" />
+  </ItemGroup>
+
+</Project>
diff --git a/src/Dcli/DcliVersion.cs b/src/Dcli/DcliVersion.cs
new file mode 100644
index 0000000..f15f743
--- /dev/null
+++ b/src/Dcli/DcliVersion.cs
@@ -0,0 +1,9 @@
+namespace Dcli;
+
+/// <summary>
+/// Assembly version marker. Feature types are added in subsequent sections.
+/// </summary>
+internal static class DcliVersion
+{
+    internal const string Value = "0.1.0";
+}
diff --git a/src/Dcli/DialogOutcome.cs b/src/Dcli/DialogOutcome.cs
new file mode 100644
index 0000000..54ad9b7
--- /dev/null
+++ b/src/Dcli/DialogOutcome.cs
@@ -0,0 +1,41 @@
+namespace Dcli;
+
+/// <summary>
+/// The outcome of an awaitable dialog — how the user closed it.
+/// </summary>
+/// <remarks>
+/// <para>
+/// <see cref="Submitted"/> and <see cref="Cancelled"/> are produced by the v1 list-based dialogs
+/// (<see cref="Terminal.SelectAsync"/>, <see cref="Terminal.MultiSelectAsync"/>,
+/// <see cref="Terminal.ChoiceAsync"/>). <see cref="Back"/> exists for API/wizard-flow
+/// compatibility but is not produced by any v1 dialog (no key maps to it in this release).
+/// </para>
+/// </remarks>
+public enum DialogOutcome
+{
+    /// <summary>The user confirmed the selection (Enter).</summary>
+    Submitted,
+
+    /// <summary>
+    /// Reserved for wizard-flow "go back" semantics. Not produced by any v1 dialog.
+    /// </summary>
+    Back,
+
+    /// <summary>The user dismissed without confirming (Escape or cancellation token).</summary>
+    Cancelled,
+}
+
+/// <summary>
+/// The result of an awaitable dialog, combining the <see cref="DialogOutcome"/> with a typed
+/// value that is valid when <see cref="Outcome"/> is <see cref="DialogOutcome.Submitted"/>.
+/// </summary>
+/// <typeparam name="T">
+/// The type of the selected value: <see cref="int"/> for single-select dialogs,
+/// <see cref="int"/>[] for multi-select.
+/// </typeparam>
+/// <param name="Outcome">How the dialog was closed.</param>
+/// <param name="Value">
+/// The selected value when <see cref="Outcome"/> is <see cref="DialogOutcome.Submitted"/>;
+/// <see langword="default"/> otherwise.
+/// </param>
+public readonly record struct DialogResult<T>(DialogOutcome Outcome, T Value);
diff --git a/src/Dcli/DialogRequests.cs b/src/Dcli/DialogRequests.cs
new file mode 100644
index 0000000..f94d328
--- /dev/null
+++ b/src/Dcli/DialogRequests.cs
@@ -0,0 +1,44 @@
+namespace Dcli;
+
+/// <summary>
+/// Parameters for <see cref="Terminal.SelectAsync"/> — a single-select list dialog.
+/// </summary>
+/// <param name="Items">The list items to display. An empty list is allowed; Submit on an
+/// empty list returns <see cref="DialogOutcome.Submitted"/> with value <c>-1</c>.</param>
+/// <param name="Title">Optional leading title row rendered above the list.</param>
+public sealed record SelectRequest(IReadOnlyList<Line> Items, Line? Title = null);
+
+/// <summary>
+/// Parameters for <see cref="Terminal.MultiSelectAsync"/> — a multi-select list dialog where
+/// Space toggles individual items.
+/// </summary>
+/// <param name="Items">The list items to display.</param>
+/// <param name="Title">Optional leading title row rendered above the list.</param>
+public sealed record MultiSelectRequest(IReadOnlyList<Line> Items, Line? Title = null);
+
+/// <summary>
+/// Parameters for <see cref="Terminal.ChoiceAsync"/> — a single-select choice dialog.
+/// Semantically equivalent to <see cref="SelectRequest"/>; the separate type is provided for
+/// caller clarity when presenting mutually-exclusive options with an explanatory prompt.
+/// </summary>
+/// <param name="Options">The choice options to display.</param>
+/// <param name="Prompt">Optional leading prompt row rendered above the options.</param>
+public sealed record ChoiceRequest(IReadOnlyList<Line> Options, Line? Prompt = null);
+
+/// <summary>
+/// Parameters for <see cref="Terminal.InputAsync"/> — a free-text entry dialog.
+/// </summary>
+/// <param name="Prompt">
+/// Optional leading prompt row rendered above the input field. When <see langword="null"/>,
+/// no prompt row is shown and the full overlay budget goes to the input field.
+/// </param>
+/// <param name="Default">
+/// Optional pre-filled text. The caret starts at the end of this text.
+/// When <see langword="null"/> or empty, the field starts blank.
+/// </param>
+/// <param name="IsSecret">
+/// When <see langword="true"/>, each character is replaced by a bullet glyph (U+2022 •)
+/// in the rendered overlay, preserving column-width arithmetic. The <see cref="DialogResult{T}.Value"/>
+/// always carries the real (unmasked) entered text.
+/// </param>
+public sealed record InputRequest(Line? Prompt = null, string? Default = null, bool IsSecret = false);
diff --git a/src/Dcli/DisplayWidth.cs b/src/Dcli/DisplayWidth.cs
new file mode 100644
index 0000000..11b0185
--- /dev/null
+++ b/src/Dcli/DisplayWidth.cs
@@ -0,0 +1,180 @@
+using System.Globalization;
+using System.Text;
+
+namespace Dcli;
+
+/// <summary>
+/// Per-Unicode-scalar display-width measurement (wcwidth semantics).
+/// <para>
+/// Rules applied, in priority order:
+/// <list type="number">
+///   <item>Control characters (U+0000–U+001F, U+007F) → 0 (not stripped; just not counted as columns).</item>
+///   <item>Zero-width characters (ZWSP U+200B, ZWNJ U+200C, ZWJ U+200D, BOM U+FEFF, SHY U+00AD,
+///     and scalars whose <see cref="UnicodeCategory"/> is <see cref="UnicodeCategory.NonSpacingMark"/>,
+///     <see cref="UnicodeCategory.EnclosingMark"/>, or <see cref="UnicodeCategory.Format"/>) → 0.</item>
+///   <item>East-Asian Wide / Fullwidth scalars (see <see cref="IsWide"/>) → 2.</item>
+///   <item>Tab (U+0009) → advances to the next multiple of <see cref="TabWidth"/> from column 0.
+///     <see cref="Measure(string)"/> assumes a start column of 0; use
+///     <see cref="MeasureWithColumn"/> when tabs appear mid-line.</item>
+///   <item>Everything else → 1.</item>
+/// </list>
+/// </para>
+/// <para>
+/// This type is internal: it is a rendering primitive, not a committed public API surface.
+/// Callers within the assembly (and the test project via InternalsVisibleTo) may use it freely.
+/// </para>
+/// </summary>
+internal static class DisplayWidth
+{
+    /// <summary>Number of columns a tab advances to (next multiple of this value).</summary>
+    internal const int TabWidth = 8;
+
+    /// <summary>
+    /// Returns the display width (in terminal columns) of a single Unicode scalar value.
+    /// Tab is measured as if it starts at column 0 (width = <see cref="TabWidth"/>).
+    /// For tab handling relative to a known column, use <see cref="MeasureWithColumn"/>.
+    /// </summary>
+    internal static int Measure(Rune rune)
+    {
+        int value = rune.Value;
+
+        // Control characters
+        if (value < 0x0020 || value == 0x007F)
+        {
+            if (value == 0x0009) // TAB
+            {
+                // Start-column-0 assumption: expands to full TabWidth.
+                return TabWidth;
+            }
+            return 0;
+        }
+
+        // Explicitly zero-width scalars
+        if (value is 0x00AD or 0x200B or 0x200C or 0x200D or 0xFEFF)
+            return 0;
+
+        // Combining marks and format category characters
+        UnicodeCategory cat = Rune.GetUnicodeCategory(rune);
+        if (cat is UnicodeCategory.NonSpacingMark
+                or UnicodeCategory.EnclosingMark
+                or UnicodeCategory.Format)
+            return 0;
+
+        // East-Asian Wide / Fullwidth
+        if (IsWide(value))
+            return 2;
+
+        return 1;
+    }
+
+    /// <summary>
+    /// Returns the display width of <paramref name="rune"/> given that it starts at
+    /// <paramref name="column"/>. Only differs from <see cref="Measure(Rune)"/> for tab characters.
+    /// </summary>
+    internal static int MeasureWithColumn(Rune rune, int column)
+    {
+        if (rune.Value == 0x0009) // TAB
+        {
+            int next = (column / TabWidth + 1) * TabWidth;
+            return next - column;
+        }
+        return Measure(rune);
+    }
+
+    /// <summary>
+    /// Returns the total display width of <paramref name="text"/> in terminal columns.
+    /// Tab characters are expanded assuming a start column of 0; for accurate tab handling
+    /// mid-line use <see cref="MeasureWithColumn"/> per scalar.
+    /// </summary>
+    internal static int Measure(string text)
+    {
+        int width = 0;
+        int col = 0;
+        foreach (Rune rune in text.EnumerateRunes())
+        {
+            int w = MeasureWithColumn(rune, col);
+            width += w;
+            col += w;
+        }
+        return width;
+    }
+
+    // -----------------------------------------------------------------------------------------
+    // East-Asian Width table
+    //
+    // Source: Unicode Standard Annex #11 "East Asian Width"
+    // (https://www.unicode.org/reports/tr11/) and Markus Kuhn's wcwidth reference implementation.
+    // Covers Unicode 15.1 Wide (W) and Fullwidth (F) ranges.  Narrow (Na), Ambiguous (A),
+    // Halfwidth (H), and Neutral (N) all map to width 1 here — the correct choice for modern
+    // Unicode-aware terminals that treat ambiguous as narrow.
+    //
+    // Ranges are stored as (inclusive_low, inclusive_high) pairs in a flat sorted int array.
+    // -----------------------------------------------------------------------------------------
+
+    private static ReadOnlySpan<int> WideRanges =>
+    [
+        // Fullwidth Latin / punctuation / etc. (F)
+        0x1100, 0x115F,   // Hangul Jamo
+        0x2329, 0x232A,   // Angle brackets (legacy)
+        0x2E80, 0x303E,   // CJK Radicals, Kangxi, CJK Symbols & Punctuation
+        0x3041, 0x33BF,   // Hiragana, Katakana, Bopomofo, Hangul Compatibility Jamo, Kanbun, Bopomofo Extended, Katakana Phonetic Extensions, Enclosed CJK, CJK Compatibility
+        0x33FF, 0x33FF,   // CJK Compatibility
+        0x3400, 0x4DBF,   // CJK Unified Ideographs Extension A
+        0x4E00, 0x9FFF,   // CJK Unified Ideographs
+        0xA000, 0xA4CF,   // Yi
+        0xA960, 0xA97F,   // Hangul Jamo Extended-A
+        0xAC00, 0xD7AF,   // Hangul Syllables
+        0xF900, 0xFAFF,   // CJK Compatibility Ideographs
+        0xFE10, 0xFE19,   // Vertical forms
+        0xFE30, 0xFE4F,   // CJK Compatibility Forms, Small Form Variants
+        0xFF01, 0xFF60,   // Fullwidth Forms
+        0xFFE0, 0xFFE6,   // Fullwidth Signs
+        // Supplementary Multilingual Plane
+        0x1B000, 0x1B12F, // Kana Supplement, Extended-A
+        0x1B130, 0x1B16F, // Kana Extended-B  (Unicode 15)
+        0x1B170, 0x1B2FF, // Nushu
+        0x1F004, 0x1F004, // Mahjong tile (wide emoji)
+        0x1F0CF, 0x1F0CF, // Playing card black joker
+        0x1F100, 0x1F10A, // Enclosed Alphanumeric Supplement (wide)
+        0x1F110, 0x1F12D, // Enclosed Alphanumeric Supplement
+        0x1F130, 0x1F169, // Enclosed Alphanumeric Supplement
+        0x1F170, 0x1F1AC, // Enclosed Alphanumeric Supplement
+        0x1F1E0, 0x1F1FF, // Regional Indicator Symbols (treated as wide for width = 2 per scalar)
+        0x1F200, 0x1F2FF, // Enclosed Ideographic Supplement
+        0x1F300, 0x1F64F, // Misc Symbols and Pictographs, Emoticons
+        0x1F680, 0x1F6FF, // Transport and Map Symbols
+        0x1F700, 0x1F77F, // Alchemical Symbols (some wide)
+        0x1F780, 0x1F7FF, // Geometric Shapes Extended
+        0x1F800, 0x1F8FF, // Supplemental Arrows-C
+        0x1F900, 0x1F9FF, // Supplemental Symbols and Pictographs
+        0x1FA00, 0x1FA6F, // Chess Symbols
+        0x1FA70, 0x1FAFF, // Symbols and Pictographs Extended-A
+        0x20000, 0x2FFFD, // CJK Unified Ideographs Extension B–F, CJK Compatibility Ideographs Supplement
+        0x30000, 0x3FFFD, // CJK Unified Ideographs Extension G–H (Unicode 13+)
+    ];
+
+    /// <summary>
+    /// Returns <see langword="true"/> if <paramref name="value"/> is an East-Asian Wide or
+    /// Fullwidth codepoint that occupies two terminal columns.
+    /// </summary>
+    internal static bool IsWide(int value)
+    {
+        ReadOnlySpan<int> ranges = WideRanges;
+        // Binary search over pair-indexed ranges.
+        int lo = 0;
+        int hi = ranges.Length / 2 - 1;
+        while (lo <= hi)
+        {
+            int mid = (lo + hi) >> 1;
+            int low = ranges[mid * 2];
+            int high = ranges[mid * 2 + 1];
+            if (value < low)
+                hi = mid - 1;
+            else if (value > high)
+                lo = mid + 1;
+            else
+                return true;
+        }
+        return false;
+    }
+}
diff --git a/src/Dcli/Format.cs b/src/Dcli/Format.cs
new file mode 100644
index 0000000..d5a6931
--- /dev/null
+++ b/src/Dcli/Format.cs
@@ -0,0 +1,30 @@
+namespace Dcli;
+
+/// <summary>
+/// Text formatting attributes that can be applied to a <see cref="Segment"/>.
+/// Values are power-of-two so they can be combined with bitwise OR.
+/// </summary>
+[Flags]
+public enum Format
+{
+    /// <summary>No formatting attributes.</summary>
+    None = 0,
+
+    /// <summary>Bold / increased intensity.</summary>
+    Bold = 1,
+
+    /// <summary>Italic.</summary>
+    Italic = 2,
+
+    /// <summary>Underline.</summary>
+    Underline = 4,
+
+    /// <summary>Dim / decreased intensity (faint).</summary>
+    Dim = 8,
+
+    /// <summary>Reverse video: swap foreground and background colors.</summary>
+    Reverse = 16,
+
+    /// <summary>Strikethrough (crossed-out text).</summary>
+    Strikethrough = 32,
+}
diff --git a/src/Dcli/ITerminal.cs b/src/Dcli/ITerminal.cs
new file mode 100644
index 0000000..dba194a
--- /dev/null
+++ b/src/Dcli/ITerminal.cs
@@ -0,0 +1,214 @@
+using System.Threading.Channels;
+
+namespace Dcli;
+
+// ─────────────────────────────────────────────────────────────────────────────
+// Sub-surface interfaces
+// ─────────────────────────────────────────────────────────────────────────────
+
+/// <summary>
+/// Consumer-facing interface for appending content to the scrollback live window.
+/// </summary>
+/// <remarks>
+/// All methods are fire-and-forget; they post commands and return before the render loop
+/// applies them or paints a frame.
+/// </remarks>
+public interface IScrollback
+{
+    /// <summary>
+    /// Appends a single styled line to the scrollback live window.
+    /// </summary>
+    /// <param name="line">The line to append.</param>
+    void Append(Line line);
+
+    /// <summary>
+    /// Begins a new live block in the scrollback, returning a handle for incremental mutation.
+    /// </summary>
+    /// <returns>A handle whose methods post fire-and-forget commands to the render loop.</returns>
+    ILiveBlock BeginLive();
+
+    /// <summary>
+    /// Begins a new collapsible block, returning a handle for one-time expansion.
+    /// </summary>
+    /// <param name="summary">The summary line shown while the block is collapsed.</param>
+    /// <param name="hiddenLines">The lines revealed when the block is expanded.</param>
+    /// <returns>A handle whose <see cref="ICollapsible.Expand"/> posts a command to the render loop.</returns>
+    ICollapsible BeginCollapsible(Line summary, IReadOnlyList<Line> hiddenLines);
+}
+
+/// <summary>
+/// Consumer-facing interface for programmatic control of the input editor.
+/// </summary>
+/// <remarks>
+/// Methods do not emit <see cref="InputChanged"/> — that event is reserved for user-driven edits.
+/// </remarks>
+public interface IInput
+{
+    /// <summary>
+    /// Replaces the entire input buffer with <paramref name="text"/> and moves the caret to the end.
+    /// Does not emit <see cref="InputChanged"/>.
+    /// </summary>
+    /// <param name="text">The text to place in the editor.</param>
+    void SetText(string text);
+
+    /// <summary>
+    /// Clears the input buffer and moves the caret to position 0.
+    /// Does not emit <see cref="InputChanged"/>.
+    /// </summary>
+    void Clear();
+}
+
+/// <summary>
+/// Consumer-facing interface for setting the status bar rows at the bottom of the fixed region.
+/// </summary>
+/// <remarks>
+/// The status bar is always fully rendered regardless of height budget — it is never truncated.
+/// </remarks>
+public interface IStatus
+{
+    /// <summary>
+    /// Replaces the status bar content with the given rows.
+    /// An empty argument list clears the status bar.
+    /// </summary>
+    /// <param name="rows">The rows to display in the status bar.</param>
+    void SetRows(params Line[] rows);
+
+    /// <summary>
+    /// Replaces the status bar content with the given rows.
+    /// Passing an empty list clears the status bar.
+    /// </summary>
+    /// <param name="rows">The rows to display in the status bar.</param>
+    void SetRows(IReadOnlyList<Line> rows);
+}
+
+/// <summary>
+/// Consumer-facing interface for showing and hiding the autocomplete dropdown overlay.
+/// </summary>
+/// <remarks>
+/// <see cref="Show"/> is a no-op when a modal overlay (dialog) is currently active.
+/// <see cref="Hide"/> clears the overlay only when the active overlay is the autocomplete.
+/// </remarks>
+public interface IAutocomplete
+{
+    /// <summary>
+    /// Shows the autocomplete dropdown with the given candidates.
+    /// No-op when a modal overlay is active.
+    /// </summary>
+    /// <param name="candidates">The candidates to display, ordered by preference.</param>
+    void Show(IReadOnlyList<AutocompleteCandidate> candidates);
+
+    /// <summary>
+    /// Hides the autocomplete dropdown without modifying the input buffer.
+    /// No-op when the active overlay is not an autocomplete.
+    /// </summary>
+    void Hide();
+}
+
+// ─────────────────────────────────────────────────────────────────────────────
+// Top-level façade interface
+// ─────────────────────────────────────────────────────────────────────────────
+
+/// <summary>
+/// The dcli terminal façade — the root abstraction a consumer depends on.
+/// </summary>
+/// <remarks>
+/// <para>
+/// The concrete implementation is <see cref="Terminal"/>; obtain one via
+/// <see cref="Terminal.StartAsync(TerminalOptions, CancellationToken)"/>. Consumer code should
+/// depend only on this interface so that tests can substitute a hand-written fake without
+/// a real terminal, raw-mode session, or static state to reset.
+/// </para>
+/// <para>
+/// <strong>No static singletons:</strong> all state is per-instance. A test that creates a fake
+/// <see cref="ITerminal"/> starts from a clean slate with no global bookkeeping.
+/// </para>
+/// </remarks>
+public interface ITerminal : IAsyncDisposable
+{
+    /// <summary>
+    /// Scrollback content surface: append lines, live blocks, and collapsibles.
+    /// </summary>
+    IScrollback Scrollback { get; }
+
+    /// <summary>
+    /// Input editor surface: programmatic text control.
+    /// </summary>
+    IInput Input { get; }
+
+    /// <summary>
+    /// Status bar surface: set the sacred status rows at the bottom of the fixed region.
+    /// </summary>
+    IStatus Status { get; }
+
+    /// <summary>
+    /// Autocomplete overlay surface: show and hide the completion dropdown.
+    /// </summary>
+    IAutocomplete Autocomplete { get; }
+
+    /// <summary>
+    /// The outbound terminal event stream.
+    /// </summary>
+    /// <remarks>
+    /// Drain this channel on a separate consumer thread or task. A consumer that never reads
+    /// accumulates events but does not stall the render loop.
+    /// </remarks>
+    ChannelReader<TerminalEvent> Events { get; }
+
+    /// <summary>
+    /// Returns the current terminal size. May return a fixed or mocked value in test fakes.
+    /// </summary>
+    /// <returns>A tuple of <c>(Columns, Rows)</c> representing the terminal dimensions.</returns>
+    (int Columns, int Rows) GetTerminalSize();
+
+    /// <summary>
+    /// Opens a single-select list dialog and awaits the user's choice.
+    /// </summary>
+    /// <param name="req">The request describing the items and optional title.</param>
+    /// <param name="cancellationToken">
+    /// Cancels the dialog and returns <see cref="DialogOutcome.Cancelled"/>.
+    /// </param>
+    /// <returns>
+    /// <see cref="DialogOutcome.Submitted"/> with the zero-based selected index, or
+    /// <see cref="DialogOutcome.Cancelled"/>.
+    /// </returns>
+    Task<DialogResult<int>> SelectAsync(SelectRequest req, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Opens a multi-select list dialog and awaits the user's selection.
+    /// </summary>
+    /// <param name="req">The request describing the items and optional title.</param>
+    /// <param name="cancellationToken">
+    /// Cancels the dialog and returns <see cref="DialogOutcome.Cancelled"/>.
+    /// </param>
+    /// <returns>
+    /// <see cref="DialogOutcome.Submitted"/> with the checked indices in ascending order, or
+    /// <see cref="DialogOutcome.Cancelled"/> with an empty array.
+    /// </returns>
+    Task<DialogResult<int[]>> MultiSelectAsync(MultiSelectRequest req, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Opens a single-select choice dialog and awaits the user's choice.
+    /// </summary>
+    /// <param name="req">The request describing the options and optional prompt.</param>
+    /// <param name="cancellationToken">
+    /// Cancels the dialog and returns <see cref="DialogOutcome.Cancelled"/>.
+    /// </param>
+    /// <returns>
+    /// <see cref="DialogOutcome.Submitted"/> with the zero-based selected index, or
+    /// <see cref="DialogOutcome.Cancelled"/>.
+    /// </returns>
+    Task<DialogResult<int>> ChoiceAsync(ChoiceRequest req, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Opens a free-text input dialog and awaits the user's entry.
+    /// </summary>
+    /// <param name="req">The request describing the optional prompt, default text, and masking.</param>
+    /// <param name="cancellationToken">
+    /// Cancels the dialog and returns <see cref="DialogOutcome.Cancelled"/>.
+    /// </param>
+    /// <returns>
+    /// <see cref="DialogOutcome.Submitted"/> with the entered text (possibly empty), or
+    /// <see cref="DialogOutcome.Cancelled"/>.
+    /// </returns>
+    Task<DialogResult<string>> InputAsync(InputRequest req, CancellationToken cancellationToken = default);
+}
diff --git a/src/Dcli/InputEvent.cs b/src/Dcli/InputEvent.cs
new file mode 100644
index 0000000..79d964d
--- /dev/null
+++ b/src/Dcli/InputEvent.cs
@@ -0,0 +1,255 @@
+using System.Text;
+
+namespace Dcli;
+
+/// <summary>
+/// Discriminated union of all events that the VT input pipeline can produce.
+/// Use pattern matching on the concrete subtypes: <see cref="KeyEvent"/>,
+/// <see cref="PasteEvent"/>, and <see cref="ResizeEvent"/>.
+/// </summary>
+public abstract record InputEvent;
+
+// ─────────────────────────────────────────────────────────────────────────────
+// KeyEvent
+// ─────────────────────────────────────────────────────────────────────────────
+
+/// <summary>
+/// A single key press, synthesised by <c>VtInputParser</c> from the raw byte stream.
+/// </summary>
+/// <param name="Code">
+/// The key that was pressed, either a Unicode scalar value or a named terminal key.
+/// </param>
+/// <param name="Modifiers">
+/// Modifier keys active during the press (Ctrl, Alt, Shift, or combinations thereof).
+/// </param>
+public sealed record KeyEvent(KeyCode Code, Modifiers Modifiers) : InputEvent;
+
+// ─────────────────────────────────────────────────────────────────────────────
+// PasteEvent
+// ─────────────────────────────────────────────────────────────────────────────
+
+/// <summary>
+/// A pasted block of text delivered via bracketed-paste mode
+/// (<c>ESC [ 2 0 0 ~</c> … <c>ESC [ 2 0 1 ~</c>).
+/// <para>
+/// <c>VtInputParser</c> accumulates all bytes between the start and end markers into
+/// a single event. The raw bytes are decoded as UTF-8; feed-call boundaries inside the paste
+/// body are transparent. A lone ESC that is not followed by the end marker is included verbatim
+/// as <c>U+001B</c> in the decoded text.
+/// </para>
+/// </summary>
+/// <param name="Text">The full pasted text, decoded from UTF-8.</param>
+public sealed record PasteEvent(string Text) : InputEvent;
+
+// ─────────────────────────────────────────────────────────────────────────────
+// ResizeEvent
+// ─────────────────────────────────────────────────────────────────────────────
+
+/// <summary>
+/// Signals that the terminal window has been resized.
+/// <para>
+/// This event is <em>not</em> synthesised from bytes by <c>VtInputParser</c>.
+/// It originates from <c>SIGWINCH</c> (POSIX) or a <c>WINDOW_BUFFER_SIZE_EVENT</c>
+/// (Windows) and is injected into the event stream by the platform resize watcher.
+/// </para>
+/// </summary>
+/// <param name="Columns">New terminal width in columns.</param>
+/// <param name="Rows">New terminal height in rows.</param>
+public sealed record ResizeEvent(int Columns, int Rows) : InputEvent;
+
+// ─────────────────────────────────────────────────────────────────────────────
+// NamedKey
+// ─────────────────────────────────────────────────────────────────────────────
+
+/// <summary>
+/// Terminal keys that do not correspond to a single printable Unicode scalar value.
+/// </summary>
+public enum NamedKey
+{
+    /// <summary>Enter / Return (0x0D in raw mode, or CSI sequences ending in 'M').</summary>
+    Enter,
+
+    /// <summary>Horizontal tab (0x09 in raw mode; distinct from Ctrl+I on the wire).</summary>
+    Tab,
+
+    /// <summary>Backspace (0x7F in raw mode or 0x08 on older configurations).</summary>
+    Backspace,
+
+    /// <summary>Escape (lone 0x1B after the ESC-disambiguation timeout).</summary>
+    Escape,
+
+    /// <summary>Up-arrow (<c>ESC [ A</c> or <c>ESC O A</c>).</summary>
+    Up,
+
+    /// <summary>Down-arrow (<c>ESC [ B</c> or <c>ESC O B</c>).</summary>
+    Down,
+
+    /// <summary>Right-arrow (<c>ESC [ C</c> or <c>ESC O C</c>).</summary>
+    Right,
+
+    /// <summary>Left-arrow (<c>ESC [ D</c> or <c>ESC O D</c>).</summary>
+    Left,
+
+    /// <summary>Home (<c>ESC [ H</c>, <c>ESC O H</c>, or <c>ESC [ 1 ~</c> / <c>ESC [ 7 ~</c>).</summary>
+    Home,
+
+    /// <summary>End (<c>ESC [ F</c>, <c>ESC O F</c>, or <c>ESC [ 4 ~</c> / <c>ESC [ 8 ~</c>).</summary>
+    End,
+
+    /// <summary>Page-Up (<c>ESC [ 5 ~</c>).</summary>
+    PageUp,
+
+    /// <summary>Page-Down (<c>ESC [ 6 ~</c>).</summary>
+    PageDown,
+
+    /// <summary>Insert (<c>ESC [ 2 ~</c>).</summary>
+    Insert,
+
+    /// <summary>Delete (<c>ESC [ 3 ~</c>).</summary>
+    Delete,
+
+    /// <summary>F1 (<c>ESC O P</c> or <c>ESC [ 1 ; &lt;m&gt; P</c>).</summary>
+    F1,
+
+    /// <summary>F2 (<c>ESC O Q</c> or <c>ESC [ 1 ; &lt;m&gt; Q</c>).</summary>
+    F2,
+
+    /// <summary>F3 (<c>ESC O R</c> or <c>ESC [ 1 ; &lt;m&gt; R</c>).</summary>
+    F3,
+
+    /// <summary>F4 (<c>ESC O S</c> or <c>ESC [ 1 ; &lt;m&gt; S</c>).</summary>
+    F4,
+
+    /// <summary>F5 (<c>ESC [ 1 5 ~</c>).</summary>
+    F5,
+
+    /// <summary>F6 (<c>ESC [ 1 7 ~</c>).</summary>
+    F6,
+
+    /// <summary>F7 (<c>ESC [ 1 8 ~</c>).</summary>
+    F7,
+
+    /// <summary>F8 (<c>ESC [ 1 9 ~</c>).</summary>
+    F8,
+
+    /// <summary>F9 (<c>ESC [ 2 0 ~</c>).</summary>
+    F9,
+
+    /// <summary>F10 (<c>ESC [ 2 1 ~</c>).</summary>
+    F10,
+
+    /// <summary>F11 (<c>ESC [ 2 3 ~</c>).</summary>
+    F11,
+
+    /// <summary>F12 (<c>ESC [ 2 4 ~</c>).</summary>
+    F12,
+
+    /// <summary>Shift+Tab (<c>ESC [ Z</c>); also called "Back Tab".</summary>
+    BackTab,
+}
+
+// ─────────────────────────────────────────────────────────────────────────────
+// Modifiers
+// ─────────────────────────────────────────────────────────────────────────────
+
+/// <summary>
+/// Modifier keys that may accompany a key press.
+/// Values are powers of two so they can be combined with bitwise OR.
+/// </summary>
+[Flags]
+public enum Modifiers
+{
+    /// <summary>No modifier keys.</summary>
+    None = 0,
+
+    /// <summary>The Ctrl key was held.</summary>
+    Ctrl = 1,
+
+    /// <summary>The Alt (Meta) key was held, or an ESC prefix was received.</summary>
+    Alt = 2,
+
+    /// <summary>
+    /// The Shift key was held.
+    /// Only appears on <see cref="KeyCode.Named"/> keys (e.g. Shift+Arrow, BackTab).
+    /// Shift is never synthesised for printable <see cref="KeyCode.FromRune(Rune)"/> runes — the
+    /// upper-case letter itself encodes the shift implicitly.
+    /// Shift is never reported together with Ctrl (the terminal collapses them on the wire).
+    /// </summary>
+    Shift = 4,
+}
+
+// ─────────────────────────────────────────────────────────────────────────────
+// KeyCode
+// ─────────────────────────────────────────────────────────────────────────────
+
+/// <summary>
+/// Discriminated value identifying which key was pressed.
+/// Either a Unicode scalar (<see cref="FromRune(Rune)"/>) or a named terminal key
+/// (<see cref="Named(NamedKey)"/>).
+/// </summary>
+public readonly record struct KeyCode
+{
+    /// <summary>Identifies which representation this code uses.</summary>
+    public enum KeyCodeKind
+    {
+        /// <summary>A Unicode scalar value (printable character or Ctrl-modified letter).</summary>
+        UnicodeScalar,
+
+        /// <summary>A named terminal key from <see cref="NamedKey"/>.</summary>
+        Named,
+    }
+
+    private readonly KeyCodeKind _kind;
+    private readonly Rune _rune;       // valid when Kind == UnicodeScalar
+    private readonly NamedKey _named;  // valid when Kind == Named
+
+    private KeyCode(KeyCodeKind kind, Rune rune, NamedKey named)
+    {
+        _kind = kind;
+        _rune = rune;
+        _named = named;
+    }
+
+    /// <summary>Which representation this code uses.</summary>
+    public KeyCodeKind Kind => _kind;
+
+    /// <summary>
+    /// Creates a <see cref="KeyCode"/> representing a Unicode scalar value.
+    /// </summary>
+    /// <param name="rune">The Unicode scalar.</param>
+    /// <returns>A <see cref="KeyCode"/> with <see cref="KeyCodeKind.UnicodeScalar"/> kind.</returns>
+    public static KeyCode FromRune(Rune rune) => new(KeyCodeKind.UnicodeScalar, rune, default);
+
+    /// <summary>
+    /// Creates a <see cref="KeyCode"/> representing a named terminal key.
+    /// </summary>
+    /// <param name="key">The named key.</param>
+    /// <returns>A <see cref="KeyCode"/> with <see cref="KeyCodeKind.Named"/> kind.</returns>
+    public static KeyCode Named(NamedKey key) => new(KeyCodeKind.Named, default, key);
+
+    /// <summary>
+    /// For <see cref="KeyCodeKind.UnicodeScalar"/> codes, returns the Unicode scalar.
+    /// Throws <see cref="InvalidOperationException"/> for other kinds.
+    /// </summary>
+    public Rune RuneValue =>
+        _kind == KeyCodeKind.UnicodeScalar
+            ? _rune
+            : throw new InvalidOperationException($"KeyCode kind is {_kind}, not UnicodeScalar.");
+
+    /// <summary>
+    /// For <see cref="KeyCodeKind.Named"/> codes, returns the <see cref="NamedKey"/> value.
+    /// Throws <see cref="InvalidOperationException"/> for other kinds.
+    /// </summary>
+    public NamedKey NamedValue =>
+        _kind == KeyCodeKind.Named
+            ? _named
+            : throw new InvalidOperationException($"KeyCode kind is {_kind}, not Named.");
+
+    /// <inheritdoc/>
+    public override string ToString() => _kind switch
+    {
+        KeyCodeKind.UnicodeScalar => $"UnicodeScalar({_rune})",
+        KeyCodeKind.Named => $"Named({_named})",
+        _ => "Unknown",
+    };
+}
diff --git a/src/Dcli/InputSurface.cs b/src/Dcli/InputSurface.cs
new file mode 100644
index 0000000..d58a284
--- /dev/null
+++ b/src/Dcli/InputSurface.cs
@@ -0,0 +1,83 @@
+using Dcli.Internal.RenderLoop;
+
+namespace Dcli;
+
+/// <summary>
+/// Consumer-facing surface for programmatic control of the input editor.
+/// </summary>
+/// <remarks>
+/// <para>
+/// All methods post fire-and-forget <see cref="Internal.RenderLoop.ILoopCommand"/>s; they
+/// return before the command is applied or a frame is painted.
+/// </para>
+/// <para>
+/// <strong>Thread safety:</strong> all methods are safe to call from any thread.
+/// </para>
+/// <para>
+/// <strong>No <c>InputChanged</c> on programmatic mutation:</strong> <see cref="SetText"/>
+/// and <see cref="Clear"/> do NOT emit <see cref="InputChanged"/>. That event is reserved for
+/// user-driven edits so that a consumer reacting to <see cref="InputChanged"/> (e.g. to update
+/// autocomplete candidates) cannot trigger a feedback loop from its own programmatic writes.
+/// </para>
+/// <para>
+/// <strong>Documented gaps:</strong>
+/// <list type="bullet">
+///   <item><c>Prompt</c> — a prompt prefix shown before the editable region — is not in the
+///     §10 model and is deferred to a later §10 refinement pass.</item>
+///   <item><c>ReadOnly</c> — preventing user edits — is similarly deferred.</item>
+/// </list>
+/// </para>
+/// </remarks>
+public sealed class InputSurface : IInput
+{
+    private readonly LoopEngine _loop;
+
+    internal InputSurface(LoopEngine loop)
+    {
+        ArgumentNullException.ThrowIfNull(loop);
+        _loop = loop;
+    }
+
+    /// <summary>
+    /// Replaces the entire input buffer with <paramref name="text"/> and moves the caret to
+    /// the end. Does not emit <see cref="InputChanged"/>.
+    /// </summary>
+    public void SetText(string text)
+    {
+        ArgumentNullException.ThrowIfNull(text);
+        _loop.Post(new SetTextCommand(text));
+    }
+
+    /// <summary>
+    /// Clears the input buffer and moves the caret to position 0.
+    /// Does not emit <see cref="InputChanged"/>.
+    /// </summary>
+    public void Clear()
+    {
+        _loop.Post(new ClearCommand());
+    }
+
+    // ── Commands ───────────────────────────────────────────────────────────────
+
+    private sealed class SetTextCommand : ILoopCommand
+    {
+        private readonly string _text;
+
+        internal SetTextCommand(string text) => _text = text;
+
+        void ILoopCommand.Apply(RenderModel model)
+        {
+            model.FixedRegion.Editor.SetText(_text);
+            model.MarkDirty();
+        }
+    }
+
+    private sealed class ClearCommand : ILoopCommand
+    {
+        void ILoopCommand.Apply(RenderModel model)
+        {
+            model.FixedRegion.Editor.Clear();
+            model.MarkDirty();
+        }
+    }
+}
diff --git a/src/Dcli/Internal/FixedRegion/Autocomplete.cs b/src/Dcli/Internal/FixedRegion/Autocomplete.cs
new file mode 100644
index 0000000..58826f2
--- /dev/null
+++ b/src/Dcli/Internal/FixedRegion/Autocomplete.cs
@@ -0,0 +1,166 @@
+namespace Dcli.Internal.FixedRegion;
+
+/// <summary>
+/// Autocomplete dropdown rendered below the input line.
+/// </summary>
+/// <remarks>
+/// <para>
+/// The overlay is driven by the consumer: call <see cref="Show"/> to present a candidate list
+/// and <see cref="Hide"/> to dismiss it. While visible, <see cref="HandleKey"/> intercepts
+/// navigation and acceptance keys; all other keys fall through to the input editor.
+/// </para>
+/// <para>
+/// <strong>Accept semantics:</strong> when the user confirms a candidate (Enter or Tab),
+/// its <see cref="AutocompleteCandidate.InsertText"/> is applied to the <see cref="TextBuffer"/>
+/// via <see cref="TextBuffer.SetText"/>, which replaces the entire buffer and moves the caret
+/// to the end. This is a whole-buffer replace — the spec scenario and slash-command-style
+/// completion require the candidate to supply the full replacement text. §12 may later
+/// refine this to a span-replace; if so, flag it as a behavioural change.
+/// </para>
+/// <para>
+/// The hardware cursor stays at the input caret while this overlay is active
+/// (<see cref="HidesCursor"/> is always <see langword="false"/>); selection is shown by
+/// <see cref="ScrollableList"/>'s reverse-video highlight.
+/// </para>
+/// <para>Thread safety is the caller's responsibility; the render-loop thread owns this in normal use.</para>
+/// </remarks>
+internal sealed class Autocomplete : IOverlay
+{
+    // ── State ─────────────────────────────────────────────────────────────────
+
+    private readonly TextBuffer _buffer;
+    private readonly ScrollableList _list;
+    private List<AutocompleteCandidate> _candidates = [];
+    private bool _visible;
+
+    // ── Construction ──────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Initialises a new <see cref="Autocomplete"/> overlay.
+    /// </summary>
+    /// <param name="buffer">
+    /// The input buffer to which accepted candidate insert-text is applied.
+    /// </param>
+    internal Autocomplete(TextBuffer buffer)
+    {
+        ArgumentNullException.ThrowIfNull(buffer);
+        _buffer = buffer;
+        _list = new ScrollableList(multiSelect: false);
+    }
+
+    // ── IOverlay ──────────────────────────────────────────────────────────────
+
+    /// <inheritdoc/>
+    public OverlayPlacement Placement => OverlayPlacement.BelowInput;
+
+    /// <inheritdoc/>
+    public bool HidesCursor => false;
+
+    /// <inheritdoc/>
+    public int MaxRows
+    {
+        get => _list.MaxRows;
+        set => _list.MaxRows = value;
+    }
+
+    /// <inheritdoc/>
+    /// <remarks>
+    /// <see langword="true"/> when the overlay is not currently visible (after <see cref="Hide"/>
+    /// or before the first <see cref="Show"/> call).
+    /// </remarks>
+    public bool IsDismissed => !_visible;
+
+    /// <inheritdoc/>
+    /// Autocomplete uses selection highlight; it does not own the hardware cursor.
+    public (int Row, int Col)? CaretInOverlay => null;
+
+    /// <inheritdoc/>
+    /// <remarks>
+    /// <list type="bullet">
+    ///   <item><description><c>↑</c> — moves selection up; consumed.</description></item>
+    ///   <item><description><c>↓</c> — moves selection down; consumed.</description></item>
+    ///   <item><description><c>Enter</c> / <c>Tab</c> — accepts selected candidate (applies InsertText, hides); consumed.</description></item>
+    ///   <item><description><c>Escape</c> — hides without touching the buffer; consumed.</description></item>
+    ///   <item><description>All other keys — fall through (returns <see langword="false"/>).</description></item>
+    /// </list>
+    /// </remarks>
+    public bool HandleKey(KeyEvent key)
+    {
+        if (key.Code.Kind != KeyCode.KeyCodeKind.Named)
+            return false;
+
+        switch (key.Code.NamedValue)
+        {
+            case NamedKey.Up:
+                _list.MoveUp();
+                return true;
+
+            case NamedKey.Down:
+                _list.MoveDown();
+                return true;
+
+            case NamedKey.Enter:
+            case NamedKey.Tab:
+                Accept();
+                return true;
+
+            case NamedKey.Escape:
+                Hide();
+                return true;
+
+            default:
+                return false;
+        }
+    }
+
+    /// <inheritdoc/>
+    public IReadOnlyList<Line> Render(int width) => _list.Render(width);
+
+    // ── Public API ────────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Presents the given candidates in the dropdown.
+    /// Resets the list selection to the first item. Showing an empty candidate list is valid
+    /// (renders nothing; an Accept on an empty list is a safe no-op).
+    /// </summary>
+    /// <param name="candidates">The candidates to display, ordered by preference.</param>
+    internal void Show(IReadOnlyList<AutocompleteCandidate> candidates)
+    {
+        ArgumentNullException.ThrowIfNull(candidates);
+        // Defensive copy: caller may mutate the original list; the Accept lookup must stay consistent.
+        _candidates = candidates.ToList();
+        _list.SetItems(_candidates.Select(c => c.Display).ToList());
+        _visible = true;
+    }
+
+    /// <summary>
+    /// Hides the dropdown without modifying the input buffer.
+    /// </summary>
+    internal void Hide()
+    {
+        _visible = false;
+    }
+
+    // ── Internal accessors ───────────────────────────────────────────────────
+
+    /// <summary>
+    /// Zero-based index of the currently selected candidate, or <c>-1</c> when the list is empty.
+    /// Exposed for <c>Dcli.Testing</c> snapshot construction.
+    /// </summary>
+    internal int SelectedIndex => _list.SelectedIndex;
+
+    // ── Private ───────────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Applies the selected candidate's <see cref="AutocompleteCandidate.InsertText"/> to the
+    /// buffer and hides the overlay. No-op when the list is empty.
+    /// </summary>
+    private void Accept()
+    {
+        int index = _list.SelectedIndex;
+        if (index >= 0 && index < _candidates.Count)
+            _buffer.SetText(_candidates[index].InsertText);
+
+        Hide();
+    }
+}
diff --git a/src/Dcli/Internal/FixedRegion/Dialog.cs b/src/Dcli/Internal/FixedRegion/Dialog.cs
new file mode 100644
index 0000000..2d13477
--- /dev/null
+++ b/src/Dcli/Internal/FixedRegion/Dialog.cs
@@ -0,0 +1,259 @@
+using System.Globalization;
+using System.Text;
+
+namespace Dcli.Internal.FixedRegion;
+
+/// <summary>
+/// A dialog overlay rendered above the input line.
+/// </summary>
+/// <remarks>
+/// <para>
+/// The dialog is a slot that hosts an interactive <see cref="ScrollableList"/>. It does not
+/// contain title or border chrome in v1 — the consumer maps a request to its list contents
+/// via the <see cref="List"/> property before showing the overlay.
+/// </para>
+/// <para>
+/// The dialog is closed (and <see cref="IsDismissed"/> becomes <see langword="true"/>) when
+/// <see cref="CloseRequest"/> is set by an Enter (<see cref="OverlayCloseKind.Submit"/>) or
+/// Escape (<see cref="OverlayCloseKind.Cancel"/>) key. §12 consumes <see cref="CloseRequest"/>
+/// to resolve the awaitable dialog result.
+/// </para>
+/// <para>
+/// <strong>Type-to-filter:</strong> when <see cref="TypeToFilter"/> is <see langword="true"/>,
+/// printable character keys append to <see cref="FilterText"/> and Backspace trims it. The
+/// actual item filtering is consumer-driven (§12) — the dialog records only the raw filter
+/// string so that <c>dcli</c> stays protocol-free and does not interpret item text semantics.
+/// </para>
+/// <para>Thread safety is the caller's responsibility; the render-loop thread owns this in normal use.</para>
+/// </remarks>
+internal sealed class Dialog : IModalOverlay
+{
+    // ── Construction ──────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Initialises a new <see cref="Dialog"/>.
+    /// </summary>
+    /// <param name="multiSelect">
+    /// When <see langword="true"/>, Space toggles the current item in and out of
+    /// <see cref="ScrollableList.CheckedIndices"/>; the list shows check markers.
+    /// </param>
+    /// <param name="modal">
+    /// When <see langword="true"/> (the default), the dialog consumes all keys that are not
+    /// handled by an earlier rule, preventing them from reaching the input editor. Also hides
+    /// the hardware cursor (<see cref="HidesCursor"/> returns <see langword="true"/>).
+    /// </param>
+    /// <param name="typeToFilter">
+    /// When <see langword="true"/>, printable keys append to <see cref="FilterText"/> and
+    /// Backspace trims it. When <see langword="false"/> (the default), those keys are handled
+    /// by the modal fall-through rule or fall through to the input editor.
+    /// </param>
+    /// <param name="title">
+    /// Optional leading row rendered above the list. When non-<see langword="null"/>, one row
+    /// of the <see cref="MaxRows"/> budget is reserved for it, and <c>List.MaxRows</c> is set to
+    /// at most <c>MaxRows - 1</c> so the total output never exceeds the budget.
+    /// </param>
+    internal Dialog(bool multiSelect = false, bool modal = true, bool typeToFilter = false, Line? title = null)
+    {
+        Modal = modal;
+        TypeToFilter = typeToFilter;
+        Title = title;
+        List = new ScrollableList(multiSelect);
+    }
+
+    // ── IOverlay ──────────────────────────────────────────────────────────────
+
+    /// <inheritdoc/>
+    public OverlayPlacement Placement => OverlayPlacement.AboveInput;
+
+    /// <inheritdoc/>
+    /// <remarks>
+    /// <see langword="true"/> while <see cref="Modal"/> is <see langword="true"/> and the dialog
+    /// has not yet been dismissed. Non-modal dialogs never hide the cursor.
+    /// </remarks>
+    public bool HidesCursor => Modal;
+
+    /// <inheritdoc/>
+    /// <remarks>
+    /// The setter stores the raw budget and forwards to <see cref="ScrollableList.MaxRows"/>
+    /// (minus one when a <see cref="Title"/> is present, as a hint; <see cref="Render"/> always
+    /// truncates the combined output to at most <see cref="_maxRows"/> rows regardless).
+    /// </remarks>
+    public int MaxRows
+    {
+        get => _maxRows;
+        set
+        {
+            _maxRows = value;
+            // Give the list as much of the budget as possible. When Title is present, one row
+            // is consumed by it; the list gets the remainder (≥ 0, clamped to 1 by ScrollableList).
+            List.MaxRows = Title is null ? value : Math.Max(1, value - 1);
+        }
+    }
+
+    /// <inheritdoc/>
+    /// <remarks><see langword="true"/> once <see cref="CloseRequest"/> is set.</remarks>
+    public bool IsDismissed => CloseRequest is not null;
+
+    /// <inheritdoc/>
+    /// <remarks>
+    /// Key handling precedence (first match wins):
+    /// <list type="number">
+    ///   <item><description><c>Enter</c> → <see cref="CloseRequest"/> = Submit; consumed.</description></item>
+    ///   <item><description><c>Escape</c> → <see cref="CloseRequest"/> = Cancel; consumed.</description></item>
+    ///   <item><description><c>↑</c> / <c>↓</c> → navigate the list; consumed.</description></item>
+    ///   <item><description>Space (U+0020) when <see cref="ScrollableList.MultiSelect"/> → toggle current; consumed.</description></item>
+    ///   <item><description>Printable rune (≥ U+0020, ≠ U+007F) when <see cref="TypeToFilter"/> → append to <see cref="FilterText"/>; consumed. Backspace → trim <see cref="FilterText"/>; consumed.</description></item>
+    ///   <item><description>Any remaining key when <see cref="Modal"/> → consumed (modal catch-all).</description></item>
+    ///   <item><description>Otherwise → <see langword="false"/> (falls through to the input editor).</description></item>
+    /// </list>
+    /// </remarks>
+    public bool HandleKey(KeyEvent key)
+    {
+        // 1. Enter → Submit
+        if (key.Code.Kind == KeyCode.KeyCodeKind.Named && key.Code.NamedValue == NamedKey.Enter)
+        {
+            CloseRequest = OverlayCloseKind.Submit;
+            return true;
+        }
+
+        // 2. Escape → Cancel
+        if (key.Code.Kind == KeyCode.KeyCodeKind.Named && key.Code.NamedValue == NamedKey.Escape)
+        {
+            CloseRequest = OverlayCloseKind.Cancel;
+            return true;
+        }
+
+        // 3. Arrow navigation
+        if (key.Code.Kind == KeyCode.KeyCodeKind.Named && key.Code.NamedValue == NamedKey.Up)
+        {
+            List.MoveUp();
+            return true;
+        }
+        if (key.Code.Kind == KeyCode.KeyCodeKind.Named && key.Code.NamedValue == NamedKey.Down)
+        {
+            List.MoveDown();
+            return true;
+        }
+
+        // 4. Space → toggle when multi-select (takes precedence over type-to-filter for space)
+        if (key.Code.Kind == KeyCode.KeyCodeKind.UnicodeScalar &&
+            key.Code.RuneValue.Value == ' ' &&
+            List.MultiSelect)
+        {
+            List.ToggleCurrent();
+            return true;
+        }
+
+        // 5. Type-to-filter: printable runes and Backspace
+        if (TypeToFilter)
+        {
+            if (key.Code.Kind == KeyCode.KeyCodeKind.Named && key.Code.NamedValue == NamedKey.Backspace)
+            {
+                TrimFilterText();
+                return true;
+            }
+
+            if (key.Code.Kind == KeyCode.KeyCodeKind.UnicodeScalar)
+            {
+                Rune r = key.Code.RuneValue;
+                // Printable: rune >= U+0020 and not DEL (U+007F)
+                if (r.Value >= 0x20 && r.Value != 0x7F)
+                {
+                    _filterText.Append(r.ToString());
+                    return true;
+                }
+            }
+        }
+
+        // 6. Modal catch-all: consume everything so the input editor gets nothing
+        if (Modal)
+            return true;
+
+        // 7. Non-modal fall-through
+        return false;
+    }
+
+    /// <inheritdoc/>
+    /// <remarks>
+    /// When <see cref="Title"/> is set, it is prepended as the first row before the list rows.
+    /// The total row count never exceeds <see cref="MaxRows"/>: the title is emitted first and
+    /// list rows fill whatever budget remains (possibly zero when <see cref="MaxRows"/> is 1).
+    /// </remarks>
+    public IReadOnlyList<Line> Render(int width)
+    {
+        IReadOnlyList<Line> listRows = List.Render(width);
+        if (Title is null)
+            return listRows;
+
+        // Budget: title occupies row 0; list gets up to (MaxRows - 1) additional rows.
+        int listBudget = Math.Max(0, _maxRows - 1);
+        int listCount = Math.Min(listRows.Count, listBudget);
+        Line[] result = new Line[1 + listCount];
+        result[0] = Title;
+        for (int i = 0; i < listCount; i++)
+            result[i + 1] = listRows[i];
+        return result;
+    }
+
+    // ── Public properties ─────────────────────────────────────────────────────
+
+    /// <summary>The hosted interactive list; configure items before displaying the dialog.</summary>
+    internal ScrollableList List { get; }
+
+    /// <summary>
+    /// Optional title row displayed above the list. Set via the constructor.
+    /// When non-<see langword="null"/>, <see cref="Render"/> prepends it and
+    /// <see cref="MaxRows"/> reserves one row for it.
+    /// </summary>
+    internal Line? Title { get; }
+
+    /// <summary>Whether the dialog is modal (consumes all keys, hides the cursor).</summary>
+    internal bool Modal { get; }
+
+    /// <summary>Whether printable keys are routed to <see cref="FilterText"/>.</summary>
+    internal bool TypeToFilter { get; }
+
+    /// <summary>
+    /// The current filter string entered by the user.
+    /// Starts empty; only updated when <see cref="TypeToFilter"/> is <see langword="true"/>.
+    /// The actual item filtering is consumer-driven (§12); this property records only the raw text.
+    /// </summary>
+    internal string FilterText => _filterText.ToString();
+
+    /// <summary>
+    /// How the dialog was closed, or <see langword="null"/> while still open.
+    /// Set by Enter (<see cref="OverlayCloseKind.Submit"/>) or Escape (<see cref="OverlayCloseKind.Cancel"/>).
+    /// Consumed by §12 to resolve the awaitable dialog result.
+    /// </summary>
+    public OverlayCloseKind? CloseRequest { get; private set; }
+
+    /// <inheritdoc/>
+    /// <remarks>
+    /// The list dialog never owns the hardware cursor; it uses selection highlight instead.
+    /// </remarks>
+    public (int Row, int Col)? CaretInOverlay => null;
+
+    // ── Private ───────────────────────────────────────────────────────────────
+
+    private int _maxRows = 10; // default matches ScrollableList default
+    private readonly StringBuilder _filterText = new();
+
+    /// <summary>
+    /// Removes the last grapheme cluster (text element) from <see cref="FilterText"/>.
+    /// No-op when the filter string is empty.
+    /// </summary>
+    private void TrimFilterText()
+    {
+        if (_filterText.Length == 0)
+            return;
+
+        string current = _filterText.ToString();
+        // Find the last grapheme cluster boundary using StringInfo.
+        TextElementEnumerator enumerator = StringInfo.GetTextElementEnumerator(current);
+        int lastStart = 0;
+        while (enumerator.MoveNext())
+            lastStart = enumerator.ElementIndex;
+
+        _filterText.Remove(lastStart, _filterText.Length - lastStart);
+    }
+}
diff --git a/src/Dcli/Internal/FixedRegion/FixedRegionComposer.cs b/src/Dcli/Internal/FixedRegion/FixedRegionComposer.cs
new file mode 100644
index 0000000..f06dc35
--- /dev/null
+++ b/src/Dcli/Internal/FixedRegion/FixedRegionComposer.cs
@@ -0,0 +1,203 @@
+using Dcli.Internal.RenderLoop;
+
+namespace Dcli.Internal.FixedRegion;
+
+/// <summary>
+/// Composes the fixed region (input editor + optional overlay + status line) into paint-state,
+/// enforcing the <c>MaxHeight</c> budget and setting <see cref="RenderModel.FixedRegionRows"/>,
+/// <see cref="RenderModel.CaretPosition"/>, and <see cref="RenderModel.IsCursorVisible"/>.
+/// </summary>
+/// <remarks>
+/// <para>
+/// <strong>Height budget:</strong>
+/// <c>cap = (rows &lt; 8) ? rows : clamp(appSet ?? rows/2, 8, rows)</c>
+/// where <c>appSet</c> is <see cref="RenderModel.MaxFixedHeight"/>.
+/// The fixed region consumes only the rows its content needs, up to the cap; it is never
+/// padded to the cap.
+/// </para>
+/// <para>
+/// <strong>Component stack (bottom-to-top in the frame, index 0 = topmost rendered first):</strong>
+/// <list type="number">
+///   <item>Overlay rows (above input, when <see cref="IOverlay.Placement"/> is <c>AboveInput</c>).</item>
+///   <item>Input editor rows (internally scrolled if needed).</item>
+///   <item>Overlay rows (below input, when <see cref="IOverlay.Placement"/> is <c>BelowInput</c>).</item>
+///   <item>Status rows (always sacred — always at the very bottom).</item>
+/// </list>
+/// </para>
+/// <para>
+/// <strong>Priority order when budget is tight (Decision 8):</strong>
+/// <list type="number">
+///   <item>Status is sacred — always fully shown.</item>
+///   <item>Caret line is always visible — input scrolls internally via
+///     <see cref="TextBuffer.Render(int, int?)"/>.</item>
+///   <item>The overlay absorbs the squeeze — capped at the remainder of the budget
+///     (up to <c>DefaultOverlayMaxRows</c>). In the common 1–2 row input case the
+///     overlay gets the bulk of the budget.</item>
+/// </list>
+/// </para>
+/// <para>
+/// <strong>Degenerate case (status alone ≥ cap):</strong>
+/// If <c>statusRows.Count ≥ cap</c>, there is no room for the input editor or any overlay.
+/// Status is shown in full (clamped to terminal height); the caret is hidden.
+/// </para>
+/// <para>
+/// <strong>Composition happens before scrollback PrePaint:</strong>
+/// <see cref="RenderModel.FixedRegionRows"/> must be populated before §9's PrePaint so that
+/// the live-window budget (<c>rows − fixedRegionRows.Count</c>) is correct.
+/// </para>
+/// </remarks>
+internal sealed class FixedRegionComposer
+{
+    private readonly StatusLine _status;
+
+    internal FixedRegionComposer(TextBuffer editor, StatusLine status)
+    {
+        Editor = editor;
+        _status = status;
+    }
+
+    /// <summary>
+    /// The input editor. The loop thread routes editing <see cref="KeyEvent"/>s directly to
+    /// this object and then calls <see cref="Compose"/> to recompose the fixed region.
+    /// </summary>
+    internal TextBuffer Editor { get; }
+
+    /// <summary>
+    /// The status line component. Exposed so façade commands can set
+    /// <see cref="StatusLine.Rows"/> on the loop thread via <c>model.FixedRegion.Status</c>.
+    /// </summary>
+    internal StatusLine Status => _status;
+
+    /// <summary>
+    /// Computes the cap for the fixed region given the terminal height and the optional
+    /// consumer-supplied <paramref name="appSet"/> value.
+    /// </summary>
+    /// <param name="rows">Terminal height in rows.</param>
+    /// <param name="appSet">
+    /// Consumer-supplied cap from <see cref="TerminalOptions.MaxFixedHeight"/>.
+    /// <see langword="null"/> means "use 50% of rows".
+    /// </param>
+    /// <returns>
+    /// The effective cap in rows:
+    /// <list type="bullet">
+    ///   <item><c>rows</c> when <c>rows &lt; 8</c> (tiny terminal: floor yields to terminal height).</item>
+    ///   <item><c>clamp(appSet ?? rows/2, 8, rows)</c> otherwise.</item>
+    /// </list>
+    /// </returns>
+    internal static int ComputeCap(int rows, int? appSet)
+    {
+        if (rows < 8)
+            return rows;
+
+        int desired = appSet ?? rows / 2;
+        return Math.Clamp(desired, 8, rows);
+    }
+
+    // Maximum rows the overlay may occupy when no consumer-supplied limit exists.
+    // §12 will make this configurable via TerminalOptions.
+    private const int _defaultOverlayMaxRows = 10;
+
+    /// <summary>
+    /// Composes the fixed region into the render model's paint-state fields.
+    /// Must be called <em>before</em> <c>ScrollbackModel.PrePaint</c> so that
+    /// <see cref="RenderModel.FixedRegionRows"/> is set when the live-window budget is computed.
+    /// </summary>
+    internal void Compose(RenderModel model)
+    {
+        int width = model.Columns;
+        int rows = model.Rows;
+        int cap = ComputeCap(rows, model.MaxFixedHeight);
+
+        IReadOnlyList<Line> statusRows = _status.Rows;
+        int statusCount = statusRows.Count;
+
+        // Degenerate: status alone fills or exceeds the cap — no room for the editor or overlays.
+        // Status is shown in full (clamped to terminal height); caret is hidden.
+        if (statusCount >= cap)
+        {
+            // Clamp status to terminal rows so we never write off-screen.
+            IReadOnlyList<Line> visibleStatus = statusCount > rows
+                ? statusRows.Take(rows).ToList()
+                : statusRows;
+
+            model.FixedRegionRows = visibleStatus;
+            model.EditorCaretLocal = null;
+            model.CaretPosition = null;
+            model.IsCursorVisible = false;
+            return;
+        }
+
+        // Normal path.
+        // budget = rows available for input + overlay (status is sacred, already accounted for).
+        int budget = cap - statusCount; // > 0 here
+
+        // Input keeps the caret visible: allot the full budget so the editor can scroll
+        // internally to expose the caret line. Renders exactly min(natural, budget) rows.
+        RenderResult editorResult = Editor.Render(width, budget);
+        IReadOnlyList<Line> inputRows = editorResult.VisibleRows;
+        (int editorCaretRow, int editorCaretCol) = editorResult.CaretPosition;
+
+        // Overlay absorbs the squeeze: whatever is left of the budget, capped at DefaultOverlayMaxRows.
+        // Do NOT read overlay.MaxRows back after writing it — compute overlayCap from budget and
+        // inputRows.Count to prevent stale values from blocking re-expansion when the budget grows.
+        IOverlay? overlay = model.ActiveOverlay;
+        IReadOnlyList<Line> overlayRows = [];
+        if (overlay is not null)
+        {
+            int overlayCap = Math.Clamp(budget - inputRows.Count, 0, _defaultOverlayMaxRows);
+            overlay.MaxRows = overlayCap; // sets the viewport for this frame (clamped to ≥1 by ScrollableList)
+            // Only render when the budget permits at least one row; otherwise the overlay is
+            // effectively squeezed to zero and we skip rendering to honour the cap proof.
+            if (overlayCap > 0)
+                overlayRows = overlay.Render(width); // ≤ overlayCap rows
+        }
+
+        // Assemble: [overlay if AboveInput] [input] [overlay if BelowInput] [status].
+        // Proof that total ≤ cap: overlayRows.Count ≤ budget − inputRows.Count,
+        // so overlayRows.Count + inputRows.Count ≤ budget = cap − statusCount,
+        // thus total = overlayRows.Count + inputRows.Count + statusCount ≤ cap.
+        int aboveCount = (overlay?.Placement == OverlayPlacement.AboveInput) ? overlayRows.Count : 0;
+
+        List<Line> fixedRows = new(overlayRows.Count + inputRows.Count + statusCount);
+        if (overlay?.Placement == OverlayPlacement.AboveInput)
+            fixedRows.AddRange(overlayRows);
+        fixedRows.AddRange(inputRows);
+        if (overlay?.Placement == OverlayPlacement.BelowInput)
+            fixedRows.AddRange(overlayRows);
+        fixedRows.AddRange(statusRows);
+
+        model.FixedRegionRows = fixedRows;
+
+        // Cursor placement (priority order):
+        //   1. HidesCursor (modal list dialog) → hidden.
+        //   2. Overlay supplies CaretInOverlay AND overlay rows were actually rendered → cursor
+        //      at the overlay's caret (input dialog case).
+        //   3. Normal → cursor at the main editor caret.
+        // EditorCaretLocal is relative to the START of the fixed region; the above-input overlay
+        // rows are folded in here so the loop's existing CaretPosition finalisation is correct.
+        if (overlay is not null && overlay.HidesCursor)
+        {
+            model.EditorCaretLocal = null;
+            model.CaretPosition = null;
+            model.IsCursorVisible = false;
+        }
+        else if (overlay is not null &&
+                 overlayRows.Count > 0 &&
+                 overlay.CaretInOverlay is { } overlayCaret)
+        {
+            // The overlay owns the cursor (e.g. InputDialog). overlayStartRow is 0 for
+            // AboveInput overlays and inputRows.Count for BelowInput overlays.
+            int overlayStartRow = overlay.Placement == OverlayPlacement.AboveInput
+                ? 0
+                : inputRows.Count;
+            model.EditorCaretLocal = (overlayStartRow + overlayCaret.Row, overlayCaret.Col);
+            model.IsCursorVisible = true;
+        }
+        else
+        {
+            // aboveCount offsets the caret row by any rows inserted above the input.
+            model.EditorCaretLocal = (aboveCount + editorCaretRow, editorCaretCol);
+            model.IsCursorVisible = true;
+        }
+    }
+}
diff --git a/src/Dcli/Internal/FixedRegion/IOverlay.cs b/src/Dcli/Internal/FixedRegion/IOverlay.cs
new file mode 100644
index 0000000..768eba3
--- /dev/null
+++ b/src/Dcli/Internal/FixedRegion/IOverlay.cs
@@ -0,0 +1,108 @@
+namespace Dcli.Internal.FixedRegion;
+
+// ── OverlayPlacement ──────────────────────────────────────────────────────────
+
+/// <summary>
+/// Where in the fixed region an overlay is rendered relative to the input line.
+/// </summary>
+internal enum OverlayPlacement
+{
+    /// <summary>The overlay renders above the input line (e.g. a dialog).</summary>
+    AboveInput,
+
+    /// <summary>The overlay renders below the input line (e.g. autocomplete suggestions).</summary>
+    BelowInput,
+}
+
+// ── OverlayCloseKind ──────────────────────────────────────────────────────────
+
+/// <summary>
+/// How an overlay was closed; surfaced by <see cref="Dialog.CloseRequest"/> and consumed by §12
+/// to resolve the awaitable dialog result.
+/// </summary>
+internal enum OverlayCloseKind
+{
+    /// <summary>The user confirmed (Enter).</summary>
+    Submit,
+
+    /// <summary>The user cancelled (Escape).</summary>
+    Cancel,
+}
+
+// ── IOverlay ──────────────────────────────────────────────────────────────────
+
+/// <summary>
+/// Front of the key intercept chain; contributes rows to the fixed region.
+/// </summary>
+/// <remarks>
+/// The active overlay receives keys before the input editor. It either consumes a key
+/// (returns <see langword="true"/> from <see cref="HandleKey"/>) or passes it on
+/// (returns <see langword="false"/>). At most one overlay is active at a time
+/// (<c>OverlayState</c> invariant, enforced in B-ii).
+/// </remarks>
+internal interface IOverlay
+{
+    /// <summary>Where the overlay is slotted relative to the input line.</summary>
+    OverlayPlacement Placement { get; }
+
+    /// <summary>
+    /// When <see langword="true"/> the composer tells the painter to hide the hardware cursor.
+    /// Only <see langword="true"/> while a modal <see cref="Dialog"/> is active.
+    /// </summary>
+    bool HidesCursor { get; }
+
+    /// <summary>
+    /// Viewport row cap set by the composer from the height budget.
+    /// Forwarded directly to the hosted <see cref="ScrollableList.MaxRows"/>.
+    /// </summary>
+    int MaxRows { get; set; }
+
+    /// <summary>
+    /// <see langword="true"/> once the overlay should be removed from the model.
+    /// For <see cref="Autocomplete"/> this is <c>!IsVisible</c>; for <see cref="Dialog"/> this is
+    /// <c>CloseRequest is not null</c>.
+    /// </summary>
+    bool IsDismissed { get; }
+
+    /// <summary>
+    /// Attempts to handle a key press.
+    /// </summary>
+    /// <param name="key">The key event.</param>
+    /// <returns>
+    /// <see langword="true"/> if the overlay consumed the key and the event must not be forwarded;
+    /// <see langword="false"/> if the key falls through to the next handler in the chain.
+    /// </returns>
+    bool HandleKey(KeyEvent key);
+
+    /// <summary>
+    /// Renders the overlay rows at the given terminal width.
+    /// Delegates to the hosted <see cref="ScrollableList.Render"/>.
+    /// </summary>
+    /// <param name="width">Terminal width in columns (forwarded as-is to the list).</param>
+    /// <returns>The rows to slot into the fixed region; empty when nothing to show.</returns>
+    IReadOnlyList<Line> Render(int width);
+
+    /// <summary>
+    /// The overlay's own caret position within its rendered rows (zero-based row and column),
+    /// or <see langword="null"/> when the overlay does not own the hardware cursor.
+    /// When non-<see langword="null"/> and the overlay was rendered this frame, the composer
+    /// parks the hardware cursor here instead of at the main input editor.
+    /// </summary>
+    (int Row, int Col)? CaretInOverlay { get; }
+}
+
+// ── IModalOverlay ─────────────────────────────────────────────────────────────
+
+/// <summary>
+/// An <see cref="IOverlay"/> that has a deterministic dismissal signal: a
+/// <see cref="CloseRequest"/> that is non-<see langword="null"/> once the overlay is closed
+/// (Enter → Submit, Escape → Cancel). The loop's dismiss hook reads this to invoke the
+/// awaitable completion and clear the overlay.
+/// </summary>
+internal interface IModalOverlay : IOverlay
+{
+    /// <summary>
+    /// How the overlay was closed, or <see langword="null"/> while still open.
+    /// </summary>
+    OverlayCloseKind? CloseRequest { get; }
+}
diff --git a/src/Dcli/Internal/FixedRegion/InputDialog.cs b/src/Dcli/Internal/FixedRegion/InputDialog.cs
new file mode 100644
index 0000000..edb77b7
--- /dev/null
+++ b/src/Dcli/Internal/FixedRegion/InputDialog.cs
@@ -0,0 +1,272 @@
+using System.Text;
+
+namespace Dcli.Internal.FixedRegion;
+
+/// <summary>
+/// A free-text input dialog overlay rendered above the input line.
+/// </summary>
+/// <remarks>
+/// <para>
+/// Unlike the list-based <see cref="Dialog"/>, this overlay hosts a <see cref="TextBuffer"/> and
+/// exposes a full single-line or multi-line text entry experience. The cursor is placed at the
+/// dialog's own caret via <see cref="CaretInOverlay"/> — not hidden the way a select dialog hides
+/// it (see Decision 8 / <see cref="HidesCursor"/>).
+/// </para>
+/// <para>
+/// When <see cref="IsSecret"/> is <see langword="true"/>, each character in the rendered rows is
+/// replaced by the bullet mask glyph U+2022 (•). The width of each glyph is preserved so that
+/// <see cref="CaretInOverlay"/> remains correct without any offset arithmetic.
+/// </para>
+/// <para>Thread safety is the caller's responsibility; the render-loop thread owns this in normal use.</para>
+/// </remarks>
+internal sealed class InputDialog : IModalOverlay
+{
+    private readonly TextBuffer _buffer;
+    private readonly bool _isSecret;
+    private int _maxRows = 10;
+    // Cached from the last Render call (or seeded by SeedWidth on the loop thread at open time)
+    // so HandleKey has a correct width for Home/End/Up/Down before the first paint.
+    // The value is 0 until seeded; SeedWidth is called by OpenDialogCommand.Apply so width-
+    // dependent navigation in the first key batch uses the real terminal width, not a guess.
+    private int _lastWidth;
+    private (int Row, int Col)? _lastCaret;
+
+    /// <summary>
+    /// Initialises a new <see cref="InputDialog"/>.
+    /// </summary>
+    /// <param name="prompt">Optional prompt row rendered above the text field.</param>
+    /// <param name="default">Optional pre-filled text; the caret starts at its end.</param>
+    /// <param name="isSecret">When <see langword="true"/>, rendered characters are masked.</param>
+    internal InputDialog(Line? prompt, string? @default, bool isSecret)
+    {
+        Prompt = prompt;
+        _isSecret = isSecret;
+        _buffer = new TextBuffer();
+        if (!string.IsNullOrEmpty(@default))
+            _buffer.SetText(@default);
+    }
+
+    // ── IOverlay ─────────────────────────────────────────────────────────────
+
+    /// <inheritdoc/>
+    public OverlayPlacement Placement => OverlayPlacement.AboveInput;
+
+    /// <inheritdoc/>
+    /// <remarks>
+    /// The input dialog is the focus — the hardware cursor is placed at its own caret.
+    /// <see langword="false"/> here; cursor position comes from <see cref="CaretInOverlay"/>.
+    /// </remarks>
+    public bool HidesCursor => false;
+
+    /// <inheritdoc/>
+    public int MaxRows
+    {
+        get => _maxRows;
+        set => _maxRows = value;
+    }
+
+    /// <inheritdoc/>
+    /// <remarks><see langword="true"/> once <see cref="CloseRequest"/> is set.</remarks>
+    public bool IsDismissed => CloseRequest is not null;
+
+    /// <inheritdoc/>
+    /// <remarks>
+    /// Key handling precedence (first match wins):
+    /// <list type="number">
+    ///   <item><description><c>Enter</c> → <see cref="CloseRequest"/> = Submit; consumed.</description></item>
+    ///   <item><description><c>Escape</c> → <see cref="CloseRequest"/> = Cancel; consumed.</description></item>
+    ///   <item><description>Printable rune (≥ U+0020, ≠ U+007F) → insert into buffer; consumed.</description></item>
+    ///   <item><description><c>Backspace</c> → delete before caret; consumed.</description></item>
+    ///   <item><description><c>Delete</c> → delete after caret; consumed.</description></item>
+    ///   <item><description><c>Left</c> / <c>Right</c> → move caret; consumed.</description></item>
+    ///   <item><description><c>Home</c> / <c>End</c> → move to visual row start/end; consumed.</description></item>
+    ///   <item><description><c>Up</c> / <c>Down</c> → move caret up/down one visual row; consumed.</description></item>
+    ///   <item><description>Any other key → modal catch-all; consumed (Tab, Ctrl/Alt, etc.).</description></item>
+    /// </list>
+    /// Cancellation is via Escape or the CancellationToken, never Ctrl+C.
+    /// </remarks>
+    public bool HandleKey(KeyEvent key)
+    {
+        int width = _lastWidth;
+
+        // 1. Enter → Submit
+        if (key.Code.Kind == KeyCode.KeyCodeKind.Named && key.Code.NamedValue == NamedKey.Enter)
+        {
+            CloseRequest = OverlayCloseKind.Submit;
+            return true;
+        }
+
+        // 2. Escape → Cancel
+        if (key.Code.Kind == KeyCode.KeyCodeKind.Named && key.Code.NamedValue == NamedKey.Escape)
+        {
+            CloseRequest = OverlayCloseKind.Cancel;
+            return true;
+        }
+
+        // Keys with Ctrl or Alt fall to the modal catch-all — they must not insert text or trigger
+        // editing operations. Mirrors the gate in LoopEngine.RouteEditorKey. Shift is allowed
+        // because terminals deliver shifted printable runes as ordinary UnicodeScalar events.
+        if ((key.Modifiers & (Modifiers.Ctrl | Modifiers.Alt)) != Modifiers.None)
+            return true; // consumed by modal catch-all
+
+        // 3. Printable rune (Ctrl/Alt already excluded above)
+        if (key.Code.Kind == KeyCode.KeyCodeKind.UnicodeScalar)
+        {
+            Rune r = key.Code.RuneValue;
+            if (r.Value >= 0x20 && r.Value != 0x7F)
+            {
+                _buffer.Insert(r);
+                return true;
+            }
+        }
+
+        // 4–8. Named navigation keys
+        if (key.Code.Kind == KeyCode.KeyCodeKind.Named)
+        {
+            switch (key.Code.NamedValue)
+            {
+                case NamedKey.Backspace:
+                    _buffer.Backspace();
+                    return true;
+
+                case NamedKey.Delete:
+                    _buffer.Delete();
+                    return true;
+
+                case NamedKey.Left:
+                    _buffer.MoveLeft();
+                    return true;
+
+                case NamedKey.Right:
+                    _buffer.MoveRight();
+                    return true;
+
+                case NamedKey.Home:
+                    _buffer.MoveHome(width);
+                    return true;
+
+                case NamedKey.End:
+                    _buffer.MoveEnd(width);
+                    return true;
+
+                case NamedKey.Up:
+                    _buffer.MoveUp(width);
+                    return true;
+
+                case NamedKey.Down:
+                    _buffer.MoveDown(width);
+                    return true;
+            }
+        }
+
+        // 9. Modal catch-all — consume everything else (Tab, Ctrl/Alt combos, unknown keys).
+        return true;
+    }
+
+    /// <inheritdoc/>
+    /// <remarks>
+    /// When <see cref="Prompt"/> is non-<see langword="null"/>, it is prepended as the first row.
+    /// When <see cref="IsSecret"/> is <see langword="true"/>, each Rune in the visible buffer rows
+    /// is replaced by one mask glyph per display column occupied by that Rune, so
+    /// <see cref="CaretInOverlay"/> needs no offset correction.
+    /// </remarks>
+    public IReadOnlyList<Line> Render(int width)
+    {
+        _lastWidth = width;
+
+        int promptRows = Prompt is not null ? 1 : 0;
+        int bufferAllotment = Math.Max(1, _maxRows - promptRows);
+
+        RenderResult r = _buffer.Render(width, allottedHeight: bufferAllotment);
+        IReadOnlyList<Line> bufferRows = _isSecret ? MaskRows(r.VisibleRows) : r.VisibleRows;
+
+        // Caret within the assembled rows: offset by prompt row if present.
+        _lastCaret = (r.CaretPosition.Row + promptRows, r.CaretPosition.Col);
+
+        // Assemble and truncate to MaxRows.
+        int totalCount = promptRows + bufferRows.Count;
+        Line[] result = new Line[Math.Min(totalCount, _maxRows)];
+        int idx = 0;
+
+        if (Prompt is not null && idx < result.Length)
+            result[idx++] = Prompt;
+
+        for (int i = 0; i < bufferRows.Count && idx < result.Length; i++)
+            result[idx++] = bufferRows[i];
+
+        return result;
+    }
+
+    // ── IModalOverlay ─────────────────────────────────────────────────────────
+
+    /// <inheritdoc/>
+    public OverlayCloseKind? CloseRequest { get; private set; }
+
+    // ── IOverlay ─────────────────────────────────────────────────────────────
+
+    /// <inheritdoc/>
+    /// <remarks>
+    /// Returns the prompt-offset caret from the last <see cref="Render"/> call,
+    /// or <see langword="null"/> before the first render.
+    /// </remarks>
+    public (int Row, int Col)? CaretInOverlay => _lastCaret;
+
+    // ── Internal surface ─────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Optional prompt row displayed above the input field. Set via the constructor.
+    /// When non-<see langword="null"/>, <see cref="Render"/> prepends it and reserves one row
+    /// of the <see cref="MaxRows"/> budget for it.
+    /// </summary>
+    internal Line? Prompt { get; }
+
+    /// <summary>Whether the input is secret (masked in the rendered rows).</summary>
+    internal bool IsSecret => _isSecret;
+
+    /// <summary>The real (unmasked) text currently in the buffer.</summary>
+    internal string Text => _buffer.Text;
+
+    /// <summary>
+    /// Seeds the cached width used by width-dependent <see cref="HandleKey"/> operations
+    /// (Home, End, Up, Down) before the first <see cref="Render"/> call.
+    /// Called by the open-dialog loop command on the loop thread immediately after the overlay
+    /// is shown, so the real terminal width is available from the first key batch.
+    /// </summary>
+    internal void SeedWidth(int width) => _lastWidth = width;
+
+    // ── Secret masking ────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Replaces every Rune in each row with one bullet glyph per display column it occupies.
+    /// This preserves column-width arithmetic so <see cref="CaretInOverlay"/> remains correct.
+    /// </summary>
+    private static Line[] MaskRows(IReadOnlyList<Line> rows)
+    {
+        Line[] masked = new Line[rows.Count];
+        for (int i = 0; i < rows.Count; i++)
+            masked[i] = MaskLine(rows[i]);
+        return masked;
+    }
+
+    private static Line MaskLine(Line line)
+    {
+        // Rebuild segment list: each Rune → DisplayWidth(rune) copies of the mask glyph.
+        List<Segment> maskedSegments = new(line.Segments.Count);
+        foreach (Segment seg in line.Segments)
+        {
+            if (string.IsNullOrEmpty(seg.Text))
+            {
+                maskedSegments.Add(seg);
+                continue;
+            }
+
+            int totalCols = 0;
+            foreach (Rune r in seg.Text.EnumerateRunes())
+                totalCols += DisplayWidth.Measure(r);
+
+            string maskedText = new string('•', totalCols);
+            maskedSegments.Add(new Segment(maskedText, seg.Style));
+        }
+        return new Line(maskedSegments);
+    }
+}
diff --git a/src/Dcli/Internal/FixedRegion/ScrollableList.cs b/src/Dcli/Internal/FixedRegion/ScrollableList.cs
new file mode 100644
index 0000000..bc954c3
--- /dev/null
+++ b/src/Dcli/Internal/FixedRegion/ScrollableList.cs
@@ -0,0 +1,284 @@
+namespace Dcli.Internal.FixedRegion;
+
+/// <summary>
+/// A bounded, scrollable, optionally multi-select list component.
+/// </summary>
+/// <remarks>
+/// <para>
+/// <strong>Selection highlight:</strong> the selected row is indicated by <see cref="Format.Reverse"/>
+/// applied to every segment in that row, preserving existing foreground, background, and other format
+/// flags (Decision 8 — reverse video, never the hardware cursor). The selected row is additionally
+/// padded to <c>width</c> columns with a reverse-styled space so the highlight reads as a full-width
+/// bar; non-selected rows are not padded.
+/// </para>
+/// <para>
+/// <strong>One item = one visual row.</strong> Each item is a <see cref="Line"/>; it is truncated
+/// (not wrapped) to the first width-aware row produced by <see cref="LineWrapper.Wrap"/>. Multi-row
+/// item wrapping is not supported — this keeps selection navigation and auto-scroll item-indexed and
+/// predictable.
+/// </para>
+/// <para>
+/// <strong>Viewport + auto-scroll:</strong> maintains a private <c>_top</c> index. On
+/// <see cref="MoveUp"/> / <see cref="MoveDown"/> the viewport scrolls by the minimum amount needed
+/// to keep <see cref="SelectedIndex"/> visible ("scroll just enough" — same anchoring as
+/// <see cref="TextBuffer"/>).
+/// </para>
+/// <para>
+/// <strong>Multi-select marker:</strong> when <see cref="MultiSelect"/> is <see langword="true"/>,
+/// every rendered row is prefixed with <c>"[x] "</c> or <c>"[ ] "</c> (4 columns each, equal width)
+/// before the item content is truncated to <c>width</c>.
+/// </para>
+/// <para>
+/// Thread safety is the caller's responsibility; the render-loop thread owns this in normal use.
+/// </para>
+/// </remarks>
+internal sealed class ScrollableList
+{
+    // ── State ────────────────────────────────────────────────────────────────────────────────
+
+    private IReadOnlyList<Line> _items = [];
+    private int _selectedIndex = -1;
+    private int _top;
+    private int _maxRows = 10;
+
+    // Sorted ascending set of checked indices; null when !MultiSelect.
+    private readonly SortedSet<int>? _checkedIndices;
+
+    // Marker text — widths must be equal.
+    private const string _checkedMarker = "[x] ";
+    private const string _uncheckedMarker = "[ ] ";
+
+    // ── Construction ─────────────────────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Initializes a new <see cref="ScrollableList"/>.
+    /// </summary>
+    /// <param name="multiSelect">
+    /// When <see langword="true"/>, <see cref="ToggleCurrent"/> adds/removes items from
+    /// <see cref="CheckedIndices"/> and every row is prefixed with a checked/unchecked marker.
+    /// When <see langword="false"/>, <see cref="ToggleCurrent"/> is a no-op and no markers are shown.
+    /// </param>
+    internal ScrollableList(bool multiSelect = false)
+    {
+        MultiSelect = multiSelect;
+        _checkedIndices = multiSelect ? [] : null;
+    }
+
+    // ── Configuration ────────────────────────────────────────────────────────────────────────
+
+    /// <summary>Whether multi-select was enabled at construction (fixed).</summary>
+    internal bool MultiSelect { get; }
+
+    /// <summary>
+    /// Maximum number of item rows shown in the viewport. Values below 1 are treated as 1.
+    /// Defaults to 10.
+    /// </summary>
+    internal int MaxRows
+    {
+        get => _maxRows;
+        set => _maxRows = Math.Max(1, value);
+    }
+
+    // ── Query ────────────────────────────────────────────────────────────────────────────────
+
+    /// <summary>Number of items in the list.</summary>
+    internal int Count => _items.Count;
+
+    /// <summary>
+    /// Zero-based index of the currently selected item, or <c>-1</c> when the list is empty.
+    /// </summary>
+    internal int SelectedIndex => _selectedIndex;
+
+    /// <summary>
+    /// The set of checked item indices in ascending order.
+    /// Always empty when <see cref="MultiSelect"/> is <see langword="false"/>.
+    /// Returns a snapshot — mutations to the returned collection do not affect the list.
+    /// </summary>
+    internal IReadOnlyCollection<int> CheckedIndices =>
+        _checkedIndices?.ToArray() ?? [];
+
+    // ── Mutation ─────────────────────────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Replaces the item list. Resets <see cref="SelectedIndex"/> to <c>0</c> (or <c>-1</c> when
+    /// empty), scrolls the viewport back to the top, and clears <see cref="CheckedIndices"/>.
+    /// </summary>
+    /// <param name="items">The new items to display.</param>
+    /// <exception cref="ArgumentNullException">
+    /// Thrown when <paramref name="items"/> is <see langword="null"/>.
+    /// </exception>
+    internal void SetItems(IReadOnlyList<Line> items)
+    {
+        ArgumentNullException.ThrowIfNull(items);
+        _items = items;
+        _selectedIndex = items.Count > 0 ? 0 : -1;
+        _top = 0;
+        _checkedIndices?.Clear();
+    }
+
+    /// <summary>
+    /// Moves the selection one item up. No-op when at index 0 or when the list is empty
+    /// (no wrap-around).
+    /// </summary>
+    internal void MoveUp()
+    {
+        if (_selectedIndex <= 0)
+            return;
+
+        _selectedIndex--;
+        ScrollToKeepSelectionVisible();
+    }
+
+    /// <summary>
+    /// Moves the selection one item down. No-op when at the last item or when the list is empty
+    /// (no wrap-around).
+    /// </summary>
+    internal void MoveDown()
+    {
+        if (_selectedIndex < 0 || _selectedIndex >= _items.Count - 1)
+            return;
+
+        _selectedIndex++;
+        ScrollToKeepSelectionVisible();
+    }
+
+    /// <summary>
+    /// Toggles whether the currently selected item is in <see cref="CheckedIndices"/>.
+    /// No-op when <see cref="MultiSelect"/> is <see langword="false"/> or when the list is empty.
+    /// </summary>
+    internal void ToggleCurrent()
+    {
+        if (_checkedIndices is null || _selectedIndex < 0)
+            return;
+
+        if (!_checkedIndices.Remove(_selectedIndex))
+            _checkedIndices.Add(_selectedIndex);
+    }
+
+    // ── Rendering ────────────────────────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Produces at most <see cref="MaxRows"/> visual rows for the current viewport.
+    /// </summary>
+    /// <param name="width">
+    /// Available display columns. Values below 1 are treated as 1.
+    /// </param>
+    /// <returns>
+    /// An ordered list of <see cref="Line"/>s — one per visible item. Empty when the list is empty.
+    /// </returns>
+    internal IReadOnlyList<Line> Render(int width)
+    {
+        if (width < 1)
+            width = 1;
+
+        if (_items.Count == 0)
+            return [];
+
+        // Reconcile the viewport anchor against the current MaxRows/Count/SelectedIndex.
+        // MaxRows may have changed between renders (height-budget squeeze/expand) without
+        // going through MoveUp/MoveDown, leaving _top stale and causing overruns or
+        // dropped-selection bugs. ScrollToKeepSelectionVisible is the single source of
+        // truth for the _top clamp.
+        ScrollToKeepSelectionVisible();
+
+        int visibleCount = Math.Min(_maxRows, _items.Count);
+        int end = _top + visibleCount;
+
+        Line[] result = new Line[visibleCount];
+
+        for (int i = _top; i < end; i++)
+        {
+            bool isSelected = i == _selectedIndex;
+            bool isChecked = _checkedIndices?.Contains(i) ?? false;
+
+            Line row = BuildRow(_items[i], isSelected, isChecked, width);
+            result[i - _top] = row;
+        }
+
+        return result;
+    }
+
+    // ── Private helpers ──────────────────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Adjusts <c>_top</c> by the minimum scroll needed to keep <see cref="SelectedIndex"/>
+    /// within the viewport <c>[_top, _top + MaxRows)</c>.
+    /// </summary>
+    private void ScrollToKeepSelectionVisible()
+    {
+        if (_selectedIndex < _top)
+        {
+            _top = _selectedIndex;
+        }
+        else if (_selectedIndex >= _top + _maxRows)
+        {
+            _top = _selectedIndex - _maxRows + 1;
+        }
+
+        // Clamp _top so we never show a partial bottom window when items allow full one.
+        int maxTop = Math.Max(0, _items.Count - _maxRows);
+        _top = Math.Min(_top, maxTop);
+    }
+
+    /// <summary>
+    /// Builds a single rendered row for an item.
+    /// </summary>
+    /// <remarks>
+    /// When <see cref="MultiSelect"/>, prefixes the row with a <c>"[x] "</c> / <c>"[ ] "</c>
+    /// marker (4 equal-width columns) before truncating to <paramref name="width"/>. If the item
+    /// is selected, every segment has <see cref="Format.Reverse"/> ORed into its style, and
+    /// a trailing reverse-styled space segment pads out to <paramref name="width"/> columns.
+    /// </remarks>
+    private Line BuildRow(Line item, bool isSelected, bool isChecked, int width)
+    {
+        // Prefix the item with the multi-select marker if needed.
+        Line source;
+        if (MultiSelect)
+        {
+            string marker = isChecked ? _checkedMarker : _uncheckedMarker;
+            // Prepend marker as first segment, followed by all item segments.
+            List<Segment> markerPlusItem = new(item.Segments.Count + 1)
+            {
+                new Segment(marker)
+            };
+            markerPlusItem.AddRange(item.Segments);
+            source = new Line(markerPlusItem);
+        }
+        else
+        {
+            source = item;
+        }
+
+        // Truncate to the first width-aware row.
+        Line truncated = LineWrapper.Wrap(source, width)[0];
+
+        if (!isSelected)
+            return truncated;
+
+        // Apply Format.Reverse to every segment.
+        Segment[] reversed = new Segment[truncated.Segments.Count];
+        int col = 0;
+        for (int i = 0; i < truncated.Segments.Count; i++)
+        {
+            Segment seg = truncated.Segments[i];
+            Style reversedStyle = seg.Style with { Format = seg.Style.Format | Format.Reverse };
+            reversed[i] = seg with { Style = reversedStyle };
+            col += DisplayWidth.Measure(seg.Text);
+        }
+
+        // Pad selected row to full width with a reverse-styled space so the highlight bar fills
+        // the entire width. The §8 painter full-clears the region, so non-selected rows need
+        // no padding.
+        if (col < width)
+        {
+            Segment[] withPad = new Segment[reversed.Length + 1];
+            reversed.CopyTo(withPad, 0);
+            withPad[reversed.Length] = new Segment(
+                new string(' ', width - col),
+                new Style(Format: Format.Reverse));
+            return new Line(withPad);
+        }
+
+        return new Line(reversed);
+    }
+}
diff --git a/src/Dcli/Internal/FixedRegion/StatusLine.cs b/src/Dcli/Internal/FixedRegion/StatusLine.cs
new file mode 100644
index 0000000..2a6a446
--- /dev/null
+++ b/src/Dcli/Internal/FixedRegion/StatusLine.cs
@@ -0,0 +1,24 @@
+namespace Dcli.Internal.FixedRegion;
+
+/// <summary>
+/// Owns the status row(s) that are always shown at the bottom of the fixed region.
+/// </summary>
+/// <remarks>
+/// <para>
+/// The status line is <em>sacred</em> (Decision 8): it is always fully rendered regardless of the
+/// height budget. When the budget is too small to show both the input editor and the status, the
+/// input shrinks (or disappears) while the status remains visible.
+/// </para>
+/// <para>
+/// A consumer sets <see cref="Rows"/> to replace the displayed content. Setting to an empty list
+/// means no status row is rendered. All rows are painted verbatim — no wrapping is applied.
+/// </para>
+/// </remarks>
+internal sealed class StatusLine
+{
+    /// <summary>
+    /// The status rows to display. May be empty (no status rendered).
+    /// Set by the consumer; read by <see cref="FixedRegionComposer"/> during composition.
+    /// </summary>
+    internal IReadOnlyList<Line> Rows { get; set; } = [];
+}
diff --git a/src/Dcli/Internal/FixedRegion/TextBuffer.cs b/src/Dcli/Internal/FixedRegion/TextBuffer.cs
new file mode 100644
index 0000000..78681df
--- /dev/null
+++ b/src/Dcli/Internal/FixedRegion/TextBuffer.cs
@@ -0,0 +1,729 @@
+using System.Globalization;
+using System.Text;
+
+namespace Dcli.Internal.FixedRegion;
+
+/// <summary>
+/// Owns the editable input text: caret position, multiline support, display-width-aware wrapping,
+/// internal scroll, and history recall. All editing operations are synchronous; thread safety is
+/// the caller's responsibility (the render-loop thread owns this in normal use).
+/// </summary>
+/// <remarks>
+/// <para>
+/// <strong>Caret representation:</strong> the caret is stored as a UTF-16 char index into
+/// <see cref="Text"/>. All public navigation/deletion methods operate in terms of
+/// <em>grapheme clusters</em> (UAX #29 extended grapheme clusters) — one arrow key = one visual
+/// character regardless of how many chars or Runes it occupies.
+/// </para>
+/// <para>
+/// <strong>Grapheme clustering:</strong> uses <see cref="StringInfo.GetTextElementEnumerator(string)"/>
+/// to enumerate extended grapheme clusters. Width of a cluster = sum of its scalars' display
+/// widths via <see cref="DisplayWidth"/>.
+/// </para>
+/// <para>
+/// <strong>Wrapping:</strong> each logical line (split on <c>\n</c>) is wrapped to visual rows
+/// using <see cref="LineWrapper.Wrap"/>. The caret maps to a <c>(visualRow, visualCol)</c> by
+/// counting cluster widths up to the caret index within its logical line, then offsetting by
+/// the wrapped-row index of the preceding logical lines.
+/// </para>
+/// <para>
+/// <strong>Internal scroll:</strong> <see cref="Render"/> accepts an <c>allottedHeight</c>. When
+/// the total visual rows exceed it, the visible window is scrolled by the minimum amount needed
+/// to keep the caret row in view (caret row always visible; scroll anchors to caret; top of
+/// window moves as little as possible).
+/// </para>
+/// <para>
+/// <strong>History recall:</strong> readline-style semantics. When the user navigates back into
+/// history (<see cref="RecallPrevious"/>), the current in-progress buffer is stashed. Navigating
+/// forward past the newest entry via <see cref="RecallNext"/> restores the stash. Calling
+/// <see cref="AddToHistory"/> clears the navigation position and the stash.
+/// </para>
+/// </remarks>
+internal sealed class TextBuffer
+{
+    // ── Text storage ────────────────────────────────────────────────────────────────────────
+
+    private string _text = string.Empty;
+
+    /// <summary>The raw text content including embedded newlines.</summary>
+    internal string Text => _text;
+
+    // ── Caret ──────────────────────────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// UTF-16 char index of the caret within <see cref="Text"/>. Always in [0, Text.Length].
+    /// </summary>
+    internal int CaretIndex { get; private set; }
+
+    /// <summary>
+    /// Sets the caret index directly. For test use only — no validation beyond clamping.
+    /// </summary>
+    internal void SetCaretIndexForTest(int index)
+    {
+        CaretIndex = Math.Clamp(index, 0, _text.Length);
+    }
+
+    // ── History ─────────────────────────────────────────────────────────────────────────────
+
+    private readonly List<string> _history = [];
+
+    /// <summary>
+    /// Index into _history of the currently recalled entry, or -1 when not in history mode.
+    /// -1 = editing the live buffer.
+    /// History navigation: 0 = oldest, _history.Count-1 = newest.
+    /// </summary>
+    private int _historyIndex = -1;
+
+    /// <summary>
+    /// The in-progress buffer stashed when the user first navigates back into history.
+    /// Restored when navigating forward past the newest entry.
+    /// </summary>
+    private string? _historyStash;
+
+    // ── Helpers: grapheme cluster iteration ─────────────────────────────────────────────────
+
+    /// <summary>
+    /// Enumerates grapheme clusters in <paramref name="text"/> as (startCharIndex, clusterString)
+    /// pairs. Uses <see cref="StringInfo.GetTextElementEnumerator(string)"/> for UAX #29 clusters.
+    /// </summary>
+    private static IEnumerable<(int Start, string Cluster)> EnumerateClusters(string text)
+    {
+        TextElementEnumerator enumerator = StringInfo.GetTextElementEnumerator(text);
+        while (enumerator.MoveNext())
+        {
+            yield return (enumerator.ElementIndex, enumerator.GetTextElement());
+        }
+    }
+
+    /// <summary>
+    /// Returns the display width of a grapheme cluster string: sum of each scalar's
+    /// <see cref="DisplayWidth.Measure(Rune)"/> value. Tabs are measured column-relative.
+    /// </summary>
+    private static int ClusterWidth(string cluster, int column)
+    {
+        int w = 0;
+        foreach (Rune rune in cluster.EnumerateRunes())
+        {
+            int runeW = DisplayWidth.MeasureWithColumn(rune, column + w);
+            w += runeW;
+        }
+        return w;
+    }
+
+    // ── Edit operations ─────────────────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Inserts a single Unicode scalar at the caret, advancing the caret past it.
+    /// </summary>
+    internal void Insert(Rune rune)
+    {
+        string s = rune.ToString();
+        _text = _text[..CaretIndex] + s + _text[CaretIndex..];
+        CaretIndex += s.Length;
+    }
+
+    /// <summary>
+    /// Inserts a string (e.g., a paste payload) at the caret, advancing the caret past it.
+    /// </summary>
+    internal void Insert(string text)
+    {
+        ArgumentNullException.ThrowIfNull(text);
+        _text = _text[..CaretIndex] + text + _text[CaretIndex..];
+        CaretIndex += text.Length;
+    }
+
+    /// <summary>
+    /// Inserts a newline at the caret (multiline support).
+    /// </summary>
+    internal void InsertNewline()
+    {
+        _text = _text[..CaretIndex] + "\n" + _text[CaretIndex..];
+        CaretIndex += 1;
+    }
+
+    /// <summary>
+    /// Deletes the grapheme cluster immediately before the caret (backspace semantics).
+    /// No-op when the caret is at position 0.
+    /// </summary>
+    internal void Backspace()
+    {
+        if (CaretIndex == 0)
+            return;
+
+        (int clusterStart, int clusterLength) = FindClusterBefore(CaretIndex);
+        _text = _text[..clusterStart] + _text[(clusterStart + clusterLength)..];
+        CaretIndex = clusterStart;
+    }
+
+    /// <summary>
+    /// Deletes the grapheme cluster immediately after the caret (delete/forward-delete semantics).
+    /// No-op when the caret is at the end of the text.
+    /// </summary>
+    internal void Delete()
+    {
+        if (CaretIndex >= _text.Length)
+            return;
+
+        string cluster = FindClusterAt(CaretIndex);
+        _text = _text[..CaretIndex] + _text[(CaretIndex + cluster.Length)..];
+        // Caret stays at same position.
+    }
+
+    /// <summary>
+    /// Replaces the entire buffer with <paramref name="newText"/> and moves the caret to the end.
+    /// Used by history recall and test helpers.
+    /// </summary>
+    internal void SetText(string newText)
+    {
+        ArgumentNullException.ThrowIfNull(newText);
+        _text = newText;
+        CaretIndex = _text.Length;
+    }
+
+    /// <summary>Clears the buffer and resets the caret to 0.</summary>
+    internal void Clear()
+    {
+        _text = string.Empty;
+        CaretIndex = 0;
+    }
+
+    // ── Caret navigation ────────────────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Moves the caret one grapheme cluster to the left.
+    /// When the caret is at the start of a logical line (after a newline), moves to the end of
+    /// the previous logical line (i.e., crosses logical-line boundaries).
+    /// No-op at position 0.
+    /// </summary>
+    internal void MoveLeft()
+    {
+        if (CaretIndex == 0)
+            return;
+
+        (int clusterStart, _) = FindClusterBefore(CaretIndex);
+        CaretIndex = clusterStart;
+    }
+
+    /// <summary>
+    /// Moves the caret one grapheme cluster to the right.
+    /// Crosses logical-line boundaries.
+    /// No-op at the end of the text.
+    /// </summary>
+    internal void MoveRight()
+    {
+        if (CaretIndex >= _text.Length)
+            return;
+
+        string cluster = FindClusterAt(CaretIndex);
+        CaretIndex += cluster.Length;
+    }
+
+    /// <summary>
+    /// Moves the caret to the start of the current visual row.
+    /// <para>
+    /// Convention: Home navigates to the start of the <em>visual row</em> the caret occupies,
+    /// not the logical line. This matches readline/terminal editor behaviour.
+    /// </para>
+    /// </summary>
+    /// <param name="width">Terminal width used for wrapping.</param>
+    internal void MoveHome(int width)
+    {
+        VisualPosition vp = ComputeVisualPosition(width);
+        // Find the char index at the start of the current visual row.
+        CaretIndex = vp.CurrentVisualRowStartCharIndex;
+    }
+
+    /// <summary>
+    /// Moves the caret to the end of the current visual row.
+    /// <para>
+    /// Convention: End navigates to the end of the <em>visual row</em> the caret occupies.
+    /// </para>
+    /// </summary>
+    /// <param name="width">Terminal width used for wrapping.</param>
+    internal void MoveEnd(int width)
+    {
+        VisualPosition vp = ComputeVisualPosition(width);
+        CaretIndex = vp.CurrentVisualRowEndCharIndex;
+    }
+
+    /// <summary>
+    /// Moves the caret up one visual row, maintaining the current visual column where possible.
+    /// Moves to position 0 when already on the first visual row.
+    /// </summary>
+    /// <param name="width">Terminal width used for wrapping.</param>
+    internal void MoveUp(int width)
+    {
+        VisualPosition vp = ComputeVisualPosition(width);
+        if (vp.VisualRow == 0)
+        {
+            CaretIndex = 0;
+            return;
+        }
+
+        // Target: row above, same visual column.
+        int targetRow = vp.VisualRow - 1;
+        int targetCol = vp.VisualCol;
+        CaretIndex = FindCaretIndexForVisualPosition(targetRow, targetCol, width);
+    }
+
+    /// <summary>
+    /// Moves the caret down one visual row, maintaining the current visual column where possible.
+    /// Moves to the end of text when already on the last visual row.
+    /// </summary>
+    /// <param name="width">Terminal width used for wrapping.</param>
+    internal void MoveDown(int width)
+    {
+        List<VisualRowInfo> rows = BuildVisualRows(width);
+        VisualPosition vp = ComputeVisualPositionFromRows(rows);
+
+        if (vp.VisualRow >= rows.Count - 1)
+        {
+            CaretIndex = _text.Length;
+            return;
+        }
+
+        int targetRow = vp.VisualRow + 1;
+        int targetCol = vp.VisualCol;
+        CaretIndex = FindCaretIndexForVisualPosition(targetRow, targetCol, width);
+    }
+
+    // ── History ─────────────────────────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Adds <paramref name="entry"/> to history. Resets history navigation position and clears
+    /// any stash. Duplicate consecutive entries are not added.
+    /// </summary>
+    internal void AddToHistory(string entry)
+    {
+        ArgumentNullException.ThrowIfNull(entry);
+        if (_history.Count == 0 || _history[^1] != entry)
+            _history.Add(entry);
+        _historyIndex = -1;
+        _historyStash = null;
+    }
+
+    /// <summary>
+    /// Recalls the previous (older) history entry, replacing the buffer.
+    /// On first call, stashes the current in-progress buffer for later restore.
+    /// No-op when already at the oldest entry.
+    /// </summary>
+    internal void RecallPrevious()
+    {
+        if (_history.Count == 0)
+            return;
+
+        if (_historyIndex == -1)
+        {
+            // First navigation into history: stash the live buffer.
+            _historyStash = _text;
+            _historyIndex = _history.Count - 1;
+        }
+        else if (_historyIndex > 0)
+        {
+            _historyIndex--;
+        }
+        else
+        {
+            // Already at oldest; no-op.
+            return;
+        }
+
+        SetText(_history[_historyIndex]);
+    }
+
+    /// <summary>
+    /// Recalls the next (newer) history entry, replacing the buffer.
+    /// When navigating past the newest entry, restores the stashed in-progress buffer.
+    /// No-op when not in history navigation mode.
+    /// </summary>
+    internal void RecallNext()
+    {
+        if (_historyIndex == -1)
+            return;
+
+        if (_historyIndex < _history.Count - 1)
+        {
+            _historyIndex++;
+            SetText(_history[_historyIndex]);
+        }
+        else
+        {
+            // Past the newest: restore stash.
+            _historyIndex = -1;
+            string stash = _historyStash ?? string.Empty;
+            _historyStash = null;
+            SetText(stash);
+        }
+    }
+
+    // ── Render ──────────────────────────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Renders the editor content at the given <paramref name="width"/>, applying an optional
+    /// height constraint. Returns the visible visual rows and the caret's position within them.
+    /// </summary>
+    /// <param name="width">Terminal width in columns (must be >= 1).</param>
+    /// <param name="allottedHeight">
+    /// Maximum number of rows the editor may occupy. When <see langword="null"/> or larger than
+    /// the total visual rows, all rows are returned (no scrolling). When smaller, the visible
+    /// window is scrolled by the minimum amount needed to keep the caret row in view.
+    /// </param>
+    /// <returns>
+    /// The visible rows as <see cref="Line"/> values (ready to paint) and the caret's
+    /// <c>(visualRow, visualCol)</c> relative to the <em>visible</em> window (0-based).
+    /// </returns>
+    internal RenderResult Render(int width, int? allottedHeight = null)
+    {
+        if (width < 1)
+            width = 1;
+
+        List<VisualRowInfo> allRows = BuildVisualRows(width);
+        VisualPosition vp = ComputeVisualPositionFromRows(allRows);
+
+        int totalRows = allRows.Count;
+        int visibleStart; // first visible row index (0-based into allRows)
+        int visibleEnd;   // exclusive
+
+        if (allottedHeight is null || allottedHeight >= totalRows)
+        {
+            visibleStart = 0;
+            visibleEnd = totalRows;
+        }
+        else
+        {
+            int h = Math.Max(1, allottedHeight.Value);
+            // Scroll minimally to keep caret row in view.
+            // Preferred: keep caret as close to center as practical, but minimum movement.
+            // The window is [visibleStart, visibleStart+h). Caret row must be within it.
+            // Minimum movement: if caret is already visible in whatever window fits, preserve
+            // the current top; otherwise snap to bring caret into view.
+            // Since we rebuild from scratch each render, we use: top = clamp(caret - h + 1, 0, max).
+            // This keeps caret at the bottom of the window when scrolling down, and at the top when
+            // scrolling up — i.e., the caret is always visible and the window moves as little as
+            // possible. This is the standard "scroll just enough" anchoring.
+            visibleStart = Math.Clamp(vp.VisualRow - h + 1, 0, totalRows - h);
+            visibleEnd = visibleStart + h;
+        }
+
+        // Build the visible row lines.
+        Line[] visibleLines = new Line[visibleEnd - visibleStart];
+        for (int i = visibleStart; i < visibleEnd; i++)
+        {
+            visibleLines[i - visibleStart] = allRows[i].Line;
+        }
+
+        int caretVisualRow = vp.VisualRow - visibleStart;
+        int caretVisualCol = vp.VisualCol;
+
+        return new RenderResult(visibleLines, (caretVisualRow, caretVisualCol));
+    }
+
+    // ── Visual row construction ──────────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// A single visual row: the <see cref="Line"/> to paint and the range of the source text
+    /// this row covers (as char indices into <see cref="Text"/>).
+    /// </summary>
+    private sealed class VisualRowInfo
+    {
+        internal Line Line { get; }
+
+        /// <summary>Char index of the first character of this visual row in <see cref="Text"/>.</summary>
+        internal int StartCharIndex { get; }
+
+        /// <summary>Char index one past the last character of this visual row in <see cref="Text"/>.</summary>
+        internal int EndCharIndex { get; }
+
+        internal VisualRowInfo(Line line, int startCharIndex, int endCharIndex)
+        {
+            Line = line;
+            StartCharIndex = startCharIndex;
+            EndCharIndex = endCharIndex;
+        }
+    }
+
+    /// <summary>
+    /// Computes all visual rows for the current text at the given width.
+    /// Each logical line (delimited by <c>\n</c>) is independently wrapped.
+    /// </summary>
+    private List<VisualRowInfo> BuildVisualRows(int width)
+    {
+        if (width < 1)
+            width = 1;
+
+        // Split text into logical lines on \n and wrap each independently.
+        List<VisualRowInfo> result = [];
+
+        // Edge case: empty text → one empty row (caret can sit here).
+        if (_text.Length == 0)
+        {
+            result.Add(new VisualRowInfo(new Line([]), 0, 0));
+            return result;
+        }
+
+        int searchFrom = 0;
+        while (true)
+        {
+            int newlinePos = _text.IndexOf('\n', searchFrom);
+
+            // Extract the logical line (the text before the next newline, or to end of string).
+            string logicalLine = newlinePos == -1
+                ? _text[searchFrom..]
+                : _text[searchFrom..newlinePos];
+
+            // Wrap the logical line into visual rows.
+            Line sourceLine = new([new Segment(logicalLine)]);
+            IReadOnlyList<Line> wrappedRows = LineWrapper.Wrap(sourceLine, width);
+
+            // Assign char ranges: each wrapped row covers a contiguous span of logicalLine.
+            int charInLogical = 0;
+            foreach (Line wRow in wrappedRows)
+            {
+                int rowChars = wRow.Segments.Sum(s => s.Text.Length);
+                int rowStart = searchFrom + charInLogical;
+                result.Add(new VisualRowInfo(wRow, rowStart, rowStart + rowChars));
+                charInLogical += rowChars;
+            }
+
+            if (newlinePos == -1)
+                break;
+
+            // Advance past the newline character. If this puts us at or past the end,
+            // the next iteration processes an empty logical line "" which produces one empty
+            // row — correct behaviour for a trailing newline (caret can sit there).
+            searchFrom = newlinePos + 1;
+        }
+
+        return result;
+    }
+
+    // ── Caret↔Visual position mapping ───────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Computed visual position of the caret: which visual row and column.
+    /// Also carries helper values for Home/End navigation.
+    /// </summary>
+    private readonly struct VisualPosition
+    {
+        /// <summary>Zero-based visual row index (across all logical lines).</summary>
+        internal int VisualRow { get; init; }
+
+        /// <summary>Zero-based visual column (display columns from left edge).</summary>
+        internal int VisualCol { get; init; }
+
+        /// <summary>
+        /// Char index of the first character on the current visual row (for Home).
+        /// </summary>
+        internal int CurrentVisualRowStartCharIndex { get; init; }
+
+        /// <summary>
+        /// Char index one past the last character on the current visual row (for End).
+        /// </summary>
+        internal int CurrentVisualRowEndCharIndex { get; init; }
+    }
+
+    private VisualPosition ComputeVisualPosition(int width)
+    {
+        List<VisualRowInfo> rows = BuildVisualRows(width);
+        return ComputeVisualPositionFromRows(rows);
+    }
+
+    /// <summary>
+    /// Returns whether the caret is on the first visual row and whether it is on the last,
+    /// given the terminal <paramref name="width"/>. Used by key routing to decide when Up/Down
+    /// should trigger history navigation instead of intra-buffer cursor movement.
+    /// </summary>
+    internal (bool IsFirst, bool IsLast) GetCaretVisualRowBounds(int width)
+    {
+        List<VisualRowInfo> rows = BuildVisualRows(width);
+        VisualPosition vp = ComputeVisualPositionFromRows(rows);
+        return (vp.VisualRow == 0, vp.VisualRow >= rows.Count - 1);
+    }
+
+    private VisualPosition ComputeVisualPositionFromRows(List<VisualRowInfo> rows)
+    {
+        // Find which visual row contains the caret.
+        // The caret sits at CaretIndex. A visual row owns char range [StartCharIndex, EndCharIndex).
+        // Special cases:
+        //   - Caret at end of text: belongs to the last row.
+        //   - Caret exactly at a non-newline wrap boundary (end of one visual row / start of next):
+        //     belongs to the NEXT row (the caret is at the start of the new visual row, col 0).
+        //     Exception: if the wrap boundary is a newline char, the caret belongs to THIS row
+        //     (the caret is positioned before the newline, which is not displayed).
+
+        int caretIdx = CaretIndex;
+
+        for (int i = 0; i < rows.Count; i++)
+        {
+            VisualRowInfo row = rows[i];
+            bool isLastRow = i == rows.Count - 1;
+
+            // The caret belongs to this row if:
+            //   caretIdx is within [StartCharIndex, EndCharIndex), OR
+            //   this is the last row and caretIdx == EndCharIndex (end of text).
+            bool inRange = caretIdx >= row.StartCharIndex &&
+                           (caretIdx < row.EndCharIndex || (isLastRow && caretIdx == row.EndCharIndex));
+
+            // Also: if caretIdx == row.EndCharIndex and this row ends at a newline position,
+            // the caret belongs to this row (before the newline, which is not displayed).
+            // The next row starts at EndCharIndex + 1.
+            if (!inRange && !isLastRow && caretIdx == row.EndCharIndex)
+            {
+                // EndCharIndex is either the newline position within the logical line or the
+                // wrap boundary. If the char at EndCharIndex in _text is '\n', the caret is
+                // at the end of this logical line (before the newline).
+                if (row.EndCharIndex < _text.Length && _text[row.EndCharIndex] == '\n')
+                    inRange = true;
+            }
+
+            if (inRange)
+            {
+                // Compute visual column: measure display width of text from row.StartCharIndex
+                // to caretIdx, using grapheme clusters and DisplayWidth.
+                string beforeCaret = _text[row.StartCharIndex..caretIdx];
+                int visualCol = MeasureDisplayWidth(beforeCaret);
+
+                return new VisualPosition
+                {
+                    VisualRow = i,
+                    VisualCol = visualCol,
+                    CurrentVisualRowStartCharIndex = row.StartCharIndex,
+                    CurrentVisualRowEndCharIndex = row.EndCharIndex,
+                };
+            }
+        }
+
+        // Fallback: caret is at end of last row.
+        if (rows.Count > 0)
+        {
+            VisualRowInfo last = rows[^1];
+            string beforeCaret = _text[last.StartCharIndex..Math.Min(caretIdx, _text.Length)];
+            int visualCol = MeasureDisplayWidth(beforeCaret);
+            return new VisualPosition
+            {
+                VisualRow = rows.Count - 1,
+                VisualCol = visualCol,
+                CurrentVisualRowStartCharIndex = last.StartCharIndex,
+                CurrentVisualRowEndCharIndex = last.EndCharIndex,
+            };
+        }
+
+        return new VisualPosition { VisualRow = 0, VisualCol = 0 };
+    }
+
+    /// <summary>
+    /// Finds the caret char index that corresponds to the given <paramref name="targetRow"/> and
+    /// <paramref name="targetCol"/> (display column), at the given <paramref name="width"/>.
+    /// If the target row is shorter than <paramref name="targetCol"/>, snaps to end of that row.
+    /// </summary>
+    private int FindCaretIndexForVisualPosition(int targetRow, int targetCol, int width)
+    {
+        List<VisualRowInfo> rows = BuildVisualRows(width);
+        if (targetRow >= rows.Count)
+            return _text.Length;
+        if (targetRow < 0)
+            return 0;
+
+        VisualRowInfo row = rows[targetRow];
+        string rowText = _text[row.StartCharIndex..row.EndCharIndex];
+
+        // Walk grapheme clusters until we reach or pass targetCol.
+        int col = 0;
+        TextElementEnumerator enumerator = StringInfo.GetTextElementEnumerator(rowText);
+        int charOffset = row.StartCharIndex;
+
+        while (enumerator.MoveNext())
+        {
+            string cluster = enumerator.GetTextElement();
+            int clusterW = ClusterWidth(cluster, col);
+            if (col + clusterW > targetCol)
+                break; // caret stays before this cluster
+            col += clusterW;
+            charOffset += cluster.Length;
+        }
+
+        return charOffset;
+    }
+
+    // ── Helpers: cluster finding ─────────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Finds the grapheme cluster immediately before <paramref name="charIndex"/> in
+    /// <see cref="Text"/>. Returns (clusterStartCharIndex, clusterLength).
+    /// </summary>
+    private (int Start, int Length) FindClusterBefore(int charIndex)
+    {
+        if (charIndex <= 0)
+            return (0, 0);
+
+        // We need to enumerate clusters up to charIndex and return the last one.
+        // StringInfo enumerates forward; we walk the prefix and take the last cluster's info.
+        string prefix = _text[..charIndex];
+        int lastStart = 0;
+        string lastCluster = string.Empty;
+
+        TextElementEnumerator enumerator = StringInfo.GetTextElementEnumerator(prefix);
+        while (enumerator.MoveNext())
+        {
+            lastStart = enumerator.ElementIndex;
+            lastCluster = enumerator.GetTextElement();
+        }
+
+        return (lastStart, lastCluster.Length);
+    }
+
+    /// <summary>
+    /// Returns the grapheme cluster string starting at <paramref name="charIndex"/> in
+    /// <see cref="Text"/>.
+    /// </summary>
+    private string FindClusterAt(int charIndex)
+    {
+        if (charIndex >= _text.Length)
+            return string.Empty;
+
+        // Enumerate clusters in the suffix starting at charIndex; return the first one.
+        string suffix = _text[charIndex..];
+        TextElementEnumerator enumerator = StringInfo.GetTextElementEnumerator(suffix);
+        if (enumerator.MoveNext())
+            return enumerator.GetTextElement();
+
+        return string.Empty;
+    }
+
+    /// <summary>
+    /// Measures the display width of a string by summing per-rune widths (column-aware for tabs).
+    /// </summary>
+    private static int MeasureDisplayWidth(string text)
+    {
+        int col = 0;
+        foreach (Rune rune in text.EnumerateRunes())
+        {
+            col += DisplayWidth.MeasureWithColumn(rune, col);
+        }
+        return col;
+    }
+}
+
+// ── Public result types ──────────────────────────────────────────────────────────────────────
+
+/// <summary>
+/// The result of <see cref="TextBuffer.Render"/>: the visible rows to paint and the caret's
+/// visual position within them.
+/// </summary>
+internal sealed class RenderResult
+{
+    /// <summary>The visible visual rows (ready to paint), possibly a subset of all rows.</summary>
+    internal IReadOnlyList<Line> VisibleRows { get; }
+
+    /// <summary>
+    /// Caret position within the visible window: (row, col), both zero-based.
+    /// Row is relative to the first visible row; col is in display columns.
+    /// </summary>
+    internal (int Row, int Col) CaretPosition { get; }
+
+    internal RenderResult(IReadOnlyList<Line> visibleRows, (int Row, int Col) caretPosition)
+    {
+        VisibleRows = visibleRows;
+        CaretPosition = caretPosition;
+    }
+}
diff --git a/src/Dcli/Internal/IRawModeSession.cs b/src/Dcli/Internal/IRawModeSession.cs
new file mode 100644
index 0000000..8243949
--- /dev/null
+++ b/src/Dcli/Internal/IRawModeSession.cs
@@ -0,0 +1,30 @@
+namespace Dcli.Internal;
+
+/// <summary>
+/// Represents an OS raw-mode session: the tty is in raw mode while this object is alive, and
+/// <see cref="Restore"/> returns it to its previous state.
+/// <para>
+/// Implementations must be idempotent and thread-safe: calling <see cref="Restore"/> (or
+/// <see cref="IDisposable.Dispose"/>) more than once — including concurrently from a signal
+/// handler, <c>ProcessExit</c>, and <see cref="IDisposable.Dispose"/> — must be safe and must
+/// not double-restore the terminal.
+/// </para>
+/// <para>
+/// This interface is the seam used by the headless test harness (§15) to substitute a no-op
+/// session without touching the render loop or input pipeline.
+/// </para>
+/// </summary>
+public interface IRawModeSession : IDisposable
+{
+    /// <summary>
+    /// Restores the terminal to the state captured before raw mode was entered.
+    /// Idempotent: safe to call from multiple threads or multiple times.
+    /// </summary>
+    void Restore();
+
+    /// <summary>
+    /// Re-applies raw mode. Called after SIGCONT (process resumed from suspension)
+    /// which may have left the tty in cooked mode.
+    /// </summary>
+    void Reapply();
+}
diff --git a/src/Dcli/Internal/Input/IInputByteSource.cs b/src/Dcli/Internal/Input/IInputByteSource.cs
new file mode 100644
index 0000000..3d9c669
--- /dev/null
+++ b/src/Dcli/Internal/Input/IInputByteSource.cs
@@ -0,0 +1,27 @@
+namespace Dcli.Internal.Input;
+
+/// <summary>
+/// Abstracts the OS-level byte source for the §6 input reader thread.
+/// <para>
+/// The contract mirrors <c>VMIN=0/VTIME=1</c> POSIX <c>read(2)</c>: a call may return
+/// anywhere from 0 to <c>buffer.Length</c> bytes. A return value of <strong>0</strong>
+/// means "timed out with no data available" — it is <em>not</em> end-of-stream.
+/// A negative return value signals an unrecoverable read error.
+/// </para>
+/// <para>
+/// Keeping this interface internal lets §15 promote it (or an equivalent) to a public
+/// <c>Dcli.Testing</c> surface with full XML docs without breaking §6.
+/// </para>
+/// </summary>
+internal interface IInputByteSource
+{
+    /// <summary>
+    /// Reads up to <paramref name="buffer"/>.Length bytes from the input source.
+    /// </summary>
+    /// <param name="buffer">Buffer to fill.</param>
+    /// <returns>
+    /// Number of bytes read (≥ 0). Zero means the timed read expired with no data; the
+    /// caller should keep the read loop running. A negative value is a fatal read error.
+    /// </returns>
+    int Read(Span<byte> buffer);
+}
diff --git a/src/Dcli/Internal/Input/InputReader.cs b/src/Dcli/Internal/Input/InputReader.cs
new file mode 100644
index 0000000..2dbf14e
--- /dev/null
+++ b/src/Dcli/Internal/Input/InputReader.cs
@@ -0,0 +1,143 @@
+using System.Threading.Channels;
+
+namespace Dcli.Internal.Input;
+
+/// <summary>
+/// Runs a dedicated long-running thread that reads bytes from an <see cref="IInputByteSource"/>,
+/// feeds them to a <see cref="VtInputParser"/>, and posts decoded <see cref="InputEvent"/>s to
+/// an injected sink.
+/// </summary>
+/// <remarks>
+/// <para>
+/// The reader is the <em>single producer</em> of input events for the render loop. It owns no
+/// UI state and never writes to stdout.
+/// </para>
+/// <para>
+/// The underlying byte source is assumed to be configured with <c>VMIN=0/VTIME=1</c>
+/// semantics (or equivalent): each <see cref="IInputByteSource.Read"/> call returns in at most
+/// ~100 ms. A 0-byte return means "timed out, no data" — the loop checks the shutdown flag and
+/// continues. This 100 ms cadence is the ESC-disambiguation window: when a bare ESC is the only
+/// pending byte at timeout, <see cref="VtInputParser.Flush"/> resolves it to
+/// <see cref="NamedKey.Escape"/>. The ~10 idle wake-ups/sec are accepted for v1.
+/// </para>
+/// <para>
+/// The sink is a <see cref="ChannelWriter{T}"/> so §7 (the render loop) can plug in its
+/// inbound channel by wrapping the posted events as needed. Tests inject a plain
+/// <c>Channel&lt;InputEvent&gt;</c>.
+/// </para>
+/// <para>
+/// Ctrl+C arrives as byte 0x03 (ISIG is cleared in raw mode) and is decoded to
+/// <see cref="KeyEvent"/>(<see cref="KeyCode.FromRune"/>('c'), <see cref="Modifiers.Ctrl"/>)
+/// like any other key. The library never terminates or interrupts the process on Ctrl+C.
+/// </para>
+/// </remarks>
+internal sealed class InputReader : IDisposable
+{
+    private readonly IInputByteSource _source;
+    private readonly ChannelWriter<InputEvent> _sink;
+    private readonly VtInputParser _parser = new();
+    private readonly Thread _thread;
+
+    // Set by Stop()/Dispose() to signal the read loop to exit.
+    private volatile bool _stopping;
+
+    // Read buffer: 4 KB is more than enough for any single read burst.
+    private const int _bufferSize = 4096;
+
+    /// <summary>
+    /// Initialises the reader and starts the dedicated input thread.
+    /// The thread is a background thread so it does not prevent process exit.
+    /// </summary>
+    /// <param name="source">The OS-level byte source (real tty fd or test fake).</param>
+    /// <param name="sink">
+    /// Channel writer to post decoded <see cref="InputEvent"/>s to.
+    /// §7 wires this to its inbound message channel.
+    /// </param>
+    internal InputReader(IInputByteSource source, ChannelWriter<InputEvent> sink)
+    {
+        _source = source;
+        _sink = sink;
+
+        _thread = new Thread(RunLoop)
+        {
+            Name = "dcli-input-reader",
+            IsBackground = true,
+        };
+        _thread.Start();
+    }
+
+    /// <summary>
+    /// Signals the read loop to stop and waits for the thread to exit.
+    /// Blocks for at most one read-timeout interval (~100 ms) plus a small margin.
+    /// </summary>
+    internal void Stop()
+    {
+        _stopping = true;
+        // The loop wakes every ~100 ms on a timed-read timeout, so joining with a
+        // generous timeout is safe. 500 ms leaves plenty of headroom without hanging.
+        // A false return from Join means the byte source is not honouring its VTIME=1
+        // timeout contract. In that case the background thread is abandoned: it is marked
+        // IsBackground so it will not prevent process exit, and blocking Dispose
+        // indefinitely would be worse than leaking an unresponsive read.
+        _thread.Join(millisecondsTimeout: 500);
+    }
+
+    /// <inheritdoc/>
+    public void Dispose() => Stop();
+
+    // ─── Read loop ───────────────────────────────────────────────────────────
+
+    private void RunLoop()
+    {
+        // TODO (future): replace the timed-read loop with poll(2)/WaitForMultipleObjects
+        // so the thread sleeps with zero CPU when the terminal is idle instead of waking
+        // ~10 times/sec. The VMIN=0/VTIME=1 contract is the v1 ESC-disambiguation mechanism.
+
+        byte[] buf = new byte[_bufferSize];
+        List<InputEvent> events = new(capacity: 16);
+
+        while (!_stopping)
+        {
+            int n = _source.Read(buf.AsSpan());
+
+            if (n < 0)
+            {
+                // Fatal read error — the tty has gone away (e.g. SSH disconnect).
+                // Stop the loop; §7/§13 handle process lifecycle.
+                break;
+            }
+
+            if (n > 0)
+            {
+                events.Clear();
+                _parser.Feed(buf.AsSpan(0, n), events);
+                PostEvents(events);
+            }
+            else
+            {
+                // 0-byte timed-read timeout. Flush a bare ESC if one is pending.
+                // Do NOT flush for partial CSI/UTF-8 — those are sequences split across
+                // the 100 ms boundary and will be completed by the next read.
+                if (_parser.HasPendingEscape)
+                {
+                    events.Clear();
+                    _parser.Flush(events);
+                    PostEvents(events);
+                }
+            }
+        }
+    }
+
+    private void PostEvents(List<InputEvent> events)
+    {
+        foreach (InputEvent ev in events)
+        {
+            // Decision 10: the inbound channel is unbounded for v1, so TryWrite always
+            // succeeds and no keystroke or paste is ever silently dropped. If a future
+            // version switches to a bounded channel it must adopt an explicit backpressure
+            // strategy (block or coalesce) rather than calling TryWrite and discarding the
+            // return value.
+            _sink.TryWrite(ev);
+        }
+    }
+}
diff --git a/src/Dcli/Internal/Input/PosixInputByteSource.cs b/src/Dcli/Internal/Input/PosixInputByteSource.cs
new file mode 100644
index 0000000..7fac44d
--- /dev/null
+++ b/src/Dcli/Internal/Input/PosixInputByteSource.cs
@@ -0,0 +1,42 @@
+using System.Runtime.InteropServices;
+using System.Runtime.Versioning;
+using Dcli.Internal.Posix;
+
+namespace Dcli.Internal.Input;
+
+/// <summary>
+/// POSIX implementation of <see cref="IInputByteSource"/>.
+/// Reads from stdin (fd 0) using <c>read(2)</c> directly, bypassing the managed
+/// <see cref="System.Console"/> stream which misinterprets a 0-byte <c>VMIN=0/VTIME&gt;0</c>
+/// timeout return as end-of-stream.
+/// <para>
+/// EINTR is retried transparently. Any other negative return is propagated as-is so the
+/// reader thread can surface it.
+/// </para>
+/// </summary>
+[SupportedOSPlatform("linux")]
+[SupportedOSPlatform("macos")]
+internal sealed class PosixInputByteSource : IInputByteSource
+{
+    // fd 0 = STDIN_FILENO (same constant value on both macOS and Linux).
+    private const int _stdinFd = 0;
+
+    /// <inheritdoc/>
+    public unsafe int Read(Span<byte> buffer)
+    {
+        if (buffer.IsEmpty)
+            return 0;
+
+        fixed (byte* ptr = buffer)
+        {
+            nint result;
+            do
+            {
+                result = PosixReadInterop.read(_stdinFd, ptr, (nuint)buffer.Length);
+            }
+            while (result < 0 && Marshal.GetLastPInvokeError() == PosixReadInterop.EINTR);
+
+            return (int)result;
+        }
+    }
+}
diff --git a/src/Dcli/Internal/Input/VtInputParser.cs b/src/Dcli/Internal/Input/VtInputParser.cs
new file mode 100644
index 0000000..0421d7b
--- /dev/null
+++ b/src/Dcli/Internal/Input/VtInputParser.cs
@@ -0,0 +1,529 @@
+using System.Buffers;
+using System.Text;
+
+namespace Dcli.Internal.Input;
+
+/// <summary>
+/// Incremental, pure-state-machine VT input parser.
+/// <para>
+/// Feed raw bytes from the terminal with <see cref="Feed"/>; decoded <see cref="InputEvent"/>
+/// values are appended to the caller-supplied sink collection. The parser retains
+/// partial state across calls, so a multi-byte UTF-8 scalar or an escape sequence that arrives
+/// split across two reads is decoded correctly.
+/// </para>
+/// <para>
+/// The parser has <em>no I/O, no clock, and no threads</em>. The §6 input reader owns the
+/// timeout and calls <see cref="Flush"/> when a timed read expires while an ESC is pending.
+/// </para>
+/// <para>
+/// Unrecognised CSI / SS3 sequences are consumed and silently dropped; the stream does not
+/// desync. The design is extensible: mouse, Kitty, and focus sequences can be added by
+/// extending <see cref="EmitCsi"/> / <see cref="EmitSs3"/> without restructuring the state
+/// machine.
+/// </para>
+/// </summary>
+internal sealed class VtInputParser
+{
+    // ─── Parser state ────────────────────────────────────────────────────────
+
+    private enum State
+    {
+        Ground,
+        Utf8,      // accumulating a multi-byte UTF-8 scalar
+        Esc,       // received 0x1B, waiting for follow-up
+        CsiParam,  // inside CSI (ESC [), accumulating parameter bytes
+        Ss3,       // inside SS3 (ESC O), waiting for final
+        Paste,     // inside bracketed paste, accumulating content
+    }
+
+    private State _state = State.Ground;
+
+    // UTF-8 accumulator (up to 4 bytes)
+    private readonly byte[] _utf8Buf = new byte[4];
+    private int _utf8Len;        // bytes stored so far
+    private int _utf8Needed;     // total bytes expected for this scalar
+
+    // CSI accumulator: parameter + intermediate bytes (e.g. "1;5" from ESC[1;5A)
+    // We store at most 64 bytes — more than enough for any real sequence.
+    private readonly byte[] _csiBuf = new byte[64];
+    private int _csiLen;
+
+    // Bracketed-paste accumulator.
+    // _pasteBuf holds the literal content bytes accumulated so far.
+    // _pasteEndCandidateLen tracks how many leading bytes of _pasteEndMarker have been
+    // matched; if the stream diverges those bytes are flushed as literal content.
+    private List<byte>? _pasteBuf;
+    private readonly byte[] _pasteEndMarker = [(byte)0x1B, (byte)'[', (byte)'2', (byte)'0', (byte)'1', (byte)'~'];
+    private int _pasteEndCandidateLen;
+
+    // ─── Public API ──────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Feeds a span of raw terminal bytes into the parser.
+    /// Decoded <see cref="InputEvent"/> values are appended to <paramref name="sink"/>.
+    /// </summary>
+    /// <param name="bytes">Raw bytes from the terminal read buffer.</param>
+    /// <param name="sink">Collection that receives decoded events.</param>
+    public void Feed(ReadOnlySpan<byte> bytes, ICollection<InputEvent> sink)
+    {
+        foreach (byte b in bytes)
+            ConsumeByte(b, sink);
+    }
+
+    /// <summary>
+    /// <see langword="true"/> when the parser is holding a bare, unresolved ESC byte
+    /// (state == <c>Esc</c>). The §6 input reader uses this to decide whether to call
+    /// <see cref="Flush"/> on a 0-byte timed-read: flush only when this is
+    /// <see langword="true"/>; do NOT flush on a partial CSI/UTF-8/paste boundary.
+    /// </summary>
+    public bool HasPendingEscape => _state == State.Esc;
+
+    /// <summary>
+    /// Resolves a pending lone ESC into a <see cref="NamedKey.Escape"/> key event.
+    /// Call this when a timed read returns no bytes while the parser is in the
+    /// <c>Esc</c> state (i.e. the §6 reader's ESC-disambiguation timeout has expired).
+    /// Has no effect when no ESC is pending.
+    /// </summary>
+    /// <param name="sink">Collection that receives the flushed event (if any).</param>
+    public void Flush(ICollection<InputEvent> sink)
+    {
+        if (_state == State.Esc)
+            sink.Add(new KeyEvent(KeyCode.Named(NamedKey.Escape), Modifiers.None));
+
+        // Any other pending state (partial UTF-8, partial CSI, paste) is silently discarded —
+        // those are malformed/incomplete sequences that cannot be completed.
+        ResetToGround();
+    }
+
+    // ─── Core state machine ──────────────────────────────────────────────────
+
+    private void ConsumeByte(byte b, ICollection<InputEvent> sink)
+    {
+        switch (_state)
+        {
+            case State.Ground:
+                ConsumeGround(b, sink);
+                break;
+
+            case State.Utf8:
+                ConsumeUtf8Continuation(b, sink);
+                break;
+
+            case State.Esc:
+                ConsumeAfterEsc(b, sink);
+                break;
+
+            case State.CsiParam:
+                ConsumeCsiParam(b, sink);
+                break;
+
+            case State.Ss3:
+                ConsumeSs3(b, sink);
+                break;
+
+            case State.Paste:
+                ConsumePaste(b, sink);
+                break;
+        }
+    }
+
+    private void ResetToGround()
+    {
+        _state = State.Ground;
+        _utf8Len = 0;
+        _utf8Needed = 0;
+        _csiLen = 0;
+        _pasteBuf = null;
+        _pasteEndCandidateLen = 0;
+    }
+
+    // ─── GROUND ──────────────────────────────────────────────────────────────
+
+    private void ConsumeGround(byte b, ICollection<InputEvent> sink)
+    {
+        if (b == 0x1B)
+        {
+            _state = State.Esc;
+            return;
+        }
+
+        if (b == 0x09) { sink.Add(Key(KeyCode.Named(NamedKey.Tab))); return; }
+        if (b == 0x0D) { sink.Add(Key(KeyCode.Named(NamedKey.Enter))); return; }
+        if (b == 0x08 || b == 0x7F) { sink.Add(Key(KeyCode.Named(NamedKey.Backspace))); return; }
+
+        // C0 control bytes 0x01–0x1A (minus 0x08/0x09/0x0D already handled above):
+        // These encode Ctrl+letter. 0x01='A'-0x40 → 'a', …, 0x1A='Z'-0x40 → 'z'.
+        // 0x00 (NUL) and 0x1C–0x1F (FS/GS/RS/US) are also control codes but lack a
+        // clear letter mapping; we emit them as Char(Rune) + Ctrl with the raw value
+        // so consumers can decide what to do.
+        if (b <= 0x1F)
+        {
+            Rune r = b switch
+            {
+                >= 0x01 and <= 0x1A => new Rune('a' + (b - 0x01)),  // Ctrl+A..Ctrl+Z
+                _ => new Rune(b),                                      // NUL, 0x1C-0x1F
+            };
+            sink.Add(new KeyEvent(KeyCode.FromRune(r), Modifiers.Ctrl));
+            return;
+        }
+
+        // DEL (0x7F) is already handled above.
+
+        // Multi-byte UTF-8 start bytes:
+        if ((b & 0xE0) == 0xC0 && b >= 0xC2) { StartUtf8(b, 2); return; }
+        if ((b & 0xF0) == 0xE0) { StartUtf8(b, 3); return; }
+        if ((b & 0xF8) == 0xF0 && b <= 0xF4) { StartUtf8(b, 4); return; }
+
+        // Invalid UTF-8 lead byte (continuation byte in GROUND, overlong, or > 0xF4):
+        // Emit U+FFFD and stay in GROUND.
+        if ((b & 0x80) != 0) { sink.Add(Key(KeyCode.FromRune(Rune.ReplacementChar))); return; }
+
+        // ASCII printable (0x20–0x7E).
+        sink.Add(Key(KeyCode.FromRune(new Rune(b))));
+    }
+
+    // ─── UTF-8 multi-byte accumulation ───────────────────────────────────────
+
+    private void StartUtf8(byte leadByte, int totalBytes)
+    {
+        _utf8Buf[0] = leadByte;
+        _utf8Len = 1;
+        _utf8Needed = totalBytes;
+        _state = State.Utf8;
+    }
+
+    private void ConsumeUtf8Continuation(byte b, ICollection<InputEvent> sink)
+    {
+        if ((b & 0xC0) != 0x80)
+        {
+            // Not a continuation byte — the sequence is malformed.
+            // Emit U+FFFD for the truncated sequence and re-process this byte from GROUND.
+            sink.Add(Key(KeyCode.FromRune(Rune.ReplacementChar)));
+            _state = State.Ground;
+            _utf8Len = 0;
+            _utf8Needed = 0;
+            ConsumeGround(b, sink);
+            return;
+        }
+
+        _utf8Buf[_utf8Len++] = b;
+
+        if (_utf8Len < _utf8Needed)
+            return; // still accumulating
+
+        // Attempt to decode the complete scalar.
+        OperationStatus status = Rune.DecodeFromUtf8(_utf8Buf.AsSpan(0, _utf8Needed), out Rune rune, out _);
+        if (status == OperationStatus.Done)
+            sink.Add(Key(KeyCode.FromRune(rune)));
+        else
+            sink.Add(Key(KeyCode.FromRune(Rune.ReplacementChar)));
+
+        _state = State.Ground;
+        _utf8Len = 0;
+        _utf8Needed = 0;
+    }
+
+    // ─── ESC dispatch ────────────────────────────────────────────────────────
+
+    private void ConsumeAfterEsc(byte b, ICollection<InputEvent> sink)
+    {
+        if (b == (byte)'[')
+        {
+            // CSI: ESC [
+            _csiLen = 0;
+            _state = State.CsiParam;
+            return;
+        }
+
+        if (b == (byte)'O')
+        {
+            // SS3: ESC O (used by xterm/macOS for arrows, Home, End, F1-F4)
+            _state = State.Ss3;
+            return;
+        }
+
+        // Alt+key prefix: ESC followed by a printable/control byte that is not '[' or 'O'.
+        // Decode the byte as if in GROUND but with Alt modifier.
+        _state = State.Ground;
+
+        if (b == 0x1B)
+        {
+            // ESC ESC: emit Escape, then start another pending ESC.
+            sink.Add(new KeyEvent(KeyCode.Named(NamedKey.Escape), Modifiers.None));
+            _state = State.Esc;
+            return;
+        }
+
+        // Produce the Alt-modified event. ConsumeGround always yields exactly one event
+        // for any non-ESC byte, so the list will have exactly one KeyEvent.
+        List<InputEvent> tmp = new(1);
+        ConsumeGround(b, tmp);
+        sink.Add(((KeyEvent)tmp[0]) with { Modifiers = ((KeyEvent)tmp[0]).Modifiers | Modifiers.Alt });
+    }
+
+    // ─── CSI parameter accumulation ──────────────────────────────────────────
+
+    // CSI parameter bytes: 0x30–0x3F (digits and ';', '<', '=', '>', '?')
+    // CSI intermediate bytes: 0x20–0x2F
+    // CSI final bytes: 0x40–0x7E
+    private void ConsumeCsiParam(byte b, ICollection<InputEvent> sink)
+    {
+        if (b is >= 0x30 and <= 0x3F || b is >= 0x20 and <= 0x2F)
+        {
+            // Parameter or intermediate byte — accumulate.
+            if (_csiLen < _csiBuf.Length)
+                _csiBuf[_csiLen++] = b;
+            // If the buffer overflows we keep going (drop excess bytes) — still safe.
+            return;
+        }
+
+        if (b is >= 0x40 and <= 0x7E)
+        {
+            // Final byte — dispatch. EmitCsi may set _state to Paste.
+            string paramStr = Encoding.ASCII.GetString(_csiBuf, 0, _csiLen);
+            _state = State.Ground;
+            _csiLen = 0;
+            EmitCsi(paramStr, b, sink);
+            return;
+        }
+
+        // Anything else (C0, DEL, etc.) in the middle of a CSI sequence is invalid —
+        // discard the sequence and process this byte from GROUND.
+        _state = State.Ground;
+        _csiLen = 0;
+        ConsumeGround(b, sink);
+    }
+
+    // ─── SS3 final ───────────────────────────────────────────────────────────
+
+    private void ConsumeSs3(byte b, ICollection<InputEvent> sink)
+    {
+        _state = State.Ground;
+        EmitSs3(b, sink);
+    }
+
+    // ─── CSI decode ──────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Decodes a complete CSI sequence. <paramref name="paramStr"/> holds the parameter/
+    /// intermediate bytes (everything between ESC[ and the final byte); <paramref name="final"/>
+    /// is the final byte. May transition <see cref="_state"/> to <see cref="State.Paste"/>
+    /// when the bracketed-paste start marker (200~) is recognised.
+    /// </summary>
+    private void EmitCsi(string paramStr, byte final, ICollection<InputEvent> sink)
+    {
+        // ── BackTab: ESC [ Z ──────────────────────────────────────────────────
+        if (final == (byte)'Z' && paramStr.Length == 0)
+        {
+            sink.Add(new KeyEvent(KeyCode.Named(NamedKey.BackTab), Modifiers.Shift));
+            return;
+        }
+
+        // ── Parse modifier from paramStr ────────────────────────────────────
+        // Standard modifier-encoded form: "1;<m><final>" or "<n>;<m>~"
+        // Modifier bitmask (m): 1 + bitmask, where bitmask = Shift(1)|Alt(2)|Ctrl(4).
+        // So m=1→None, m=2→Shift, m=3→Alt, m=4→Shift+Alt, m=5→Ctrl, m=6→Shift+Ctrl,
+        //    m=7→Alt+Ctrl, m=8→Shift+Alt+Ctrl.
+
+        Modifiers mods = Modifiers.None;
+        string coreParam = paramStr;
+
+        int semicolonIdx = paramStr.IndexOf(';', StringComparison.Ordinal);
+        if (semicolonIdx >= 0)
+        {
+            string modPart = paramStr[(semicolonIdx + 1)..];
+            coreParam = paramStr[..semicolonIdx];
+            if (int.TryParse(modPart, out int mRaw) && mRaw >= 1)
+            {
+                int bitmask = mRaw - 1;
+                if ((bitmask & 1) != 0) mods |= Modifiers.Shift;
+                if ((bitmask & 2) != 0) mods |= Modifiers.Alt;
+                if ((bitmask & 4) != 0) mods |= Modifiers.Ctrl;
+            }
+        }
+
+        // Under Ctrl, strip Shift (terminal-truth rule 5.5).
+        if ((mods & Modifiers.Ctrl) != 0)
+            mods &= ~Modifiers.Shift;
+
+        // ── Cursor-movement finals (A–D, H, F) ──────────────────────────────
+        if (final is (byte)'A' or (byte)'B' or (byte)'C' or (byte)'D' or (byte)'H' or (byte)'F')
+        {
+            NamedKey? key = final switch
+            {
+                (byte)'A' => NamedKey.Up,
+                (byte)'B' => NamedKey.Down,
+                (byte)'C' => NamedKey.Right,
+                (byte)'D' => NamedKey.Left,
+                (byte)'H' => NamedKey.Home,
+                (byte)'F' => NamedKey.End,
+                _ => null,
+            };
+            if (key is not null)
+            {
+                sink.Add(new KeyEvent(KeyCode.Named(key.Value), mods));
+                return;
+            }
+        }
+
+        // ── SS3-style F1–F4 via CSI with modifiers (ESC[1;<m>P…S) ───────────
+        if (final is (byte)'P' or (byte)'Q' or (byte)'R' or (byte)'S')
+        {
+            NamedKey? key = final switch
+            {
+                (byte)'P' => NamedKey.F1,
+                (byte)'Q' => NamedKey.F2,
+                (byte)'R' => NamedKey.F3,
+                (byte)'S' => NamedKey.F4,
+                _ => null,
+            };
+            if (key is not null)
+            {
+                sink.Add(new KeyEvent(KeyCode.Named(key.Value), mods));
+                return;
+            }
+        }
+
+        // ── Tilde-terminated sequences (ESC [ <n> ~) ─────────────────────────
+        if (final == (byte)'~')
+        {
+            if (!int.TryParse(coreParam, out int n))
+            {
+                // Non-numeric param — consume and drop, no desync.
+                return;
+            }
+
+            // ── Bracketed paste start (ESC [ 2 0 0 ~) ────────────────────────
+            if (n == 200)
+            {
+                _state = State.Paste;
+                _pasteBuf = [];
+                _pasteEndCandidateLen = 0;
+                return;
+            }
+
+            NamedKey? key = n switch
+            {
+                1 or 7 => NamedKey.Home,
+                2 => NamedKey.Insert,
+                3 => NamedKey.Delete,
+                4 or 8 => NamedKey.End,
+                5 => NamedKey.PageUp,
+                6 => NamedKey.PageDown,
+                11 => NamedKey.F1,
+                12 => NamedKey.F2,
+                13 => NamedKey.F3,
+                14 => NamedKey.F4,
+                15 => NamedKey.F5,
+                17 => NamedKey.F6,
+                18 => NamedKey.F7,
+                19 => NamedKey.F8,
+                20 => NamedKey.F9,
+                21 => NamedKey.F10,
+                23 => NamedKey.F11,
+                24 => NamedKey.F12,
+                _ => null,
+            };
+
+            if (key is not null)
+            {
+                sink.Add(new KeyEvent(KeyCode.Named(key.Value), mods));
+                return;
+            }
+
+            // Unrecognised tilde code — consume and drop (safe, no desync).
+            return;
+        }
+
+        // ── Unrecognised final — consume and drop ────────────────────────────
+        // This is the catch-all that makes the parser safe for mouse / Kitty /
+        // focus events (those use finals like 'M', 'm', 'I', 'O', etc.).
+    }
+
+    // ─── Bracketed paste accumulation ────────────────────────────────────────
+
+    /// <summary>
+    /// Consumes one byte while inside bracketed paste mode.
+    /// <para>
+    /// Paste content is literal — all bytes including control bytes and ESC sequences are
+    /// accumulated verbatim until the end marker <c>ESC [ 2 0 1 ~</c> is recognised.
+    /// The end marker may arrive split across multiple <see cref="Feed"/> calls; partial
+    /// matches are buffered and only committed to the paste body if the byte stream diverges
+    /// from the end marker before it completes.
+    /// </para>
+    /// </summary>
+    private void ConsumePaste(byte b, ICollection<InputEvent> sink)
+    {
+        // _pasteEndMarker = [0x1B, '[', '2', '0', '1', '~']
+        // _pasteEndCandidateLen tracks how many leading bytes of _pasteEndMarker have been matched.
+
+        if (_pasteEndCandidateLen > 0)
+        {
+            // We are inside a partial end-marker match. Check if this byte continues it.
+            if (b == _pasteEndMarker[_pasteEndCandidateLen])
+            {
+                _pasteEndCandidateLen++;
+                if (_pasteEndCandidateLen == _pasteEndMarker.Length)
+                {
+                    // End marker fully matched — emit the paste event.
+                    string text = Encoding.UTF8.GetString(_pasteBuf!.ToArray());
+                    sink.Add(new PasteEvent(text));
+                    ResetToGround();
+                }
+                // Otherwise: still matching, wait for more bytes.
+                return;
+            }
+
+            // Diverged from end marker — the buffered candidate bytes are literal paste content.
+            for (int i = 0; i < _pasteEndCandidateLen; i++)
+                _pasteBuf!.Add(_pasteEndMarker[i]);
+            _pasteEndCandidateLen = 0;
+
+            // Now process the current byte from the top of ConsumePaste (fall through below).
+        }
+
+        // Start of a potential end-marker match?
+        if (b == _pasteEndMarker[0])
+        {
+            _pasteEndCandidateLen = 1;
+            return;
+        }
+
+        // Ordinary paste content byte.
+        _pasteBuf!.Add(b);
+    }
+
+    // ─── SS3 decode ──────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Decodes an SS3 sequence. On macOS/xterm, arrow keys and F1–F4 arrive as
+    /// <c>ESC O A/B/C/D</c> and <c>ESC O P/Q/R/S</c> respectively.
+    /// Home/End can also arrive as <c>ESC O H/F</c>.
+    /// </summary>
+    private static void EmitSs3(byte final, ICollection<InputEvent> sink)
+    {
+        NamedKey? key = final switch
+        {
+            (byte)'A' => NamedKey.Up,
+            (byte)'B' => NamedKey.Down,
+            (byte)'C' => NamedKey.Right,
+            (byte)'D' => NamedKey.Left,
+            (byte)'H' => NamedKey.Home,
+            (byte)'F' => NamedKey.End,
+            (byte)'P' => NamedKey.F1,
+            (byte)'Q' => NamedKey.F2,
+            (byte)'R' => NamedKey.F3,
+            (byte)'S' => NamedKey.F4,
+            _ => null,
+        };
+
+        if (key is not null)
+            sink.Add(new KeyEvent(KeyCode.Named(key.Value), Modifiers.None));
+
+        // Unrecognised SS3 final — consume and drop.
+    }
+
+    // ─── Helpers ─────────────────────────────────────────────────────────────
+
+    private static KeyEvent Key(KeyCode code) => new(code, Modifiers.None);
+}
diff --git a/src/Dcli/Internal/Input/WindowsInputByteSource.cs b/src/Dcli/Internal/Input/WindowsInputByteSource.cs
new file mode 100644
index 0000000..026447b
--- /dev/null
+++ b/src/Dcli/Internal/Input/WindowsInputByteSource.cs
@@ -0,0 +1,22 @@
+using System.Runtime.Versioning;
+
+namespace Dcli.Internal.Input;
+
+/// <summary>
+/// Windows implementation of <see cref="IInputByteSource"/>.
+/// </summary>
+/// <remarks>
+/// §14.1 implements and verifies the Windows runtime path. This compile-only stub
+/// keeps the cross-platform build green on all CI agents.
+/// </remarks>
+[SupportedOSPlatform("windows")]
+internal sealed class WindowsInputByteSource : IInputByteSource
+{
+    /// <inheritdoc/>
+    public int Read(Span<byte> buffer)
+    {
+        // TODO (§14.1): implement using ReadFile / WaitForSingleObject with a timeout
+        // so the read loop mirrors the POSIX VMIN=0/VTIME=1 contract (0 = timeout, keep going).
+        throw new NotImplementedException("Windows input byte source is implemented in §14.1.");
+    }
+}
diff --git a/src/Dcli/Internal/Posix/PosixRawModeSession.cs b/src/Dcli/Internal/Posix/PosixRawModeSession.cs
new file mode 100644
index 0000000..7cfd0f5
--- /dev/null
+++ b/src/Dcli/Internal/Posix/PosixRawModeSession.cs
@@ -0,0 +1,177 @@
+using System.Runtime.InteropServices;
+using System.Runtime.Versioning;
+
+namespace Dcli.Internal.Posix;
+
+/// <summary>
+/// POSIX raw-mode session. Captures the current <c>termios</c> state, applies raw mode,
+/// and restores the original state on <see cref="Dispose"/> or <see cref="Restore"/>.
+/// Thread-safe and idempotent: restoration via <see cref="Restore"/> is a lock-free CAS so
+/// concurrent calls from Dispose, a signal handler, and ProcessExit are safe.
+/// </summary>
+[SupportedOSPlatform("linux")]
+[SupportedOSPlatform("macos")]
+internal sealed class PosixRawModeSession : IRawModeSession
+{
+    // 0 = active, 1 = restored
+    private int _restoreFlag;
+
+    // Only one of these is populated, selected at construction time by platform.
+    private readonly TermiosMac? _savedMac;
+    private readonly TermiosLinux? _savedLinux;
+
+    private PosixRawModeSession(TermiosMac saved)
+    {
+        _savedMac = saved;
+        _savedLinux = null;
+    }
+
+    private PosixRawModeSession(TermiosLinux saved)
+    {
+        _savedMac = null;
+        _savedLinux = saved;
+    }
+
+    /// <summary>
+    /// Captures the current termios state and enters raw mode.
+    /// </summary>
+    /// <exception cref="InvalidOperationException">
+    /// The underlying <c>tcgetattr</c> or <c>tcsetattr</c> syscall failed.
+    /// </exception>
+    internal static PosixRawModeSession Enter()
+    {
+        if (OperatingSystem.IsMacOS())
+            return EnterMac();
+        if (OperatingSystem.IsLinux())
+            return EnterLinux();
+
+        throw new PlatformNotSupportedException("PosixRawModeSession is only supported on macOS and Linux.");
+    }
+
+    [SupportedOSPlatform("macos")]
+    private static PosixRawModeSession EnterMac()
+    {
+        int rc = TermiosMacInterop.tcgetattr(TermiosMacFlags.STDIN_FILENO, out TermiosMac original);
+        if (rc != 0)
+            throw new InvalidOperationException($"tcgetattr failed (errno={Marshal.GetLastPInvokeError()}).");
+
+        TermiosMac raw = original;
+
+        // Clear input flags: stop translating CR→NL (ICRNL) and output flow control (IXON).
+        raw.c_iflag &= ~(TermiosMacFlags.ICRNL | TermiosMacFlags.IXON);
+
+        // Clear local flags: no canonical mode, no echo, no signals (ISIG), no extended (IEXTEN).
+        raw.c_lflag &= ~(TermiosMacFlags.ICANON | TermiosMacFlags.ECHO | TermiosMacFlags.ECHOE
+                       | TermiosMacFlags.ECHOK | TermiosMacFlags.ECHONL | TermiosMacFlags.ISIG
+                       | TermiosMacFlags.IEXTEN);
+
+        // Non-blocking timed reads: return immediately if no bytes, or after VTIME tenths-of-seconds.
+        raw.VMIN = 0;
+        raw.VTIME = 1;  // 100 ms timeout per read
+
+        rc = TermiosMacInterop.tcsetattr(TermiosMacFlags.STDIN_FILENO, TermiosMacFlags.TCSAFLUSH, in raw);
+        if (rc != 0)
+            throw new InvalidOperationException($"tcsetattr failed (errno={Marshal.GetLastPInvokeError()}).");
+
+        return new PosixRawModeSession(original);
+    }
+
+    [SupportedOSPlatform("linux")]
+    private static PosixRawModeSession EnterLinux()
+    {
+        int rc = TermiosLinuxInterop.tcgetattr(TermiosLinuxFlags.STDIN_FILENO, out TermiosLinux original);
+        if (rc != 0)
+            throw new InvalidOperationException($"tcgetattr failed (errno={Marshal.GetLastPInvokeError()}).");
+
+        TermiosLinux raw = original;
+
+        raw.c_iflag &= ~(TermiosLinuxFlags.ICRNL | TermiosLinuxFlags.IXON);
+        raw.c_lflag &= ~(TermiosLinuxFlags.ICANON | TermiosLinuxFlags.ECHO | TermiosLinuxFlags.ECHOE
+                       | TermiosLinuxFlags.ECHOK | TermiosLinuxFlags.ECHONL | TermiosLinuxFlags.ISIG
+                       | TermiosLinuxFlags.IEXTEN);
+
+        raw.VMIN = 0;
+        raw.VTIME = 1;
+
+        rc = TermiosLinuxInterop.tcsetattr(TermiosLinuxFlags.STDIN_FILENO, TermiosLinuxFlags.TCSAFLUSH, in raw);
+        if (rc != 0)
+            throw new InvalidOperationException($"tcsetattr failed (errno={Marshal.GetLastPInvokeError()}).");
+
+        return new PosixRawModeSession(original);
+    }
+
+    /// <inheritdoc/>
+    public void Restore()
+    {
+        // CAS: only the first caller proceeds; subsequent calls are no-ops.
+        if (Interlocked.Exchange(ref _restoreFlag, 1) != 0)
+            return;
+
+        if (OperatingSystem.IsMacOS() && _savedMac.HasValue)
+            RestoreMac(_savedMac.Value);
+        else if (OperatingSystem.IsLinux() && _savedLinux.HasValue)
+            RestoreLinux(_savedLinux.Value);
+    }
+
+    [SupportedOSPlatform("macos")]
+    private static void RestoreMac(TermiosMac saved)
+    {
+        // Best-effort: we are possibly in a signal handler or finalizer; ignore errors.
+        TermiosMacInterop.tcsetattr(TermiosMacFlags.STDIN_FILENO, TermiosMacFlags.TCSAFLUSH, in saved);
+    }
+
+    [SupportedOSPlatform("linux")]
+    private static void RestoreLinux(TermiosLinux saved)
+    {
+        TermiosLinuxInterop.tcsetattr(TermiosLinuxFlags.STDIN_FILENO, TermiosLinuxFlags.TCSAFLUSH, in saved);
+    }
+
+    /// <inheritdoc/>
+    public void Reapply()
+    {
+        // Re-enter raw mode after SIGCONT (resume from suspend may have left tty cooked).
+        // dcli does not own suspend: ISIG is cleared so SIGTSTP is not sent by keypress, and no
+        // SIGTSTP handler is installed. Reapply() exists for an externally-driven kill -STOP/-CONT.
+        // Only re-apply if we have not yet been restored.
+        if (Volatile.Read(ref _restoreFlag) != 0)
+            return;
+
+        if (OperatingSystem.IsMacOS())
+            ReapplyMac();
+        else if (OperatingSystem.IsLinux())
+            ReapplyLinux();
+    }
+
+    [SupportedOSPlatform("macos")]
+    private static void ReapplyMac()
+    {
+        if (TermiosMacInterop.tcgetattr(TermiosMacFlags.STDIN_FILENO, out TermiosMac current) != 0)
+            return;
+
+        current.c_iflag &= ~(TermiosMacFlags.ICRNL | TermiosMacFlags.IXON);
+        current.c_lflag &= ~(TermiosMacFlags.ICANON | TermiosMacFlags.ECHO | TermiosMacFlags.ECHOE
+                           | TermiosMacFlags.ECHOK | TermiosMacFlags.ECHONL | TermiosMacFlags.ISIG
+                           | TermiosMacFlags.IEXTEN);
+        current.VMIN = 0;
+        current.VTIME = 1;
+        TermiosMacInterop.tcsetattr(TermiosMacFlags.STDIN_FILENO, TermiosMacFlags.TCSAFLUSH, in current);
+    }
+
+    [SupportedOSPlatform("linux")]
+    private static void ReapplyLinux()
+    {
+        if (TermiosLinuxInterop.tcgetattr(TermiosLinuxFlags.STDIN_FILENO, out TermiosLinux current) != 0)
+            return;
+
+        current.c_iflag &= ~(TermiosLinuxFlags.ICRNL | TermiosLinuxFlags.IXON);
+        current.c_lflag &= ~(TermiosLinuxFlags.ICANON | TermiosLinuxFlags.ECHO | TermiosLinuxFlags.ECHOE
+                           | TermiosLinuxFlags.ECHOK | TermiosLinuxFlags.ECHONL | TermiosLinuxFlags.ISIG
+                           | TermiosLinuxFlags.IEXTEN);
+        current.VMIN = 0;
+        current.VTIME = 1;
+        TermiosLinuxInterop.tcsetattr(TermiosLinuxFlags.STDIN_FILENO, TermiosLinuxFlags.TCSAFLUSH, in current);
+    }
+
+    /// <inheritdoc/>
+    public void Dispose() => Restore();
+}
diff --git a/src/Dcli/Internal/Posix/PosixResizeWatcher.cs b/src/Dcli/Internal/Posix/PosixResizeWatcher.cs
new file mode 100644
index 0000000..aaa3cb0
--- /dev/null
+++ b/src/Dcli/Internal/Posix/PosixResizeWatcher.cs
@@ -0,0 +1,49 @@
+using System.Runtime.InteropServices;
+using System.Runtime.Versioning;
+using Dcli.Internal.RenderLoop;
+
+namespace Dcli.Internal.Posix;
+
+/// <summary>
+/// POSIX implementation of <see cref="IResizeWatcher"/>: registers for <c>SIGWINCH</c> via
+/// <see cref="PosixSignalRegistration"/> and delivers resize events to the caller-supplied callback.
+/// </summary>
+[SupportedOSPlatform("linux")]
+[SupportedOSPlatform("macos")]
+internal sealed class PosixResizeWatcher : IResizeWatcher
+{
+    private readonly ITerminalSizeSource _sizeSource;
+    private PosixSignalRegistration? _registration;
+    private int _disposed;
+
+    internal PosixResizeWatcher(ITerminalSizeSource sizeSource)
+    {
+        ArgumentNullException.ThrowIfNull(sizeSource);
+        _sizeSource = sizeSource;
+    }
+
+    /// <inheritdoc/>
+    public void Start(Action<int, int> onResize)
+    {
+        ArgumentNullException.ThrowIfNull(onResize);
+
+        _registration = PosixSignalRegistration.Create(PosixSignal.SIGWINCH,
+            context =>
+            {
+                // We handle SIGWINCH fully; do not re-raise to the runtime.
+                context.Cancel = true;
+                (int cols, int rows) = _sizeSource.GetSize();
+                onResize(cols, rows);
+            });
+    }
+
+    /// <inheritdoc/>
+    public void Dispose()
+    {
+        if (Interlocked.Exchange(ref _disposed, 1) != 0)
+            return;
+
+        _registration?.Dispose();
+        _registration = null;
+    }
+}
diff --git a/src/Dcli/Internal/Posix/PosixTerminalSizeSource.cs b/src/Dcli/Internal/Posix/PosixTerminalSizeSource.cs
new file mode 100644
index 0000000..5e842b0
--- /dev/null
+++ b/src/Dcli/Internal/Posix/PosixTerminalSizeSource.cs
@@ -0,0 +1,36 @@
+using System.Runtime.Versioning;
+using Dcli.Internal.RenderLoop;
+
+namespace Dcli.Internal.Posix;
+
+/// <summary>
+/// Queries the terminal size via <see cref="Console.WindowWidth"/> / <see cref="Console.WindowHeight"/>,
+/// which delegate to the runtime's native shim (<c>libSystem.Native</c>) rather than a hand-rolled
+/// <c>ioctl(TIOCGWINSZ)</c> P/Invoke.
+/// Returns an 80×24 fallback when the query fails (e.g. in CI without a tty).
+/// </summary>
+/// <remarks>
+/// The hand-rolled ioctl P/Invoke caused an AV on AArch64 Darwin because <c>ioctl(2)</c> is
+/// variadic and the AArch64 ABI passes variadic args on the stack; the runtime's shim is
+/// non-variadic and has no such problem. See commit message for details.
+/// </remarks>
+[SupportedOSPlatform("linux")]
+[SupportedOSPlatform("macos")]
+internal sealed class PosixTerminalSizeSource : ITerminalSizeSource
+{
+    private const int _fallbackColumns = 80;
+    private const int _fallbackRows = 24;
+
+    /// <inheritdoc/>
+    public (int Columns, int Rows) GetSize()
+    {
+        int cols, rows;
+        try { cols = Console.WindowWidth; } catch (IOException) { cols = 0; }
+        try { rows = Console.WindowHeight; } catch (IOException) { rows = 0; }
+
+        if (cols > 0 && rows > 0)
+            return (cols, rows);
+
+        return (_fallbackColumns, _fallbackRows);
+    }
+}
diff --git a/src/Dcli/Internal/Posix/Termios.cs b/src/Dcli/Internal/Posix/Termios.cs
new file mode 100644
index 0000000..f8a653d
--- /dev/null
+++ b/src/Dcli/Internal/Posix/Termios.cs
@@ -0,0 +1,174 @@
+// POSIX termios P/Invoke layer.
+//
+// Separate constant sets and struct layouts are defined per platform because macOS/BSD and Linux
+// use incompatible representations:
+//
+//   macOS (BSD): tcflag_t = ulong (64-bit), speed_t = ulong (64-bit), NCCS = 20
+//                VMIN=16, VTIME=17
+//                Source: /usr/include/sys/termios.h (XNU / LLVM libc headers, macOS 14+)
+//
+//   Linux:       tcflag_t = uint (32-bit), speed_t = uint (32-bit), NCCS = 32
+//                VMIN=6,  VTIME=5
+//                Source: <asm-generic/termbits.h> (Linux kernel headers, identical across arches
+//                except SPARC; all arches supported by .NET use the generic values)
+//
+// P/Invoke targets "libc" which resolves to libSystem.B.dylib on macOS and libc.so.6 on Linux.
+// Both export tcgetattr/tcsetattr with the same C prototype; only the struct layout differs.
+
+using System.Runtime.InteropServices;
+using System.Runtime.Versioning;
+
+namespace Dcli.Internal.Posix;
+
+// ─── macOS / BSD ─────────────────────────────────────────────────────────────
+
+/// <summary>macOS <c>termios</c> struct (tcflag_t = ulong, NCCS = 20).</summary>
+/// <remarks>
+/// Layout (verified against /usr/include/sys/termios.h, macOS 14+ / XNU):
+///   offset  0: c_iflag  (ulong, 8 bytes)
+///   offset  8: c_oflag  (ulong, 8 bytes)
+///   offset 16: c_cflag  (ulong, 8 bytes)
+///   offset 24: c_lflag  (ulong, 8 bytes)
+///   offset 32: c_cc[0..19] (20 × byte)   — NO c_line field; macOS/BSD omits it
+///   offset 52: padding  (4 bytes, implicit for 8-byte alignment of speed fields)
+///   offset 56: c_ispeed (speed_t = ulong, 8 bytes)
+///   offset 64: c_ospeed (speed_t = ulong, 8 bytes)
+///   total: 72 bytes
+/// The Linux glibc/musl struct genuinely has c_line after c_lflag; macOS/BSD does not.
+/// </remarks>
+[SupportedOSPlatform("macos")]
+[StructLayout(LayoutKind.Sequential)]
+internal struct TermiosMac
+{
+    // c_iflag / c_oflag / c_cflag / c_lflag are ulong on macOS (tcflag_t = unsigned long = 8 bytes)
+    public ulong c_iflag;
+    public ulong c_oflag;
+    public ulong c_cflag;
+    public ulong c_lflag;
+
+    // c_cc[NCCS] where NCCS=20 on macOS — no c_line gap before this array
+    public byte cc0, cc1, cc2, cc3, cc4;
+    public byte cc5, cc6, cc7, cc8, cc9;
+    public byte cc10, cc11, cc12, cc13, cc14;
+    public byte cc15, cc16, cc17, cc18, cc19;
+
+    // speed_t = unsigned long on macOS (8 bytes each); do not simplify to uint.
+    public ulong c_ispeed;
+    public ulong c_ospeed;
+
+    // VMIN=16, VTIME=17 on macOS
+    internal byte VMIN { readonly get => cc16; set => cc16 = value; }
+    internal byte VTIME { readonly get => cc17; set => cc17 = value; }
+}
+
+/// <summary>macOS termios flag constants.</summary>
+[SupportedOSPlatform("macos")]
+internal static class TermiosMacFlags
+{
+    // c_iflag bits (Source: macOS /usr/include/sys/termios.h)
+    internal const ulong ICRNL = 0x0100UL;  // map CR to NL on input
+    internal const ulong IXON = 0x0200UL;  // enable output flow control
+
+    // c_lflag bits (Source: macOS /usr/include/sys/termios.h)
+    internal const ulong ISIG = 0x0080UL;  // enable signals (INTR, QUIT, SUSP)
+    internal const ulong ICANON = 0x0100UL;  // canonical mode
+    internal const ulong ECHO = 0x0008UL;  // echo input characters
+    internal const ulong ECHOE = 0x0002UL;  // echo ERASE as BS-SP-BS
+    internal const ulong ECHOK = 0x0004UL;  // echo KILL
+    internal const ulong ECHONL = 0x0010UL;  // echo NL
+    internal const ulong IEXTEN = 0x0400UL;  // enable extended processing (e.g. DISCARD, LNEXT)
+
+    internal const int TCSAFLUSH = 2;
+    internal const int STDIN_FILENO = 0;
+}
+
+// ─── Linux ───────────────────────────────────────────────────────────────────
+
+/// <summary>Linux <c>termios</c> struct (tcflag_t = uint, NCCS = 32).</summary>
+[SupportedOSPlatform("linux")]
+[StructLayout(LayoutKind.Sequential)]
+internal struct TermiosLinux
+{
+    public uint c_iflag;
+    public uint c_oflag;
+    public uint c_cflag;
+    public uint c_lflag;
+
+    public byte c_line;  // line discipline
+
+    // c_cc[NCCS] where NCCS=32 on Linux
+    public byte cc0, cc1, cc2, cc3, cc4, cc5, cc6, cc7;
+    public byte cc8, cc9, cc10, cc11, cc12, cc13, cc14, cc15;
+    public byte cc16, cc17, cc18, cc19, cc20, cc21, cc22, cc23;
+    public byte cc24, cc25, cc26, cc27, cc28, cc29, cc30, cc31;
+
+    public uint c_ispeed;
+    public uint c_ospeed;
+
+    // VMIN=6, VTIME=5 on Linux (asm-generic/termbits.h)
+    internal byte VMIN { readonly get => cc6; set => cc6 = value; }
+    internal byte VTIME { readonly get => cc5; set => cc5 = value; }
+}
+
+/// <summary>Linux termios flag constants.</summary>
+[SupportedOSPlatform("linux")]
+internal static class TermiosLinuxFlags
+{
+    // c_iflag bits (Source: asm-generic/termbits.h)
+    internal const uint ICRNL = 0x0100U;  // map CR to NL on input
+    internal const uint IXON = 0x0400U;  // enable output flow control
+
+    // c_lflag bits (Source: asm-generic/termbits.h)
+    internal const uint ISIG = 0x0001U;  // enable signals
+    internal const uint ICANON = 0x0002U;  // canonical mode
+    internal const uint ECHO = 0x0008U;  // echo input characters
+    internal const uint ECHOE = 0x0010U;  // echo erase as BS-SP-BS
+    internal const uint ECHOK = 0x0020U;  // echo kill
+    internal const uint ECHONL = 0x0040U;  // echo NL
+    internal const uint IEXTEN = 0x8000U;  // extended processing
+
+    internal const int TCSAFLUSH = 2;
+    internal const int STDIN_FILENO = 0;
+}
+
+// ─── P/Invoke declarations ───────────────────────────────────────────────────
+
+[SupportedOSPlatform("linux")]
+internal static partial class TermiosLinuxInterop
+{
+    [DefaultDllImportSearchPaths(DllImportSearchPath.SafeDirectories)]
+    [LibraryImport("libc", EntryPoint = "tcgetattr", SetLastError = true)]
+    internal static partial int tcgetattr(int fd, out TermiosLinux termios);
+
+    [DefaultDllImportSearchPaths(DllImportSearchPath.SafeDirectories)]
+    [LibraryImport("libc", EntryPoint = "tcsetattr", SetLastError = true)]
+    internal static partial int tcsetattr(int fd, int optionalActions, in TermiosLinux termios);
+}
+
+[SupportedOSPlatform("macos")]
+internal static partial class TermiosMacInterop
+{
+    [DefaultDllImportSearchPaths(DllImportSearchPath.SafeDirectories)]
+    [LibraryImport("libc", EntryPoint = "tcgetattr", SetLastError = true)]
+    internal static partial int tcgetattr(int fd, out TermiosMac termios);
+
+    [DefaultDllImportSearchPaths(DllImportSearchPath.SafeDirectories)]
+    [LibraryImport("libc", EntryPoint = "tcsetattr", SetLastError = true)]
+    internal static partial int tcsetattr(int fd, int optionalActions, in TermiosMac termios);
+}
+
+/// <summary>
+/// POSIX <c>read(2)</c> — shared across macOS and Linux; the prototype is identical.
+/// Used by the raw-mode harness to read stdin without going through the managed stream layer,
+/// which misinterprets a 0-byte timeout return (VMIN=0/VTIME&gt;0) as end-of-stream.
+/// </summary>
+[SupportedOSPlatform("linux")]
+[SupportedOSPlatform("macos")]
+internal static partial class PosixReadInterop
+{
+    internal const int EINTR = 4;
+
+    [DefaultDllImportSearchPaths(DllImportSearchPath.SafeDirectories)]
+    [LibraryImport("libc", EntryPoint = "read", SetLastError = true)]
+    internal static unsafe partial nint read(int fd, byte* buf, nuint count);
+}
diff --git a/src/Dcli/Internal/RawModeSession.cs b/src/Dcli/Internal/RawModeSession.cs
new file mode 100644
index 0000000..df665e9
--- /dev/null
+++ b/src/Dcli/Internal/RawModeSession.cs
@@ -0,0 +1,38 @@
+using Dcli.Internal.Posix;
+using Dcli.Internal.Windows;
+
+namespace Dcli.Internal;
+
+/// <summary>
+/// Factory that creates the platform-appropriate <see cref="IRawModeSession"/>.
+/// </summary>
+internal static class RawModeSession
+{
+    /// <summary>
+    /// Runs capability detection and enters raw mode, returning an <see cref="IRawModeSession"/>
+    /// that restores the terminal on dispose.
+    /// </summary>
+    /// <param name="inputs">
+    /// Override capability inputs (for tests). Pass <see langword="null"/> to read from the
+    /// real environment.
+    /// </param>
+    /// <returns>A live raw-mode session.</returns>
+    /// <exception cref="TerminalNotSupportedException">
+    /// The terminal does not meet the minimum VT requirements.
+    /// </exception>
+    /// <exception cref="PlatformNotSupportedException">
+    /// The current platform is not supported.
+    /// </exception>
+    internal static IRawModeSession Enter(CapabilityInputs? inputs = null)
+    {
+        TerminalCapabilityDetector.EnsureVtCapable(inputs);
+
+        if (OperatingSystem.IsLinux() || OperatingSystem.IsMacOS())
+            return PosixRawModeSession.Enter();
+
+        if (OperatingSystem.IsWindows())
+            return WindowsRawModeSession.Enter();
+
+        throw new PlatformNotSupportedException("dcli only supports Linux, macOS, and Windows.");
+    }
+}
diff --git a/src/Dcli/Internal/RecordingRawModeSession.cs b/src/Dcli/Internal/RecordingRawModeSession.cs
new file mode 100644
index 0000000..1c40235
--- /dev/null
+++ b/src/Dcli/Internal/RecordingRawModeSession.cs
@@ -0,0 +1,64 @@
+namespace Dcli.Internal;
+
+/// <summary>
+/// A test-double <see cref="IRawModeSession"/> that records calls to <see cref="Restore"/> and
+/// <see cref="Reapply"/> without touching the OS. Used to verify the restore coordinator's
+/// wiring logic without a real tty.
+/// </summary>
+internal sealed class RecordingRawModeSession : IRawModeSession
+{
+    private int _restoreCallCount;
+    private int _effectiveRestoreCount;
+    private int _reapplyCallCount;
+    private long _outputLengthAtRestore = -1;
+
+    /// <summary>
+    /// Optional delegate that returns the current byte position of the output stream.
+    /// When set, <see cref="Restore"/> snapshots the stream position so tests can assert
+    /// that the ANSI restore sequence was emitted BEFORE termios restore was called.
+    /// </summary>
+    internal Func<long>? GetOutputLength { get; set; }
+
+    /// <summary>
+    /// Total number of times <see cref="Restore"/> was invoked (raw call count, including
+    /// redundant calls). Use <see cref="EffectiveRestoreCount"/> to assert the CAS-idempotent
+    /// "one real restore" invariant.
+    /// </summary>
+    internal int RestoreCallCount => Volatile.Read(ref _restoreCallCount);
+
+    /// <summary>
+    /// Number of times <see cref="Restore"/> had a real effect (i.e. was the first call).
+    /// Mirrors the CAS-idempotency of <see cref="Posix.PosixRawModeSession"/>: only the first
+    /// Restore call counts; all subsequent calls are no-ops.
+    /// </summary>
+    internal int EffectiveRestoreCount => Volatile.Read(ref _effectiveRestoreCount);
+
+    /// <summary>Total number of times <see cref="Reapply"/> was invoked.</summary>
+    internal int ReapplyCallCount => Volatile.Read(ref _reapplyCallCount);
+
+    /// <summary>
+    /// The output stream length (in characters) at the moment the first <see cref="Restore"/>
+    /// was called, or -1 if <see cref="GetOutputLength"/> was not set. Used to assert ordering:
+    /// ANSI bytes must have been written before termios restore.
+    /// </summary>
+    internal long OutputLengthAtRestore => Volatile.Read(ref _outputLengthAtRestore);
+
+    /// <inheritdoc/>
+    public void Restore()
+    {
+        // Snapshot the output length on the FIRST call to Restore (before any idempotency guard
+        // so the snapshot reflects the position when restore was first triggered).
+        long snapshot = GetOutputLength?.Invoke() ?? -1;
+        Interlocked.CompareExchange(ref _outputLengthAtRestore, snapshot, -1);
+
+        Interlocked.Increment(ref _restoreCallCount);
+        // Mirror real CAS-idempotency: only the first call is "effective".
+        Interlocked.CompareExchange(ref _effectiveRestoreCount, 1, 0);
+    }
+
+    /// <inheritdoc/>
+    public void Reapply() => Interlocked.Increment(ref _reapplyCallCount);
+
+    /// <inheritdoc/>
+    public void Dispose() => Restore();
+}
diff --git a/src/Dcli/Internal/RenderLoop/IClock.cs b/src/Dcli/Internal/RenderLoop/IClock.cs
new file mode 100644
index 0000000..911d81c
--- /dev/null
+++ b/src/Dcli/Internal/RenderLoop/IClock.cs
@@ -0,0 +1,25 @@
+namespace Dcli.Internal.RenderLoop;
+
+/// <summary>
+/// Abstracts wall-clock access so the render loop's cadence (frame-rate cap, idle detection)
+/// can be exercised deterministically in tests without real time passing.
+/// </summary>
+/// <remarks>
+/// The real implementation delegates to <see cref="System.Diagnostics.Stopwatch"/> /
+/// <c>Task.Delay</c>. The virtual-clock test implementation
+/// lets tests advance time explicitly so throttle / coalescing / idle scenarios run
+/// without any wall-clock sleeps.
+/// </remarks>
+internal interface IClock
+{
+    /// <summary>Returns the current instant as a <see cref="TimeSpan"/> since an arbitrary epoch.</summary>
+    TimeSpan Now { get; }
+
+    /// <summary>
+    /// Returns a <see cref="System.Threading.Tasks.Task"/> that completes at <paramref name="deadline"/>,
+    /// or immediately if <paramref name="deadline"/> is already in the past.
+    /// </summary>
+    /// <param name="deadline">The target instant (same epoch as <see cref="Now"/>).</param>
+    /// <param name="cancellationToken">Cancels the wait.</param>
+    Task WaitUntilAsync(TimeSpan deadline, CancellationToken cancellationToken);
+}
diff --git a/src/Dcli/Internal/RenderLoop/IOutputSink.cs b/src/Dcli/Internal/RenderLoop/IOutputSink.cs
new file mode 100644
index 0000000..15bc835
--- /dev/null
+++ b/src/Dcli/Internal/RenderLoop/IOutputSink.cs
@@ -0,0 +1,38 @@
+namespace Dcli.Internal.RenderLoop;
+
+/// <summary>
+/// Abstracts the destination for rendered output, allowing §8's real escape-sequence paint
+/// to be swapped in without changing the loop's cadence logic, and allowing tests to capture
+/// paint calls deterministically.
+/// </summary>
+/// <remarks>
+/// §8 will replace the minimal sentinel implementation with a real frame renderer.
+/// Tests inject an in-memory capturing sink that records each paint call.
+/// The real implementation writes to stdout via a buffered writer.
+/// </remarks>
+internal interface IOutputSink
+{
+    /// <summary>
+    /// Called by the render loop exactly once per coalesced frame when the model is dirty
+    /// and the minimum frame interval has elapsed.
+    /// </summary>
+    /// <param name="model">The current render model snapshot (read-only for the sink).</param>
+    void Paint(RenderModel model);
+
+    /// <summary>
+    /// Emits the minimal ANSI restore sequence and flushes the output destination.
+    /// Called from the render-loop <c>finally</c> block on every exit path so that cursor
+    /// visibility, synchronized-output mode, and SGR state are cleaned up before the
+    /// terminal session is released.
+    /// </summary>
+    /// <remarks>
+    /// The sequence emitted is:
+    /// <list type="bullet">
+    ///   <item><c>ESC[?2026l</c> — synchronized-output OFF (in case the process died mid-frame)</item>
+    ///   <item><c>ESC[?25h</c> — cursor visible (undoes any <c>ESC[?25l</c> from dialog / parking)</item>
+    ///   <item><c>ESC[0m</c> — SGR reset (clear any lingering colour/style attributes)</item>
+    /// </list>
+    /// Implementations must be idempotent — the method may be called more than once.
+    /// </remarks>
+    void EmitRestoreSequence();
+}
diff --git a/src/Dcli/Internal/RenderLoop/IResizeWatcher.cs b/src/Dcli/Internal/RenderLoop/IResizeWatcher.cs
new file mode 100644
index 0000000..e82803c
--- /dev/null
+++ b/src/Dcli/Internal/RenderLoop/IResizeWatcher.cs
@@ -0,0 +1,42 @@
+namespace Dcli.Internal.RenderLoop;
+
+/// <summary>
+/// An OS edge that notifies callers when the terminal window size changes.
+/// </summary>
+/// <remarks>
+/// <para>
+/// The callback supplied to <see cref="Start"/> may fire on a POSIX signal-handler thread.
+/// It must only perform thread-safe work — in production, posting a
+/// <see cref="Dcli.ResizeEvent"/> to the loop's inbound channel via
+/// <c>loop.InputWriter.TryWrite</c> (an unbounded channel write is safe from any thread).
+/// </para>
+/// <para>
+/// Callers must dispose the watcher to deregister the OS hook and ensure no callback fires
+/// after disposal.
+/// </para>
+/// </remarks>
+internal interface IResizeWatcher : IDisposable
+{
+    /// <summary>
+    /// Arms the OS resize hook and registers <paramref name="onResize"/> as the callback.
+    /// The callback receives <c>(columns, rows)</c> for the new terminal size.
+    /// </summary>
+    /// <param name="onResize">
+    /// Called on every terminal resize. May be invoked on a signal-handler thread; must be
+    /// thread-safe and must not block.
+    /// </param>
+    void Start(Action<int, int> onResize);
+}
+
+/// <summary>
+/// A no-op <see cref="IResizeWatcher"/> for headless tests and unknown-platform fallback.
+/// Never fires the callback.
+/// </summary>
+internal sealed class NoopResizeWatcher : IResizeWatcher
+{
+    /// <inheritdoc/>
+    public void Start(Action<int, int> onResize) { }
+
+    /// <inheritdoc/>
+    public void Dispose() { }
+}
diff --git a/src/Dcli/Internal/RenderLoop/ITerminalSizeSource.cs b/src/Dcli/Internal/RenderLoop/ITerminalSizeSource.cs
new file mode 100644
index 0000000..630939c
--- /dev/null
+++ b/src/Dcli/Internal/RenderLoop/ITerminalSizeSource.cs
@@ -0,0 +1,15 @@
+namespace Dcli.Internal.RenderLoop;
+
+/// <summary>
+/// Supplies the initial terminal dimensions for the render model's volatile size snapshot.
+/// </summary>
+/// <remarks>
+/// §13 will wire a real SIGWINCH-aware implementation. Tests inject a fixed-size stub.
+/// The loop reads the initial size once at start-up and thereafter receives updates via
+/// resize messages on the inbound channel.
+/// </remarks>
+internal interface ITerminalSizeSource
+{
+    /// <summary>Returns the current terminal size as (columns, rows).</summary>
+    (int Columns, int Rows) GetSize();
+}
diff --git a/src/Dcli/Internal/RenderLoop/LoopEngine.cs b/src/Dcli/Internal/RenderLoop/LoopEngine.cs
new file mode 100644
index 0000000..add20a9
--- /dev/null
+++ b/src/Dcli/Internal/RenderLoop/LoopEngine.cs
@@ -0,0 +1,702 @@
+using System.Collections.Concurrent;
+using System.Text;
+using System.Threading.Channels;
+using Dcli.Internal.FixedRegion;
+using Dcli.Internal.Input;
+
+namespace Dcli.Internal.RenderLoop;
+
+/// <summary>
+/// The single-writer render loop actor.
+/// </summary>
+/// <remarks>
+/// <para>
+/// <strong>Threading contract — the load-bearing correctness property:</strong><br/>
+/// One dedicated OS thread (<c>dcli-render-loop</c>) owns ALL mutable UI state. Every mutation
+/// of <see cref="_model"/> happens inside <see cref="RunLoop"/> on that thread. No other thread
+/// ever reads or writes the model fields. No locks are needed because there is exactly one writer.
+/// </para>
+/// <para>
+/// The blocking point in the loop is <see cref="BlockUntilMessageOrDeadline"/>. When the model
+/// is clean the loop parks via <c>WaitToReadAsync().AsTask().GetAwaiter().GetResult()</c> — zero
+/// CPU until a message arrives. When dirty it races the channel wait against
+/// <c>IClock.WaitUntilAsync(deadline)</c> via <c>Task.WaitAny</c>, so the deadline goes through
+/// the virtual clock and cadence tests remain fully deterministic. Both calls are synchronous
+/// blocking calls on the loop thread — the dedicated thread is never yielded to the pool. Every
+/// line of <see cref="RunLoop"/> — including every read and write of <see cref="_model"/> —
+/// executes on the single loop thread.
+/// </para>
+/// <para>
+/// Producers call <see cref="Post"/> (non-blocking <c>TryWrite</c>) and return immediately before
+/// the command is applied or a frame is painted.
+/// </para>
+/// <para>
+/// <strong>Outbound channel:</strong> the loop writes via <c>TryWrite</c> on an unbounded channel.
+/// A consumer that never drains the channel accumulates events but does not stall painting.
+/// </para>
+/// </remarks>
+internal sealed class LoopEngine : IDisposable
+{
+    // ── Configuration ────────────────────────────────────────────────────────
+
+    /// <summary>Minimum interval between successive frames (~16 ms ≈ 60 fps ceiling).</summary>
+    internal static readonly TimeSpan DefaultMinFrameInterval = TimeSpan.FromMilliseconds(16);
+
+    // ── Inbound channel ──────────────────────────────────────────────────────
+    // Unbounded (Decision 10): apply is cheap; drains fast; no message is ever dropped.
+    private readonly Channel<LoopMessage> _inbound = Channel.CreateUnbounded<LoopMessage>(
+        new UnboundedChannelOptions { SingleReader = true });
+
+    // ── Outbound channel ─────────────────────────────────────────────────────
+    // Unbounded: TryWrite never blocks; a slow consumer accumulates without stalling the loop.
+    private readonly Channel<TerminalEvent> _outbound = Channel.CreateUnbounded<TerminalEvent>(
+        new UnboundedChannelOptions { SingleWriter = true });
+
+    // ── Pending settle completions ────────────────────────────────────────────
+    // SettleCommand enqueues a TCS here; the loop thread signals it after the next paint.
+    // ConcurrentQueue because producers write from any thread, loop reads on loop thread.
+    private readonly ConcurrentQueue<TaskCompletionSource> _pendingSettles = new();
+
+    // ── Injected edges ───────────────────────────────────────────────────────
+    private readonly IClock _clock;
+    private readonly IOutputSink _sink;
+    private readonly TimeSpan _minFrameInterval;
+
+    // Optional hook called from RunLoop's finally so that an unhandled loop-thread
+    // exception still restores the terminal. The RestoreCoordinator covers signals
+    // and ProcessExit; this covers the loop-thread-crash path.
+    private readonly Action? _restoreOnExit;
+
+    // ── Loop state ───────────────────────────────────────────────────────────
+    private readonly RenderModel _model;
+    private readonly Thread _thread;
+    private readonly CancellationTokenSource _cts = new();
+
+    // Signals loop-thread exit to any awaiter (e.g. DisposeAsync fault rethrow).
+    // RunContinuationsAsynchronously: continuations must not resume on the loop thread.
+    private readonly TaskCompletionSource _loopTerminated =
+        new(TaskCreationOptions.RunContinuationsAsynchronously);
+
+    // ── Volatile terminal-size snapshot (Decision 10: snapshot reads) ────────
+    // Updated on the loop thread; read from any thread via GetTerminalSize().
+    private volatile int _snapshotColumns;
+    private volatile int _snapshotRows;
+
+    // Disposal guard.
+    private int _disposed;
+
+    // Instrumentation: how many times the loop entered its blocking wait.
+    // Exposed for idle-efficiency tests; not part of the public surface.
+    private int _waitCount;
+
+    /// <summary>
+    /// Initialises and starts the render loop on its dedicated thread.
+    /// </summary>
+    /// <param name="sizeSource">Provides the initial terminal dimensions.</param>
+    /// <param name="clock">Supplies "now" and deadline waits.</param>
+    /// <param name="sink">Destination for rendered frames.</param>
+    /// <param name="minFrameInterval">
+    /// Minimum time between successive paints. Defaults to <see cref="DefaultMinFrameInterval"/>.
+    /// </param>
+    /// <param name="restoreOnExit">
+    /// Optional action called from <c>RunLoop</c>'s <c>finally</c> block so that an unhandled
+    /// exception on the loop thread still restores the terminal. Must be idempotent (it may also
+    /// be called by <see cref="Dispose"/> and the <see cref="RestoreCoordinator"/>).
+    /// </param>
+    /// <param name="maxFixedHeight">
+    /// Optional consumer-supplied cap on the fixed-region height in rows.
+    /// Forwarded to <see cref="RenderModel.MaxFixedHeight"/>; enforced by §10.
+    /// </param>
+    internal LoopEngine(
+        ITerminalSizeSource sizeSource,
+        IClock clock,
+        IOutputSink sink,
+        TimeSpan? minFrameInterval = null,
+        Action? restoreOnExit = null,
+        int? maxFixedHeight = null)
+    {
+        ArgumentNullException.ThrowIfNull(sizeSource);
+        ArgumentNullException.ThrowIfNull(clock);
+        ArgumentNullException.ThrowIfNull(sink);
+
+        _clock = clock;
+        _sink = sink;
+        _minFrameInterval = minFrameInterval ?? DefaultMinFrameInterval;
+        _restoreOnExit = restoreOnExit;
+        _model = new RenderModel(sizeSource, maxFixedHeight);
+
+        _snapshotColumns = _model.Columns;
+        _snapshotRows = _model.Rows;
+
+        _thread = new Thread(RunLoop)
+        {
+            Name = "dcli-render-loop",
+            IsBackground = true,
+        };
+        _thread.Start();
+    }
+
+    // ── Public surface ───────────────────────────────────────────────────────
+
+    /// <summary>
+    /// The outbound event stream. The consumer drains this on its own thread.
+    /// The loop never reads this reader.
+    /// </summary>
+    internal ChannelReader<TerminalEvent> OutboundEvents => _outbound.Reader;
+
+    /// <summary>
+    /// How many times the loop entered its blocking wait.
+    /// A truly-idle loop increments this once and parks; a busy-poll increments it thousands of times.
+    /// </summary>
+    internal int WaitCount => Volatile.Read(ref _waitCount);
+
+    /// <summary>
+    /// A task that completes when the loop thread exits.
+    /// Completes successfully on a clean shutdown; faulted when the loop died due to an
+    /// unhandled exception (e.g. a crashing sink or command). Use to detect loop death.
+    /// </summary>
+    internal Task LoopTerminated => _loopTerminated.Task;
+
+    /// <summary>
+    /// A <see cref="ChannelWriter{T}"/> that wraps incoming <see cref="InputEvent"/>s as
+    /// <see cref="InputMessage"/>s before posting to the inbound channel.
+    /// Wire §6's <see cref="InputReader"/> to this writer.
+    /// </summary>
+    internal ChannelWriter<InputEvent> InputWriter => new InputWrappingWriter(_inbound.Writer);
+
+    /// <summary>
+    /// Posts a fire-and-forget command to the loop. Returns before the command is applied
+    /// or the frame is painted (Decision 10: API is fire-and-forget).
+    /// </summary>
+    internal void Post(ILoopCommand command)
+    {
+        // TryWrite on an unbounded channel always succeeds.
+        _inbound.Writer.TryWrite(new CommandMessage(command));
+    }
+
+    /// <summary>
+    /// Returns the last-known terminal size from the volatile snapshot.
+    /// Does not round-trip to the loop thread (Decision 10: snapshot read).
+    /// </summary>
+    internal (int Columns, int Rows) GetTerminalSize() => (_snapshotColumns, _snapshotRows);
+
+    // ── Internal settle helper (used by cadence tests; §15 will generalize) ──
+
+    /// <summary>
+    /// Drains all currently-enqueued inbound messages, produces at most one coalesced frame
+    /// (if dirty and the deadline allows), then completes the returned task.
+    /// </summary>
+    /// <remarks>
+    /// Provides deterministic cadence-test synchronization without wall-clock sleeps.
+    /// §15's <c>HeadlessTerminal.SettleAsync</c> will generalize this into a public contract.
+    ///
+    /// A <see cref="SettleCommand"/> sentinel is enqueued. When the loop applies it, the
+    /// <see cref="RenderModel"/> is marked dirty and the TCS is queued for post-paint signaling.
+    /// After the paint step in <see cref="RunLoop"/> the loop drains all pending settle TCSes,
+    /// ensuring the caller's task completes only after the frame (if any) has been produced.
+    /// </remarks>
+    internal Task SettleAsync(CancellationToken cancellationToken = default)
+    {
+        TaskCompletionSource tcs = new(TaskCreationOptions.RunContinuationsAsynchronously);
+        _inbound.Writer.TryWrite(new CommandMessage(new SettleCommand(_pendingSettles, tcs)));
+        return tcs.Task.WaitAsync(cancellationToken);
+    }
+
+    /// <inheritdoc/>
+    public void Dispose()
+    {
+        if (Interlocked.Exchange(ref _disposed, 1) != 0)
+            return;
+
+        _cts.Cancel();
+        // Complete the channel so WaitToReadAsync unblocks immediately; the clock wait is
+        // cancelled via the loop token, so both waits in BlockUntilMessageOrDeadline exit.
+        _inbound.Writer.TryComplete();
+        // If the loop wedges beyond this window, ANSI restore is best-effort skipped; the
+        // signal handler's own _session.Restore() call still guarantees termios restore.
+        _thread.Join(timeout: TimeSpan.FromSeconds(2));
+        _cts.Dispose();
+    }
+
+    // ── Loop body ─────────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// The render loop. Runs entirely on the dedicated <c>dcli-render-loop</c> thread.
+    /// Every read and write of <see cref="_model"/> occurs here.
+    /// </summary>
+    private void RunLoop()
+    {
+        CancellationToken ct = _cts.Token;
+        TimeSpan nextPaintDeadline = _clock.Now; // first dirty state may paint immediately
+
+        Exception? loopFault = null;
+
+        try
+        {
+            while (!ct.IsCancellationRequested)
+            {
+                // Block the loop thread until a message is available, or — only when dirty —
+                // until the next paint deadline arrives. Clean model: park indefinitely on
+                // the channel (zero CPU). Returns false when cancelled or channel is completed.
+                if (!BlockUntilMessageOrDeadline(nextPaintDeadline, waitForDeadline: _model.IsDirty, ct))
+                    break;
+
+                // Drain all available messages. Thread discipline: all ApplyMessage calls run
+                // here on the loop thread — the only mutator of _model.
+                while (_inbound.Reader.TryRead(out LoopMessage? msg))
+                {
+                    ApplyMessage(msg);
+                }
+
+                // Paint only when dirty and the minimum frame interval has elapsed.
+                TimeSpan now = _clock.Now;
+                if (_model.IsDirty && now >= nextPaintDeadline)
+                {
+                    // §10 integration point: compose the fixed region (input + status) first.
+                    // This sets FixedRegionRows, which §9's PrePaint reads to compute the live
+                    // window budget (maxHeight = rows − fixedRegionRows.Count).
+                    _model.FixedRegion.Compose(_model);
+
+                    // §9 integration point: recompute LiveWindowRows and NewlyCommittedRows
+                    // from the live object list, applying the commit-horizon overflow rule.
+                    // Must run after fixed-region compose and before Paint.
+                    _model.Scrollback.PrePaint(_model);
+
+                    // §10 caret finalisation: offset the editor-local caret row by
+                    // LiveWindowRows.Count to produce the full-frame caret position.
+                    if (_model.EditorCaretLocal is (int editorRow, int editorCol))
+                        _model.CaretPosition = (_model.LiveWindowRows.Count + editorRow, editorCol);
+                    else
+                        _model.CaretPosition = null;
+
+                    _sink.Paint(_model);
+                    // Reset NewlyCommittedRows to empty after the frame — committed rows must
+                    // be emitted exactly once (never rewritten on the next frame).
+                    _model.Scrollback.ClearCommitted(_model);
+                    _model.ClearDirty();
+                    nextPaintDeadline = now + _minFrameInterval;
+                }
+
+                // Signal any pending settle waiters (after the paint so they observe the frame).
+                DrainSettles();
+            }
+
+            // Signal any settles that are still pending at shutdown.
+            DrainSettles();
+        }
+#pragma warning disable CA1031 // background actor-thread root: must not propagate; finally restores terminal
+        catch (Exception ex)
+        {
+            // An unhandled exception on the loop thread (e.g. a crash in the sink or a
+            // command's Apply). The exception must NOT propagate to the thread root — that
+            // would crash the process. The finally block below handles restoration and faults
+            // the loop-terminated task so callers (e.g. DisposeAsync) can observe the crash.
+            loopFault = ex;
+            FaultSettles(ex);
+        }
+#pragma warning restore CA1031
+        finally
+        {
+            // Emit the ANSI restore sequence before releasing the terminal session.
+            // This runs on the loop thread — the single writer — so there is no stdout
+            // interleaving risk. Order: sync-output OFF → cursor visible → SGR reset → flush.
+            // Must happen before termios restore (via _restoreOnExit) so the bytes reach
+            // the terminal while it is still in its current mode.
+            // A broken-pipe or disposed-stdout fault must NOT skip the termios restore below —
+            // ANSI emission is best-effort; termios restore is the load-bearing guarantee.
+            try { _sink.EmitRestoreSequence(); }
+            catch (Exception ansiFault) when (ansiFault is IOException or ObjectDisposedException) { }
+
+            // Restore the terminal regardless of how the loop exits (normal stop, exception,
+            // or external cancellation). The action is idempotent — Dispose, signals, and
+            // ProcessExit all converge on the same CAS-guarded IRawModeSession.Restore.
+            // Catch broadly: at shutdown, a swallowed exception here is preferable to crashing
+            // into a still-raw terminal.
+#pragma warning disable CA1031 // shutdown finally: broad catch intentional; see comment above
+            try { _restoreOnExit?.Invoke(); }
+            catch (Exception) { }
+#pragma warning restore CA1031
+
+            // Signal loop-thread exit. Fault if the loop crashed; succeed on clean shutdown.
+            // RunContinuationsAsynchronously ensures awaiters do not resume on the loop thread.
+            if (loopFault is not null)
+                _loopTerminated.TrySetException(loopFault);
+            else
+                _loopTerminated.TrySetResult();
+        }
+    }
+
+    /// <summary>Applies one inbound message to the render model. Loop-thread only.</summary>
+    private void ApplyMessage(LoopMessage msg)
+    {
+        switch (msg)
+        {
+            case InputMessage inputMsg:
+                ApplyInputEvent(inputMsg.Event);
+                break;
+
+            case CommandMessage cmdMsg:
+                cmdMsg.Command.Apply(_model);
+                break;
+        }
+    }
+
+    /// <summary>
+    /// Routes an input event to the editor or falls through to the outbound channel.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// <strong>Editing keys</strong> (consumed — mutate the editor, mark dirty, do NOT emit):
+    /// printable Unicode scalars → <c>Insert</c>; Backspace → <c>Backspace</c>;
+    /// Delete → <c>Delete</c>; Left/Right → <c>MoveLeft/MoveRight</c>;
+    /// Home/End → <c>MoveHome/MoveEnd</c>; Up/Down → history navigation when the caret is on
+    /// the first/last visual row respectively, otherwise <c>MoveUp/MoveDown</c>.
+    /// </para>
+    /// <para>
+    /// <strong>Fall-through keys</strong> (not consumed here — emitted on the outbound channel):
+    /// Enter, Tab, Ctrl+*, Alt+*, and any key not listed above. §12 handles submit-on-Enter,
+    /// completion, and the full event story.
+    /// </para>
+    /// <para>
+    /// <strong>History-nav convention:</strong> Up navigates to the previous (older) history
+    /// entry when the caret is on the <em>first</em> visual row; otherwise moves the caret up
+    /// within the editor. Down navigates to the next (newer) entry when the caret is on the
+    /// <em>last</em> visual row; otherwise moves the caret down.
+    /// </para>
+    /// </remarks>
+    private void ApplyInputEvent(InputEvent ev)
+    {
+        _model.MarkDirty();
+
+        switch (ev)
+        {
+            case KeyEvent ke:
+                bool consumed = false;
+
+                // Intercept-chain: active overlay is the FRONT (first refusal).
+                if (_model.ActiveOverlay is { } overlay)
+                {
+                    consumed = overlay.HandleKey(ke);
+                    if (overlay.IsDismissed)
+                    {
+                        // Invoke the pending completion before clearing so the TCS is resolved
+                        // while the overlay state (CloseRequest and any selection fields) is still
+                        // readable. ClearOverlay() nulls both the overlay and the delegate.
+                        _model.PendingModalCompletion?.Invoke();
+                        _model.ClearOverlay();
+                    }
+                }
+
+                if (!consumed)
+                {
+                    // Capture the editor text before routing so we can detect changes.
+                    // Only relevant when no overlay consumed the key (overlay edits don't
+                    // drive InputChanged — they have their own semantics).
+                    TextBuffer editor = _model.FixedRegion.Editor;
+                    string textBefore = editor.Text;
+
+                    consumed = RouteEditorKey(ke);
+
+                    // Emit InputChanged when the editor text changed due to user input.
+                    // This covers Insert, Backspace, Delete, and history recall (Up/Down),
+                    // but excludes pure caret movement (Left/Right/Home/End/MoveUp/MoveDown).
+                    // PasteEvent is not currently routed to the editor — wiring paste is out of
+                    // Chunk C scope; paste would need a separate InputChanged trigger when added.
+                    if (consumed && editor.Text != textBefore)
+                        _outbound.Writer.TryWrite(new InputChanged(editor.Text));
+                }
+
+                if (!consumed)
+                {
+                    // Plain Enter with no Ctrl/Alt modifier and no overlay: submit the line.
+                    // Behaviour:
+                    //   - Emits InputSubmitted(text) — always, even for an empty buffer.
+                    //   - Adds the text to history (skips empty entries).
+                    //   - Clears the editor (readline-style: the buffer is empty after submit).
+                    //   - Does NOT also emit KeyPressed(Enter) — submit is consumed here.
+                    // Modified Enter (Ctrl+Enter, Alt+Enter) falls through as KeyPressed
+                    // because RouteEditorKey already returns false for Ctrl/Alt modifiers.
+                    if (ke.Code.Kind == KeyCode.KeyCodeKind.Named &&
+                        ke.Code.NamedValue == NamedKey.Enter &&
+                        ke.Modifiers == Modifiers.None)
+                    {
+                        TextBuffer editor = _model.FixedRegion.Editor;
+                        string submittedText = editor.Text;
+                        _outbound.Writer.TryWrite(new InputSubmitted(submittedText));
+                        if (submittedText.Length > 0)
+                            editor.AddToHistory(submittedText);
+                        editor.Clear();
+                        // MarkDirty already called at the top of ApplyInputEvent.
+                    }
+                    else
+                    {
+                        _outbound.Writer.TryWrite(new KeyPressed(ke));
+                    }
+                }
+                break;
+
+            case ResizeEvent re:
+                _model.UpdateSize(re.Columns, re.Rows);
+                _snapshotColumns = re.Columns;
+                _snapshotRows = re.Rows;
+                _outbound.Writer.TryWrite(new Resized(re.Columns, re.Rows));
+                break;
+        }
+    }
+
+    /// <summary>
+    /// Attempts to route <paramref name="ke"/> to the input editor.
+    /// Returns <see langword="true"/> when the key was consumed (editor mutated or navigation
+    /// performed); <see langword="false"/> when the key should fall through to the outbound channel.
+    /// </summary>
+    private bool RouteEditorKey(KeyEvent ke)
+    {
+        // Keys with Ctrl or Alt modifiers fall through (§12 owns submit / completion / etc.).
+        if ((ke.Modifiers & (Modifiers.Ctrl | Modifiers.Alt)) != Modifiers.None)
+            return false;
+
+        int width = _model.Columns;
+        var editor = _model.FixedRegion.Editor;
+
+        if (ke.Code.Kind == KeyCode.KeyCodeKind.UnicodeScalar)
+        {
+            Rune rune = ke.Code.RuneValue;
+            // Only insert displayable characters — skip control characters (U+0000..U+001F, U+007F).
+            if (rune.Value >= 0x20 && rune.Value != 0x7F)
+            {
+                editor.Insert(rune);
+                return true;
+            }
+            return false;
+        }
+
+        if (ke.Code.Kind == KeyCode.KeyCodeKind.Named)
+        {
+            switch (ke.Code.NamedValue)
+            {
+                case NamedKey.Backspace:
+                    editor.Backspace();
+                    return true;
+
+                case NamedKey.Delete:
+                    editor.Delete();
+                    return true;
+
+                case NamedKey.Left:
+                    editor.MoveLeft();
+                    return true;
+
+                case NamedKey.Right:
+                    editor.MoveRight();
+                    return true;
+
+                case NamedKey.Home:
+                    editor.MoveHome(width);
+                    return true;
+
+                case NamedKey.End:
+                    editor.MoveEnd(width);
+                    return true;
+
+                case NamedKey.Up:
+                    {
+                        (bool isFirst, _) = editor.GetCaretVisualRowBounds(width);
+                        if (isFirst)
+                            editor.RecallPrevious();
+                        else
+                            editor.MoveUp(width);
+                        return true;
+                    }
+
+                case NamedKey.Down:
+                    {
+                        (_, bool isLast) = editor.GetCaretVisualRowBounds(width);
+                        if (isLast)
+                            editor.RecallNext();
+                        else
+                            editor.MoveDown(width);
+                        return true;
+                    }
+
+                // Enter, Tab, and all others fall through to §12.
+                default:
+                    return false;
+            }
+        }
+
+        return false;
+    }
+
+    /// <summary>
+    /// Signals all pending settle TCSes. Called after each paint attempt on the loop thread.
+    /// </summary>
+    private void DrainSettles()
+    {
+        while (_pendingSettles.TryDequeue(out TaskCompletionSource? tcs))
+        {
+            tcs.TrySetResult();
+        }
+    }
+
+    /// <summary>
+    /// Faults all pending settle TCSes with <paramref name="fault"/>. Called from the
+    /// catch block when the loop dies so no settle awaiter hangs forever. Also drains
+    /// any <see cref="SettleCommand"/>s still in the inbound channel (i.e. enqueued after
+    /// the last drain but before the exception propagated) and faults them too.
+    /// </summary>
+    private void FaultSettles(Exception fault)
+    {
+        // Fault TCSes that were already moved to the pending queue.
+        while (_pendingSettles.TryDequeue(out TaskCompletionSource? tcs))
+        {
+            tcs.TrySetException(fault);
+        }
+
+        // Also drain the inbound channel for SettleCommands that arrived after the last
+        // drain but before the exception fired (e.g. enqueued while Paint was blocking).
+        while (_inbound.Reader.TryRead(out LoopMessage? msg))
+        {
+            if (msg is CommandMessage { Command: SettleCommand settle })
+                settle.FaultExternally(fault);
+        }
+    }
+
+    // ── Blocking wait ─────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Blocks the loop thread until a message is available on the inbound channel, or —
+    /// when <paramref name="waitForDeadline"/> is <see langword="true"/> — until
+    /// <paramref name="deadline"/> is reached via <see cref="IClock.WaitUntilAsync"/>.
+    /// Returns <see langword="true"/> when a message is (likely) available or the deadline
+    /// was reached; <see langword="false"/> on cancellation or channel completion.
+    /// </summary>
+    /// <param name="deadline">The next paint deadline (only used when <paramref name="waitForDeadline"/> is true).</param>
+    /// <param name="waitForDeadline">
+    /// <see langword="true"/> when the model is dirty and a deadline wake is needed;
+    /// <see langword="false"/> when the model is clean — parks indefinitely on the channel.
+    /// </param>
+    /// <param name="ct">Cancellation token.</param>
+    /// <remarks>
+    /// <para>
+    /// Clean-idle path: <c>WaitToReadAsync().AsTask().GetAwaiter().GetResult()</c> parks the
+    /// OS thread in the kernel wait until the channel is signalled — zero CPU while idle.
+    /// </para>
+    /// <para>
+    /// Dirty path: <c>Task.WaitAny</c> races the channel wait against
+    /// <c>IClock.WaitUntilAsync(deadline)</c>. The deadline goes through <see cref="IClock"/>
+    /// so the virtual-clock test double can advance time deterministically without real time passing.
+    /// </para>
+    /// <para>
+    /// Both paths are synchronous blocking calls on the loop thread — no <c>await</c>, no
+    /// thread-pool continuation, no mutation on any thread other than the dedicated loop thread.
+    /// </para>
+    /// </remarks>
+    private bool BlockUntilMessageOrDeadline(TimeSpan deadline, bool waitForDeadline, CancellationToken ct)
+    {
+        if (ct.IsCancellationRequested || _inbound.Reader.Completion.IsCompleted)
+            return false;
+
+        // Fast path: message already waiting.
+        if (_inbound.Reader.TryPeek(out _))
+            return true;
+
+        Interlocked.Increment(ref _waitCount);
+
+        if (!waitForDeadline)
+        {
+            // Model is clean — no paint pending. Park on the channel until a message arrives.
+            // GetResult() blocks the OS thread in the kernel; zero CPU while idle.
+            try
+            {
+                bool hasData = _inbound.Reader.WaitToReadAsync(ct).AsTask().GetAwaiter().GetResult();
+                return hasData;
+            }
+            catch (OperationCanceledException)
+            {
+                return false;
+            }
+        }
+        else
+        {
+            // Model is dirty — race the channel wait against the paint deadline.
+            // The deadline goes through IClock so the virtual clock controls cadence in tests.
+            //
+            // A per-iteration linked CTS lets us cancel the losing task's waiter so it
+            // doesn't orphan a live Task.Delay / VirtualClock TCS registration.
+            // The linked CTS is disposed (which cancels it) in the finally block.
+            using CancellationTokenSource iterationCts =
+                CancellationTokenSource.CreateLinkedTokenSource(ct);
+
+            Task waitToReadTask = _inbound.Reader.WaitToReadAsync(iterationCts.Token).AsTask();
+            Task deadlineTask = _clock.WaitUntilAsync(deadline, iterationCts.Token);
+
+            try
+            {
+                // WaitAny unblocks when either task completes (including via cancellation).
+                // Cancellation propagates through the tasks themselves (iterationCts.Token is
+                // passed to both); passing CancellationToken.None here avoids a redundant
+                // OperationCanceledException throw from WaitAny itself.
+                Task.WaitAny([waitToReadTask, deadlineTask], CancellationToken.None);
+            }
+            finally
+            {
+                // Cancel the losing waiter. The tasks swallow OperationCanceledException
+                // internally, so no unobserved-task warning is produced.
+                iterationCts.Cancel();
+            }
+
+            // Cancellation or channel completion handled by the next iteration check.
+            return !ct.IsCancellationRequested && !_inbound.Reader.Completion.IsCompleted;
+        }
+    }
+
+    // ── SettleCommand ──────────────────────────────────────────────────────────
+
+    private sealed class SettleCommand : ILoopCommand
+    {
+        private readonly ConcurrentQueue<TaskCompletionSource> _settleQueue;
+        private readonly TaskCompletionSource _tcs;
+
+        internal SettleCommand(ConcurrentQueue<TaskCompletionSource> settleQueue, TaskCompletionSource tcs)
+        {
+            _settleQueue = settleQueue;
+            _tcs = tcs;
+        }
+
+        void ILoopCommand.Apply(RenderModel model)
+        {
+            // Mark the model dirty so the loop evaluates the paint condition after draining.
+            model.MarkDirty();
+            // Enqueue the TCS; it will be signaled by DrainSettles() after the paint.
+            _settleQueue.Enqueue(_tcs);
+        }
+
+        /// <summary>
+        /// Faults the settle TCS directly. Called when the loop crashes before this command
+        /// was applied (i.e. it was still in the inbound channel at crash time).
+        /// </summary>
+        internal void FaultExternally(Exception fault) => _tcs.TrySetException(fault);
+    }
+
+    // ── InputWrappingWriter ────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Adapts <see cref="ChannelWriter{LoopMessage}"/> to <see cref="ChannelWriter{InputEvent}"/>
+    /// by wrapping each event in an <see cref="InputMessage"/>.
+    /// </summary>
+    private sealed class InputWrappingWriter : ChannelWriter<InputEvent>
+    {
+        private readonly ChannelWriter<LoopMessage> _inner;
+
+        internal InputWrappingWriter(ChannelWriter<LoopMessage> inner) => _inner = inner;
+
+        public override bool TryWrite(InputEvent item) =>
+            _inner.TryWrite(new InputMessage(item));
+
+        public override ValueTask<bool> WaitToWriteAsync(CancellationToken cancellationToken = default) =>
+            _inner.WaitToWriteAsync(cancellationToken);
+
+        public override ValueTask WriteAsync(InputEvent item, CancellationToken cancellationToken = default) =>
+            _inner.WriteAsync(new InputMessage(item), cancellationToken);
+    }
+}
diff --git a/src/Dcli/Internal/RenderLoop/LoopMessage.cs b/src/Dcli/Internal/RenderLoop/LoopMessage.cs
new file mode 100644
index 0000000..a3a23c0
--- /dev/null
+++ b/src/Dcli/Internal/RenderLoop/LoopMessage.cs
@@ -0,0 +1,55 @@
+using Dcli.Internal.Input;
+
+namespace Dcli.Internal.RenderLoop;
+
+// ─────────────────────────────────────────────────────────────────────────────
+// Inbound message union
+//
+// All mutations of the render model flow through this FIFO channel. Input events
+// from InputReader and API commands from the consumer share one channel so ordering
+// is deterministic and both are reflected in the same coalesced frame.
+// ─────────────────────────────────────────────────────────────────────────────
+
+/// <summary>
+/// Discriminated union of all messages that enter the render loop via the inbound channel.
+/// The loop applies each message on its own dedicated thread in FIFO order.
+/// </summary>
+/// <remarks>
+/// Two families:
+/// <list type="bullet">
+///   <item><see cref="InputMessage"/> — wraps a decoded <see cref="InputEvent"/> from §6.</item>
+///   <item><see cref="ILoopCommand"/> — API commands from the consumer (§9–§12 add commands
+///     by implementing this interface; the loop does not need to be modified).</item>
+/// </list>
+/// </remarks>
+internal abstract record LoopMessage;
+
+/// <summary>
+/// Wraps a decoded input event from the §6 <see cref="InputReader"/> for
+/// delivery to the render loop thread.
+/// </summary>
+/// <param name="Event">The decoded input event.</param>
+internal sealed record InputMessage(InputEvent Event) : LoopMessage;
+
+/// <summary>
+/// A command issued by the consumer via the fire-and-forget API.
+/// </summary>
+/// <remarks>
+/// Implement this interface to add new API commands (§9–§12) without modifying the loop.
+/// <see cref="Apply"/> is always called on the render loop thread so implementors may mutate
+/// the render model directly without locks.
+/// </remarks>
+internal interface ILoopCommand
+{
+    /// <summary>
+    /// Applies the command to <paramref name="model"/>.
+    /// Called exclusively on the render loop thread; no synchronization is required.
+    /// </summary>
+    void Apply(RenderModel model);
+}
+
+/// <summary>
+/// Wraps an <see cref="ILoopCommand"/> as a <see cref="LoopMessage"/> for inbound-channel delivery.
+/// </summary>
+/// <param name="Command">The command to apply.</param>
+internal sealed record CommandMessage(ILoopCommand Command) : LoopMessage;
diff --git a/src/Dcli/Internal/RenderLoop/NoopOutputSink.cs b/src/Dcli/Internal/RenderLoop/NoopOutputSink.cs
new file mode 100644
index 0000000..d6ad4f0
--- /dev/null
+++ b/src/Dcli/Internal/RenderLoop/NoopOutputSink.cs
@@ -0,0 +1,24 @@
+namespace Dcli.Internal.RenderLoop;
+
+/// <summary>
+/// A no-op <see cref="IOutputSink"/> used as the real stdout placeholder until §8
+/// implements actual escape-sequence painting.
+/// </summary>
+/// <remarks>
+/// Swapped out in §8 for the real frame renderer. Writing nothing is safe — it prevents
+/// garbage being emitted to a real terminal while the painting layer is not yet built.
+/// </remarks>
+internal sealed class NoopOutputSink : IOutputSink
+{
+    /// <inheritdoc/>
+    public void Paint(RenderModel model)
+    {
+        // §8 replaces this with real VT frame painting.
+    }
+
+    /// <inheritdoc/>
+    public void EmitRestoreSequence()
+    {
+        // No-op: this sink writes nothing to stdout.
+    }
+}
diff --git a/src/Dcli/Internal/RenderLoop/OpenDialogCommand.cs b/src/Dcli/Internal/RenderLoop/OpenDialogCommand.cs
new file mode 100644
index 0000000..98a1413
--- /dev/null
+++ b/src/Dcli/Internal/RenderLoop/OpenDialogCommand.cs
@@ -0,0 +1,111 @@
+using Dcli.Internal.FixedRegion;
+
+namespace Dcli.Internal.RenderLoop;
+
+/// <summary>
+/// Loop command that opens a modal overlay and registers a parameterless completion delegate.
+/// </summary>
+/// <remarks>
+/// <para>
+/// The command carries a prepared <see cref="IModalOverlay"/> and a closure-captured
+/// parameterless <see cref="Action"/> that captures the overlay reference, reads its outcome on
+/// the loop thread, and completes the caller's <see cref="TaskCompletionSource{T}"/>. The
+/// generic TCS is captured by the closure so the inbound channel remains untyped.
+/// </para>
+/// <para>
+/// <strong>Reject rule:</strong> if any <see cref="IModalOverlay"/> is already active when this
+/// command applies, the call's task is faulted with <see cref="InvalidOperationException"/>.
+/// An active <see cref="Autocomplete"/> is suppressed (replaced) — modal overlays take priority.
+/// </para>
+/// </remarks>
+internal sealed class OpenDialogCommand : ILoopCommand
+{
+    private readonly IModalOverlay _overlay;
+    private readonly Action _completion;
+    private readonly Action _reject;
+
+    /// <summary>
+    /// Initialises the command.
+    /// </summary>
+    /// <param name="overlay">The prepared modal overlay.</param>
+    /// <param name="completion">
+    /// Parameterless delegate invoked on the loop thread when the overlay is dismissed.
+    /// The closure captures the overlay and reads its outcome to complete the caller's TCS.
+    /// </param>
+    /// <param name="reject">
+    /// Delegate invoked when an <see cref="IModalOverlay"/> is already active. Faults the
+    /// caller's TCS with <see cref="InvalidOperationException"/>.
+    /// </param>
+    internal OpenDialogCommand(IModalOverlay overlay, Action completion, Action reject)
+    {
+        _overlay = overlay;
+        _completion = completion;
+        _reject = reject;
+    }
+
+    /// <inheritdoc/>
+    void ILoopCommand.Apply(RenderModel model)
+    {
+        if (model.ActiveOverlay is IModalOverlay)
+        {
+            // A modal overlay is already active — reject the new open request.
+            _reject();
+            return;
+        }
+
+        // ShowModal unconditionally assigns ActiveOverlay, suppressing any active Autocomplete.
+        model.ShowModal(_overlay, _completion);
+
+        // Seed the real terminal width into InputDialog so that width-dependent key handling
+        // (Home/End/Up/Down) is correct for keys arriving in the same batch as this open command,
+        // before the first Render call has a chance to update _lastWidth.
+        if (_overlay is InputDialog inputDialog)
+            inputDialog.SeedWidth(model.Columns);
+
+        model.MarkDirty();
+    }
+}
+
+/// <summary>
+/// Loop command that cancels the active modal overlay if it is still the expected one.
+/// </summary>
+/// <remarks>
+/// Posted by a <see cref="CancellationToken"/> registration when the token is cancelled while
+/// a modal overlay is open. If the overlay has already been dismissed (naturally or by another
+/// cancel), this is a no-op.
+/// </remarks>
+internal sealed class CancelDialogCommand : ILoopCommand
+{
+    private readonly IModalOverlay _expectedOverlay;
+    private readonly Action _cancelCompletion;
+
+    /// <summary>
+    /// Initialises the command.
+    /// </summary>
+    /// <param name="expectedOverlay">
+    /// The modal overlay this cancel is intended for. If the active overlay is a different object,
+    /// the command is a no-op (another overlay may have opened after this one closed).
+    /// </param>
+    /// <param name="cancelCompletion">
+    /// Action that completes the caller's TCS with <see cref="DialogOutcome.Cancelled"/> and
+    /// disposes the CancellationToken registration to prevent leaks. Invoked on the loop thread.
+    /// </param>
+    internal CancelDialogCommand(IModalOverlay expectedOverlay, Action cancelCompletion)
+    {
+        _expectedOverlay = expectedOverlay;
+        _cancelCompletion = cancelCompletion;
+    }
+
+    /// <inheritdoc/>
+    void ILoopCommand.Apply(RenderModel model)
+    {
+        // Only act if the exact overlay we were created for is still the active overlay.
+        if (!ReferenceEquals(model.ActiveOverlay, _expectedOverlay))
+            return;
+
+        // Invoke the completion (completes TCS with Cancelled) before clearing.
+        _cancelCompletion();
+        model.ClearOverlay();
+        model.MarkDirty();
+    }
+}
diff --git a/src/Dcli/Internal/RenderLoop/RenderModel.cs b/src/Dcli/Internal/RenderLoop/RenderModel.cs
new file mode 100644
index 0000000..767b2fd
--- /dev/null
+++ b/src/Dcli/Internal/RenderLoop/RenderModel.cs
@@ -0,0 +1,208 @@
+using Dcli.Internal.FixedRegion;
+using Dcli.Internal.Scrollback;
+
+namespace Dcli.Internal.RenderLoop;
+
+/// <summary>
+/// Mutable UI state owned exclusively by the render loop thread.
+/// </summary>
+/// <remarks>
+/// <para>
+/// <strong>Thread discipline:</strong> this object must only ever be read or written on the
+/// render loop thread — the single dedicated thread running <see cref="LoopEngine"/>.
+/// No locks are needed because the single-writer loop is the only mutator.
+/// </para>
+/// <para>
+/// <strong>Paint-state fields</strong> (<see cref="LiveWindowRows"/>, <see cref="FixedRegionRows"/>,
+/// <see cref="CaretPosition"/>, <see cref="IsCursorVisible"/>, <see cref="NewlyCommittedRows"/>):
+/// populated by §9 (scrollback) and §10 (fixed region). §8's <see cref="VtFrameRenderer"/> reads
+/// them directly — the painter emits whatever the model holds. Tests set them directly to produce
+/// deterministic golden frames.
+/// </para>
+/// <para>
+/// <strong>Dirty flag:</strong> the loop sets <see cref="IsDirty"/> to <see langword="true"/>
+/// whenever any state changes (command applied, input event processed). The flag is cleared
+/// after each paint. A frame is only produced when dirty AND the minimum frame interval has
+/// elapsed, giving the throttled-coalescing cadence.
+/// </para>
+/// <para>
+/// <strong>Terminal size snapshot:</strong> <see cref="Columns"/> and <see cref="Rows"/> are
+/// read by consumers via a volatile snapshot (Decision 10 — "value reads served from snapshot,
+/// not round-trip"). The snapshot is initialised from the injected
+/// <see cref="ITerminalSizeSource"/> and updated by resize messages. Resize events flow in
+/// via <c>PosixResizeWatcher</c> (§13.1).
+/// </para>
+/// <para>
+/// <strong>MaxFixedHeight:</strong> recorded from <see cref="TerminalOptions.MaxFixedHeight"/>
+/// but not yet enforced. §10 will apply the fixed-region MaxHeight budget
+/// (<c>clamp(appSet ?? 50%, 8, rows)</c>) when the fixed-region painter is implemented.
+/// </para>
+/// </remarks>
+internal sealed class RenderModel
+{
+    /// <summary>
+    /// Initialises the render model with the terminal dimensions reported by
+    /// <paramref name="sizeSource"/>.
+    /// </summary>
+    /// <param name="sizeSource">Provides the initial terminal dimensions.</param>
+    /// <param name="maxFixedHeight">
+    /// Optional consumer-supplied cap on the fixed-region height (rows). Recorded here;
+    /// §10's painter will enforce it.
+    /// </param>
+    internal RenderModel(ITerminalSizeSource sizeSource, int? maxFixedHeight = null)
+    {
+        ArgumentNullException.ThrowIfNull(sizeSource);
+        (Columns, Rows) = sizeSource.GetSize();
+        MaxFixedHeight = maxFixedHeight;
+    }
+
+    /// <summary>
+    /// Whether any mutation has occurred since the last paint. Cleared by
+    /// <see cref="ClearDirty"/>; set by callers applying commands or input events.
+    /// </summary>
+    internal bool IsDirty { get; private set; }
+
+    /// <summary>Current terminal width in columns (from the latest size snapshot).</summary>
+    internal int Columns { get; private set; }
+
+    /// <summary>Current terminal height in rows (from the latest size snapshot).</summary>
+    internal int Rows { get; private set; }
+
+    /// <summary>
+    /// Consumer-supplied cap on the fixed-region height in rows.
+    /// <see langword="null"/> means no explicit cap. Recorded but not yet enforced;
+    /// §10 will apply the full <c>clamp(appSet ?? 50%, 8, rows)</c> budget.
+    /// </summary>
+    internal int? MaxFixedHeight { get; }
+
+    // ── Paint-state (loop-thread-owned, no locks) ────────────────────────────
+    // Populated by §9 (scrollback) and §10 (fixed region).
+    // The painter reads them verbatim; §8 tests set them directly.
+
+    /// <summary>
+    /// Pre-wrapped visual rows for the live window, already fitted to <see cref="Columns"/>.
+    /// Each row is a list of styled segments. §9 fills this; painter emits them as-is.
+    /// </summary>
+    internal IReadOnlyList<Line> LiveWindowRows { get; set; } = [];
+
+    /// <summary>
+    /// Pre-composed rows for the fixed region (input bar, status line, overlays).
+    /// Each row is a list of styled segments. §10/§11 fill this; painter emits them as-is.
+    /// </summary>
+    internal IReadOnlyList<Line> FixedRegionRows { get; set; } = [];
+
+    /// <summary>
+    /// Desired hardware-cursor position within the frame (zero-based, relative to the first
+    /// row of the frame). <see langword="null"/> defers to <see cref="IsCursorVisible"/>.
+    /// §10 sets this to the input-editor caret.
+    /// </summary>
+    internal (int Row, int Col)? CaretPosition { get; set; }
+
+    /// <summary>
+    /// Whether the hardware cursor should be visible at end of frame.
+    /// <see langword="true"/> (default) = place cursor at <see cref="CaretPosition"/> and show it.
+    /// <see langword="false"/> = hide cursor (modal-dialog case). §11 drives this via
+    /// <see cref="ActiveOverlay"/>.<see cref="IOverlay.HidesCursor"/>.
+    /// </summary>
+    internal bool IsCursorVisible { get; set; } = true;
+
+    /// <summary>
+    /// Rows that became committed (scrolled above the anchor) this frame.
+    /// §9's <see cref="ScrollbackModel.PrePaint"/> fills this before each paint;
+    /// <see cref="ScrollbackModel.ClearCommitted"/> resets it to empty after the frame so
+    /// committed rows are emitted exactly once and never rewritten.
+    /// </summary>
+    internal IReadOnlyList<Line> NewlyCommittedRows { get; set; } = [];
+
+    /// <summary>
+    /// The scrollback model (§9). Owned by the render loop; called from the loop's
+    /// apply→paint cycle via <see cref="ScrollbackModel.PrePaint"/> and
+    /// <see cref="ScrollbackModel.ClearCommitted"/>.
+    /// </summary>
+    internal ScrollbackModel Scrollback { get; } = new();
+
+    /// <summary>
+    /// The fixed-region composer (§10). Populates <see cref="FixedRegionRows"/> and
+    /// <see cref="EditorCaretLocal"/> each paint cycle, before scrollback PrePaint.
+    /// </summary>
+    internal FixedRegionComposer FixedRegion { get; } = new(new TextBuffer(), new StatusLine());
+
+    /// <summary>
+    /// The editor-relative caret position set by <see cref="FixedRegionComposer.Compose"/>:
+    /// (row within the visible input rows, display col). <see langword="null"/> when the caret
+    /// is hidden (degenerate tiny-terminal or modal dialog — §11). The loop thread offsets the
+    /// row by <see cref="LiveWindowRows"/>.Count after scrollback PrePaint to produce
+    /// <see cref="CaretPosition"/>.
+    /// </summary>
+    internal (int Row, int Col)? EditorCaretLocal { get; set; }
+
+    // ── Active overlay (§11) ──────────────────────────────────────────────────
+
+    /// <summary>
+    /// The currently active overlay, or <see langword="null"/> when none is shown.
+    /// At most one overlay is active at a time (the OverlayState invariant).
+    /// <see cref="FixedRegionComposer.Compose"/> slots this into the fixed region each frame.
+    /// </summary>
+    internal IOverlay? ActiveOverlay { get; private set; }
+
+    /// <summary>
+    /// Shows an autocomplete overlay. No-op when a modal overlay (<see cref="IModalOverlay"/>) is
+    /// already active (modal takes priority — opening autocomplete under a modal dialog would
+    /// violate the single-active-overlay invariant and the modal-suppresses-autocomplete rule).
+    /// </summary>
+    internal void ShowAutocomplete(Autocomplete autocomplete)
+    {
+        if (ActiveOverlay is IModalOverlay)
+            return;
+        ActiveOverlay = autocomplete;
+    }
+
+    /// <summary>
+    /// Shows a modal overlay and registers its parameterless completion delegate.
+    /// Suppresses any active autocomplete (modal takes priority).
+    /// </summary>
+    /// <param name="overlay">The modal overlay to show.</param>
+    /// <param name="completion">
+    /// Called on the loop thread when the overlay is dismissed. The closure captures the overlay
+    /// reference and reads its state (<see cref="IModalOverlay.CloseRequest"/> and any other
+    /// overlay-specific fields) to complete the caller's <see cref="System.Threading.Tasks.TaskCompletionSource{T}"/>.
+    /// </param>
+    internal void ShowModal(IModalOverlay overlay, Action? completion = null)
+    {
+        ActiveOverlay = overlay;
+        PendingModalCompletion = completion;
+    }
+
+    /// <summary>
+    /// The loop-thread-owned completion delegate for the currently active modal overlay.
+    /// Set by <see cref="ShowModal"/> alongside the overlay; cleared by <see cref="ClearOverlay"/>.
+    /// Invoked by the loop's dismiss hook before clearing the overlay.
+    /// </summary>
+    internal Action? PendingModalCompletion { get; private set; }
+
+    /// <summary>
+    /// Clears the active overlay and its pending completion delegate.
+    /// Called by the loop after <see cref="IOverlay.IsDismissed"/> becomes <see langword="true"/>
+    /// (autocomplete hidden, modal overlay Enter/Escape, or external token cancellation).
+    /// </summary>
+    internal void ClearOverlay()
+    {
+        ActiveOverlay = null;
+        PendingModalCompletion = null;
+    }
+
+    // ── Dirtyness ─────────────────────────────────────────────────────────────
+
+    /// <summary>Marks the model as dirty so the loop will produce a frame.</summary>
+    internal void MarkDirty() => IsDirty = true;
+
+    /// <summary>Clears the dirty flag after a frame has been painted.</summary>
+    internal void ClearDirty() => IsDirty = false;
+
+    /// <summary>Updates the terminal size in the model.</summary>
+    internal void UpdateSize(int columns, int rows)
+    {
+        Columns = columns;
+        Rows = rows;
+    }
+}
diff --git a/src/Dcli/Internal/RenderLoop/SgrTranslator.cs b/src/Dcli/Internal/RenderLoop/SgrTranslator.cs
new file mode 100644
index 0000000..9654bc6
--- /dev/null
+++ b/src/Dcli/Internal/RenderLoop/SgrTranslator.cs
@@ -0,0 +1,212 @@
+using System.Text;
+
+namespace Dcli.Internal.RenderLoop;
+
+/// <summary>
+/// Translates a <see cref="Style"/> to an ANSI SGR escape sequence string.
+/// </summary>
+/// <remarks>
+/// Callers: <see cref="VtFrameRenderer"/> on the loop thread; unit tests directly.
+/// When <see cref="TerminalCapabilities.HasTruecolor"/> is <see langword="false"/>, RGB colors
+/// are downgraded to the nearest xterm 256-indexed palette entry.
+/// </remarks>
+internal sealed class SgrTranslator
+{
+    // Named-color offset tables (base + bright).
+    // Named colors 0–7 → 30–37 fg / 40–47 bg.
+    // Named colors 8–15 → 90–97 fg / 100–107 bg.
+    private const int _fgBase = 30;
+    private const int _bgBase = 40;
+    private const int _fgBrightBase = 90;
+    private const int _bgBrightBase = 100;
+
+    // xterm 256-color cube: indices 16–231 are a 6×6×6 RGB cube.
+    // Cube-level RGB values at each of the 6 steps:
+    private static readonly int[] _cubeSteps = [0, 95, 135, 175, 215, 255];
+
+    private readonly TerminalCapabilities _capabilities;
+
+    /// <summary>Returns an SGR reset sequence (<c>ESC[0m</c>).</summary>
+    internal const string Reset = "\x1b[0m";
+
+    /// <summary>
+    /// Initialises the translator with the given capabilities.
+    /// </summary>
+    /// <param name="capabilities">
+    /// Capabilities detected from the environment. Pass <see cref="TerminalCapabilities.Default"/>
+    /// in headless tests.
+    /// </param>
+    internal SgrTranslator(TerminalCapabilities capabilities)
+    {
+        _capabilities = capabilities;
+    }
+
+    /// <summary>
+    /// Appends the opening SGR codes for <paramref name="style"/> to <paramref name="sb"/>.
+    /// Writes nothing when the style is entirely default (no-op fast path).
+    /// </summary>
+    /// <param name="style">The style to translate.</param>
+    /// <param name="sb">Destination builder.</param>
+    /// <returns>
+    /// <see langword="true"/> if any SGR codes were appended (caller should emit
+    /// <see cref="Reset"/> after the styled run); <see langword="false"/> when the style is
+    /// the default (no SGR emitted, no reset needed).
+    /// </returns>
+    internal bool AppendOpenSgr(Style style, StringBuilder sb)
+    {
+        // Fast path: nothing to encode.
+        if (style.Foreground is null && style.Background is null && style.Format == Format.None)
+            return false;
+
+        sb.Append("\x1b[");
+
+        bool needSemi = false;
+
+        // Format flags — emit in a defined order so sequences are deterministic.
+        if (style.Format.HasFlag(Format.Bold)) { AppendCode(sb, 1, ref needSemi); }
+        if (style.Format.HasFlag(Format.Dim)) { AppendCode(sb, 2, ref needSemi); }
+        if (style.Format.HasFlag(Format.Italic)) { AppendCode(sb, 3, ref needSemi); }
+        if (style.Format.HasFlag(Format.Underline)) { AppendCode(sb, 4, ref needSemi); }
+        if (style.Format.HasFlag(Format.Reverse)) { AppendCode(sb, 7, ref needSemi); }
+        if (style.Format.HasFlag(Format.Strikethrough)) { AppendCode(sb, 9, ref needSemi); }
+
+        // Foreground color.
+        if (style.Foreground.HasValue)
+            AppendColorCode(sb, style.Foreground.Value, isForeground: true, ref needSemi);
+
+        // Background color.
+        if (style.Background.HasValue)
+            AppendColorCode(sb, style.Background.Value, isForeground: false, ref needSemi);
+
+        sb.Append('m');
+        return true;
+    }
+
+    /// <summary>
+    /// Returns the complete SGR opening sequence for <paramref name="style"/>, or an empty string
+    /// when the style is the default. Convenience wrapper around <see cref="AppendOpenSgr"/>.
+    /// </summary>
+    internal string ToOpenSgr(Style style)
+    {
+        StringBuilder sb = new();
+        AppendOpenSgr(style, sb);
+        return sb.ToString();
+    }
+
+    // ── Private helpers ───────────────────────────────────────────────────────
+
+    private static void AppendCode(StringBuilder sb, int code, ref bool needSemi)
+    {
+        if (needSemi) sb.Append(';');
+        sb.Append(code);
+        needSemi = true;
+    }
+
+    private void AppendColorCode(StringBuilder sb, Color color, bool isForeground, ref bool needSemi)
+    {
+        switch (color.Kind)
+        {
+            case Color.ColorKind.Named:
+                {
+                    int idx = (int)color.NamedValue;
+                    int code = idx < 8
+                        ? (isForeground ? _fgBase : _bgBase) + idx
+                        : (isForeground ? _fgBrightBase : _bgBrightBase) + (idx - 8);
+                    AppendCode(sb, code, ref needSemi);
+                    break;
+                }
+            case Color.ColorKind.Indexed:
+                {
+                    // 256-color: ESC[38;5;n m  or  ESC[48;5;n m
+                    if (needSemi) sb.Append(';');
+                    sb.Append(isForeground ? "38;5;" : "48;5;");
+                    sb.Append(color.IndexValue);
+                    needSemi = true;
+                    break;
+                }
+            case Color.ColorKind.Rgb:
+                {
+                    if (_capabilities.HasTruecolor)
+                    {
+                        // Truecolor: ESC[38;2;r;g;b m  or  ESC[48;2;r;g;b m
+                        if (needSemi) sb.Append(';');
+                        sb.Append(isForeground ? "38;2;" : "48;2;");
+                        sb.Append(color.R); sb.Append(';');
+                        sb.Append(color.G); sb.Append(';');
+                        sb.Append(color.B);
+                        needSemi = true;
+                    }
+                    else
+                    {
+                        // Downgrade to nearest xterm 256-indexed color.
+                        int idx = NearestXterm256(color.R, color.G, color.B);
+                        if (needSemi) sb.Append(';');
+                        sb.Append(isForeground ? "38;5;" : "48;5;");
+                        sb.Append(idx);
+                        needSemi = true;
+                    }
+                    break;
+                }
+        }
+    }
+
+    /// <summary>
+    /// Maps an RGB triple to the nearest xterm 256-palette index (16–255).
+    /// Compares the best match from the 6×6×6 cube (indices 16–231) against
+    /// the 24-level greyscale ramp (indices 232–255) and returns the closer one.
+    /// </summary>
+    internal static int NearestXterm256(byte r, byte g, byte b)
+    {
+        // ── 6×6×6 cube (indices 16–231) ──────────────────────────────────────
+        int cr = NearestCubeIndex(r);
+        int cg = NearestCubeIndex(g);
+        int cb = NearestCubeIndex(b);
+        int cubeIndex = 16 + 36 * cr + 6 * cg + cb;
+
+        int cubeDist = SquaredDist(r, g, b, _cubeSteps[cr], _cubeSteps[cg], _cubeSteps[cb]);
+
+        // ── 24-level greyscale ramp (indices 232–255): grey = 8 + i*10 ───────
+        // Find the nearest grey level.
+        int greyLevel = NearestGreyLevel(r, g, b);
+        int greyValue = 8 + greyLevel * 10;
+        int greyIndex = 232 + greyLevel;
+
+        int greyDist = SquaredDist(r, g, b, greyValue, greyValue, greyValue);
+
+        return greyDist <= cubeDist ? greyIndex : cubeIndex;
+    }
+
+    private static int NearestCubeIndex(int channel)
+    {
+        // Snap to the nearest of the 6 cube step values.
+        int best = 0;
+        int bestDist = int.MaxValue;
+        for (int i = 0; i < _cubeSteps.Length; i++)
+        {
+            int d = Math.Abs(channel - _cubeSteps[i]);
+            if (d < bestDist)
+            {
+                bestDist = d;
+                best = i;
+            }
+        }
+        return best;
+    }
+
+    private static int NearestGreyLevel(int r, int g, int b)
+    {
+        // The grey ramp uses perceptual average for the target grey.
+        int avg = (r + g + b) / 3;
+        // grey(i) = 8 + i*10, for i in [0,23], so i = clamp(round((avg - 8) / 10), 0, 23).
+        int level = (avg - 8 + 5) / 10; // round-half-up
+        return Math.Clamp(level, 0, 23);
+    }
+
+    private static int SquaredDist(int r, int g, int b, int pr, int pg, int pb)
+    {
+        int dr = r - pr;
+        int dg = g - pg;
+        int db = b - pb;
+        return dr * dr + dg * dg + db * db;
+    }
+}
diff --git a/src/Dcli/Internal/RenderLoop/SystemClock.cs b/src/Dcli/Internal/RenderLoop/SystemClock.cs
new file mode 100644
index 0000000..cfa99d6
--- /dev/null
+++ b/src/Dcli/Internal/RenderLoop/SystemClock.cs
@@ -0,0 +1,24 @@
+using System.Diagnostics;
+
+namespace Dcli.Internal.RenderLoop;
+
+/// <summary>
+/// Real-time <see cref="IClock"/> implementation backed by <see cref="Stopwatch"/>.
+/// </summary>
+internal sealed class SystemClock : IClock
+{
+    private static readonly Stopwatch _epoch = Stopwatch.StartNew();
+
+    /// <inheritdoc/>
+    public TimeSpan Now => _epoch.Elapsed;
+
+    /// <inheritdoc/>
+    public async Task WaitUntilAsync(TimeSpan deadline, CancellationToken cancellationToken)
+    {
+        TimeSpan remaining = deadline - Now;
+        if (remaining > TimeSpan.Zero)
+        {
+            await Task.Delay(remaining, cancellationToken).ConfigureAwait(false);
+        }
+    }
+}
diff --git a/src/Dcli/Internal/RenderLoop/VtFrameRenderer.cs b/src/Dcli/Internal/RenderLoop/VtFrameRenderer.cs
new file mode 100644
index 0000000..c55cfeb
--- /dev/null
+++ b/src/Dcli/Internal/RenderLoop/VtFrameRenderer.cs
@@ -0,0 +1,285 @@
+using System.Text;
+
+namespace Dcli.Internal.RenderLoop;
+
+/// <summary>
+/// The real <see cref="IOutputSink"/> implementation that emits VT escape sequences for one frame.
+/// </summary>
+/// <remarks>
+/// <para>
+/// <strong>Thread discipline:</strong> <see cref="Paint"/> is called exclusively on the render loop
+/// thread. The injected writer must not be written from any other thread.
+/// </para>
+/// <para>
+/// <strong>Cross-frame cursor accounting (8.1):</strong> between frames the hardware cursor
+/// rests at the <em>caret row</em> when the cursor is visible and a caret position is set
+/// (because <see cref="ParkCursor"/> moves it there), or on the last row of the re-render region
+/// when the cursor is hidden or no caret position is provided. Each frame:
+/// (1) moves up to the anchor (top of the previous region) using the stored resting row,
+/// (2) emits any newly-committed rows with a trailing newline so they become frozen native-scrollback,
+/// (3) clears from that point to end-of-screen (<c>ESC[0J</c>) to erase the stale region,
+/// (4) repaints the current region.
+/// Committed rows are never touched by subsequent frames — the anchor descends by the committed
+/// count, keeping frozen content strictly above.
+/// </para>
+/// <para>
+/// <strong>Synchronized-output fence (8.2):</strong> each frame is wrapped in
+/// <c>ESC[?2026h</c> / <c>ESC[?2026l</c>. Terminals that do not recognise the DEC private
+/// mode silently ignore the sequence — this is the "no-op safely where unsupported" guarantee;
+/// no capability detection is needed.
+/// </para>
+/// <para>
+/// <strong>Cursor parking/hiding (8.3):</strong> at end of frame the hardware cursor is either
+/// positioned at the model caret (normal/autocomplete case) and shown, or hidden
+/// (<c>ESC[?25l</c>) when <see cref="RenderModel.IsCursorVisible"/> is <see langword="false"/>
+/// (modal-dialog case).
+/// </para>
+/// <para>
+/// <strong>8.4 diff-reconciler seam:</strong> the region repaint is isolated in
+/// <see cref="RepaintRegion"/>. A future per-line diff reconciler would replace that method,
+/// comparing against a retained previous-frame buffer to rewrite only changed rows. v1 is a
+/// full clear-then-repaint within the synchronized fence.
+/// </para>
+/// </remarks>
+internal sealed class VtFrameRenderer : IOutputSink
+{
+    // VT escape-sequence constants.
+    private const string _syncStart = "\x1b[?2026h";  // synchronized output: begin
+    private const string _syncEnd = "\x1b[?2026l";    // synchronized output: end
+    private const string _eraseEol = "\x1b[K";        // clear to end-of-line
+    private const string _eraseToEos = "\x1b[0J";     // erase from cursor to end-of-screen
+    private const string _cursorHide = "\x1b[?25l";   // DECTCEM hide
+    private const string _cursorShow = "\x1b[?25h";   // DECTCEM show
+
+    // Restore sequence emitted on every shutdown path.
+    private const string _restoreSequence = "\x1b[?2026l\x1b[?25h\x1b[0m";
+
+    private readonly TextWriter _writer;
+    private readonly SgrTranslator _sgr;
+    private readonly StringBuilder _buf = new(capacity: 4096);
+
+    // ── Cross-frame painter state ─────────────────────────────────────────────
+    // After each Paint the cursor rests at _lastRestingRow (0-based, relative to the
+    // region top). When the cursor is visible and a caret position is set, ParkCursor
+    // moves it to the caret row, so _lastRestingRow == caretRow. Otherwise the cursor
+    // rests on the last row of the region (_lastPaintedHeight - 1).
+    // On the next frame we move _lastRestingRow rows up to reach the anchor.
+
+    private int _lastPaintedHeight;  // rows in the previous frame's re-render region
+    private int _lastRestingRow;     // 0-based row within the previous region where the cursor rests
+    private bool _isFirstFrame = true; // true until the first Paint call completes
+
+    /// <summary>
+    /// Initialises the renderer with the given output destination and terminal capabilities.
+    /// </summary>
+    /// <param name="writer">
+    /// The byte/text destination. Use <see cref="System.IO.StringWriter"/> for tests;
+    /// use a <see cref="System.IO.StreamWriter"/> around stdout for production.
+    /// </param>
+    /// <param name="capabilities">
+    /// Detected terminal capabilities that influence color encoding.
+    /// Pass <see cref="TerminalCapabilities.Default"/> for headless tests.
+    /// </param>
+    internal VtFrameRenderer(TextWriter writer, TerminalCapabilities capabilities = default)
+    {
+        ArgumentNullException.ThrowIfNull(writer);
+        _writer = writer;
+        _sgr = new SgrTranslator(capabilities);
+    }
+
+    /// <inheritdoc/>
+    public void Paint(RenderModel model)
+    {
+        _buf.Clear();
+
+        // ── Open synchronized-output fence (8.2) ─────────────────────────────
+        _buf.Append(_syncStart);
+
+        if (_isFirstFrame)
+        {
+            // First frame: paint at the current cursor position — no move-up needed.
+            EmitCommittedRows(model.NewlyCommittedRows);
+            RepaintRegion(model);
+        }
+        else
+        {
+            // ── Move to anchor (top of previous re-render region) ─────────────
+            // The cursor rests at _lastRestingRow (the caret row when the cursor was
+            // visible and positioned, or the last row otherwise). Moving up by exactly
+            // _lastRestingRow rows reaches the anchor without intruding into frozen
+            // committed content above it.
+            int stepsUp = _lastRestingRow;
+            if (stepsUp > 0)
+            {
+                _buf.Append("\x1b[");
+                _buf.Append(stepsUp);
+                _buf.Append('A'); // CUU: cursor up
+            }
+            // Return to column 1.
+            _buf.Append('\r');
+
+            // ── Emit newly-committed rows ─────────────────────────────────────
+            // These print at the anchor and scroll up into native terminal history.
+            // After emission the anchor has descended by the committed-row count,
+            // so subsequent frames never touch them again.
+            EmitCommittedRows(model.NewlyCommittedRows);
+
+            // ── Clear old re-render region ────────────────────────────────────
+            // Erase from the new anchor position (just below committed rows) to
+            // end-of-screen, removing the stale previous frame content.
+            _buf.Append(_eraseToEos);
+
+            // ── Repaint current region ────────────────────────────────────────
+            // 8.4 seam: RepaintRegion() is the isolated site where a future per-line
+            // diff reconciler would compare against the previous frame buffer and only
+            // rewrite changed rows. v1 is a full clear-then-repaint; the clear above
+            // already happened, so RepaintRegion just writes the new rows.
+            RepaintRegion(model);
+        }
+
+        // ── Cursor parking / hiding (8.3) ────────────────────────────────────
+        int totalRows = model.LiveWindowRows.Count + model.FixedRegionRows.Count;
+        ParkCursor(model, totalRows);
+
+        // ── Close synchronized-output fence (8.2) ────────────────────────────
+        _buf.Append(_syncEnd);
+
+        // ── Flush to destination — one flush per frame ───────────────────────
+        _writer.Write(_buf);
+        _writer.Flush();
+
+        // ── Update painter state for next frame ───────────────────────────────
+        // _lastRestingRow is the 0-based row within this frame's region where the
+        // cursor now rests. ParkCursor moves it to the caret row when the cursor is
+        // visible and CaretPosition is set; otherwise it stays on the last row.
+        _lastPaintedHeight = totalRows;
+        _lastRestingRow = (model.IsCursorVisible && model.CaretPosition.HasValue)
+            ? model.CaretPosition.Value.Row
+            : (totalRows - 1);
+        _isFirstFrame = false;
+    }
+
+    /// <inheritdoc/>
+    public void EmitRestoreSequence()
+    {
+        _writer.Write(_restoreSequence);
+        _writer.Flush();
+    }
+
+    // ── Private helpers ───────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Emits the rows that have just been committed to native terminal scrollback.
+    /// Each row is styled, followed by <c>ESC[K</c> (erase-to-EOL) and a trailing
+    /// <c>\n</c> so the terminal advances the line into its scrollback history.
+    /// After this method returns the cursor is at column 1 of the new anchor row.
+    /// </summary>
+    private void EmitCommittedRows(IReadOnlyList<Line> committed)
+    {
+        foreach (Line line in committed)
+        {
+            EmitLine(line);
+            _buf.Append(_eraseEol);
+            _buf.Append('\n'); // advance into native scrollback — this row is now frozen
+        }
+    }
+
+    /// <summary>
+    /// Repaints the bounded re-render region: live-window rows then fixed-region rows.
+    /// The final row of the frame is emitted without a trailing newline. After this
+    /// method returns, <see cref="ParkCursor"/> moves the cursor to the caret row
+    /// (when visible and set) or leaves it on the last row (when hidden or no caret).
+    /// </summary>
+    /// <remarks>
+    /// <strong>8.4 diff-reconciler seam:</strong> this method owns the repaint. A future
+    /// implementation would accept a retained previous-frame snapshot and only emit rows
+    /// that differ, rather than the current full-clear-then-repaint approach.
+    /// </remarks>
+    private void RepaintRegion(RenderModel model)
+    {
+        bool liveIsFrameFinalSection = model.FixedRegionRows.Count == 0;
+        EmitRows(model.LiveWindowRows, isFrameFinalSection: liveIsFrameFinalSection);
+        EmitRows(model.FixedRegionRows, isFrameFinalSection: true);
+    }
+
+    /// <summary>
+    /// Positions the hardware cursor at the model caret (visible case) or hides it
+    /// (modal-dialog / cursor-hidden case). Called after all content rows are emitted,
+    /// with the cursor resting on the last row of the frame.
+    /// </summary>
+    private void ParkCursor(RenderModel model, int totalRows)
+    {
+        if (!model.IsCursorVisible)
+        {
+            _buf.Append(_cursorHide);
+            return;
+        }
+
+        _buf.Append(_cursorShow);
+        if (model.CaretPosition.HasValue)
+        {
+            // Position the cursor: row 0 is the first row of the re-render region.
+            // The cursor currently rests on the last row (index totalRows-1).
+            (int row, int col) = model.CaretPosition.Value;
+            int stepsUp = totalRows - 1 - row;
+            if (stepsUp > 0)
+            {
+                _buf.Append("\x1b[");
+                _buf.Append(stepsUp);
+                _buf.Append('A'); // CUU: cursor up
+            }
+            // Move to column (CHA: ESC[nG, 1-based).
+            _buf.Append("\x1b[");
+            _buf.Append(col + 1);
+            _buf.Append('G');
+        }
+    }
+
+    /// <summary>
+    /// Emits a sequence of pre-wrapped rows. Each row ends with <c>ESC[K</c> (erase-to-EOL)
+    /// to wipe stale trailing cells. Every row except the very last row of the entire frame
+    /// is followed by a newline; the final row of the frame is emitted with no trailing
+    /// newline so the cursor initially rests on the last row. <see cref="ParkCursor"/>
+    /// then repositions to the caret row (when visible and set), making
+    /// <c>stepsUp = totalRows - 1 - caretRow</c> correct for that repositioning.
+    /// </summary>
+    /// <param name="rows">The rows to emit.</param>
+    /// <param name="isFrameFinalSection">
+    /// <see langword="true"/> when this is the last section of the frame (the one whose
+    /// last element is the overall last row). When <see langword="true"/>, the trailing
+    /// newline is suppressed on the final row of <paramref name="rows"/>.
+    /// </param>
+    private void EmitRows(IReadOnlyList<Line> rows, bool isFrameFinalSection)
+    {
+        for (int i = 0; i < rows.Count; i++)
+        {
+            EmitLine(rows[i]);
+            _buf.Append(_eraseEol);
+            bool isFinalRowOfFrame = isFrameFinalSection && i == rows.Count - 1;
+            if (!isFinalRowOfFrame)
+                _buf.Append('\n');
+        }
+    }
+
+    /// <summary>
+    /// Emits all segments of a single <see cref="Line"/> with SGR styling.
+    /// Resets SGR after each styled segment so attributes do not bleed.
+    /// An unstyled segment (default <see cref="Style"/>) emits no SGR at all.
+    /// </summary>
+    private void EmitLine(Line line)
+    {
+        foreach (Segment segment in line.Segments)
+        {
+            bool hasStyle = _sgr.AppendOpenSgr(segment.Style, _buf);
+            _buf.Append(segment.Text);
+            if (hasStyle)
+                _buf.Append(SgrTranslator.Reset);
+        }
+    }
+
+    /// <summary>
+    /// The number of rows painted in the most recent <see cref="Paint"/> call.
+    /// Zero before the first paint.
+    /// </summary>
+    internal int LastPaintedHeight => _lastPaintedHeight;
+}
diff --git a/src/Dcli/Internal/RestoreCoordinator.cs b/src/Dcli/Internal/RestoreCoordinator.cs
new file mode 100644
index 0000000..4e79256
--- /dev/null
+++ b/src/Dcli/Internal/RestoreCoordinator.cs
@@ -0,0 +1,162 @@
+using System.Runtime.InteropServices;
+using System.Runtime.Versioning;
+
+namespace Dcli.Internal;
+
+/// <summary>
+/// Wires a last-resort restoration net around an <see cref="IRawModeSession"/>:
+/// <list type="bullet">
+///   <item>On POSIX: SIGINT, SIGTERM, SIGQUIT → restore terminal, then re-raise the signal so
+///     the process terminates with the correct exit status.</item>
+///   <item>On POSIX: SIGCONT → re-apply raw mode (resume after suspend left the tty cooked).</item>
+///   <item>All platforms: <c>AppDomain.CurrentDomain.ProcessExit</c> → restore terminal.</item>
+/// </list>
+/// <para>
+/// This class is internal and testable: the <see cref="IRawModeSession"/> is injected, so
+/// tests substitute a <see cref="RecordingRawModeSession"/> and drive the observable actions
+/// (<see cref="SimulateTerminateSignal"/> / <see cref="SimulateContinueSignal"/> /
+/// <see cref="OnProcessExit"/>) directly without needing to construct real OS signal contexts.
+/// </para>
+/// <para>
+/// Idempotency: all paths converge on <see cref="IRawModeSession.Restore"/>, which is itself
+/// idempotent, so concurrent invocations from multiple handlers are safe.
+/// </para>
+/// </summary>
+internal sealed class RestoreCoordinator : IDisposable
+{
+    private readonly IRawModeSession _session;
+    private readonly List<IDisposable> _registrations = [];
+    private int _disposed;
+
+    // Wired after loop creation (SetHaltAction). When set, signal-driven restore halts the
+    // loop first — so the loop's finally emits the ANSI restore on the single-writer thread —
+    // before falling back to the idempotent session restore below.
+    private volatile Action? _haltLoop;
+
+    private RestoreCoordinator(IRawModeSession session)
+    {
+        _session = session;
+    }
+
+    /// <summary>
+    /// Wires the loop-halt action called on every terminate-signal path so the loop thread
+    /// emits the ANSI restore sequence before the termios restore occurs.
+    /// Must be called once, after the loop is created, before any signal can fire.
+    /// The supplied action must be idempotent.
+    /// </summary>
+    internal void SetHaltAction(Action haltLoop)
+    {
+        ArgumentNullException.ThrowIfNull(haltLoop);
+        _haltLoop = haltLoop;
+    }
+
+    /// <summary>
+    /// Creates and wires a coordinator for <paramref name="session"/>, registering all signal
+    /// handlers and the ProcessExit hook.
+    /// </summary>
+    internal static RestoreCoordinator Wire(IRawModeSession session)
+    {
+        RestoreCoordinator coord = new(session);
+        coord.Register();
+        return coord;
+    }
+
+    private void Register()
+    {
+        // AppDomain.ProcessExit fires on normal exit and Environment.Exit; always registered.
+        AppDomain.CurrentDomain.ProcessExit += OnProcessExit;
+
+        // POSIX-only signals.
+        if (OperatingSystem.IsLinux() || OperatingSystem.IsMacOS())
+            RegisterPosixSignals();
+    }
+
+    [SupportedOSPlatform("linux")]
+    [SupportedOSPlatform("macos")]
+    private void RegisterPosixSignals()
+    {
+        // Terminate signals: restore terminal then let the process die with the default disposition.
+        // Cancel=false (the default) allows the signal to propagate so the exit status is correct.
+        _registrations.Add(PosixSignalRegistration.Create(PosixSignal.SIGINT, OnTerminateSignal));
+        _registrations.Add(PosixSignalRegistration.Create(PosixSignal.SIGTERM, OnTerminateSignal));
+        _registrations.Add(PosixSignalRegistration.Create(PosixSignal.SIGQUIT, OnTerminateSignal));
+
+        // SIGCONT: resume from an externally-driven kill -STOP/-CONT may have left the tty cooked.
+        // dcli does not install a SIGTSTP handler (ISIG is cleared, so Ctrl-Z does not generate it);
+        // Reapply() handles the case where an external agent suspended and resumed the process.
+        _registrations.Add(PosixSignalRegistration.Create(PosixSignal.SIGCONT, OnContinueSignal));
+    }
+
+    // ─── OS signal callbacks ─────────────────────────────────────────────────
+
+    [SupportedOSPlatform("linux")]
+    [SupportedOSPlatform("macos")]
+    private void OnTerminateSignal(PosixSignalContext context)
+    {
+        // context.Cancel remains false; signal propagates → process dies.
+        SimulateTerminateSignal();
+    }
+
+    [SupportedOSPlatform("linux")]
+    [SupportedOSPlatform("macos")]
+    private void OnContinueSignal(PosixSignalContext context)
+    {
+        context.Cancel = true;  // We fully handle SIGCONT; do not re-raise.
+        SimulateContinueSignal();
+    }
+
+    // ─── Testable action methods ─────────────────────────────────────────────
+    // These are the unit-testable equivalents of the OS callbacks.
+    // Tests call them directly; the OS callbacks above delegate to them.
+
+    /// <summary>
+    /// Halts the loop (so its <c>finally</c> emits the ANSI restore on the single-writer thread)
+    /// then restores the terminal session — the action taken on SIGINT/SIGTERM/SIGQUIT.
+    /// Exposed for unit-testing the handler logic without a real OS signal.
+    /// </summary>
+    internal void SimulateTerminateSignal()
+    {
+        // Halting the loop causes its finally block to run EmitRestoreSequence() on the
+        // loop thread (the single stdout writer), preserving the single-writer discipline.
+        // _haltLoop is null only in tests that construct RestoreCoordinator directly without
+        // wiring a loop — those tests exercise termios restore only, which is sufficient.
+        _haltLoop?.Invoke();
+
+        // Idempotent fallback: covers the case where _haltLoop was not wired (bare tests)
+        // and ensures termios is restored even if the loop halt races with another exit path.
+        _session.Restore();
+    }
+
+    /// <summary>
+    /// Re-applies raw mode — the action taken on SIGCONT.
+    /// Exposed for unit-testing the handler logic without a real OS signal.
+    /// </summary>
+    internal void SimulateContinueSignal() => _session.Reapply();
+
+    // ─── ProcessExit handler ─────────────────────────────────────────────────
+
+    /// <summary>
+    /// Called by <c>AppDomain.ProcessExit</c>. Restores the terminal.
+    /// </summary>
+    internal void OnProcessExit(object? sender, EventArgs e) => _session.Restore();
+
+    // ─── Dispose ─────────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Deregisters all signal/ProcessExit hooks and calls <see cref="IRawModeSession.Restore"/>.
+    /// </summary>
+    public void Dispose()
+    {
+        if (Interlocked.Exchange(ref _disposed, 1) != 0)
+            return;
+
+        _session.Restore();
+
+        AppDomain.CurrentDomain.ProcessExit -= OnProcessExit;
+
+        foreach (IDisposable reg in _registrations)
+            reg.Dispose();
+
+        _registrations.Clear();
+    }
+}
diff --git a/src/Dcli/Internal/Scrollback/Collapsible.cs b/src/Dcli/Internal/Scrollback/Collapsible.cs
new file mode 100644
index 0000000..f46a755
--- /dev/null
+++ b/src/Dcli/Internal/Scrollback/Collapsible.cs
@@ -0,0 +1,96 @@
+namespace Dcli.Internal.Scrollback;
+
+/// <summary>
+/// A one-way collapsible line-object: starts collapsed (showing a summary line), can expand
+/// exactly once to reveal hidden lines, and can never re-collapse.
+/// </summary>
+/// <remarks>
+/// <para>
+/// <strong>One-way / monotonic height:</strong> once <see cref="Expand"/> is called the block
+/// is permanently expanded. A second call to <see cref="Expand"/> is a no-op. There is no
+/// collapse operation — the spec forbids re-collapse.
+/// </para>
+/// <para>
+/// <strong>Oversized expansion:</strong> if the expanded height would exceed the live-window
+/// cap the expansion is handled by <see cref="ScrollbackModel.ExpandCollapsible"/> instead
+/// of being performed inline. The collapsible stays a collapsed marker in the live list and
+/// the hidden lines are emitted into the committed flow exactly once.
+/// </para>
+/// <para>
+/// <strong>Freeze-collapsed-at-horizon:</strong> a collapsible that commits while still
+/// collapsed is dropped from <c>_liveObjects</c>. Any subsequent <see cref="Expand"/> call
+/// is a no-op because the caller no longer holds a reference that is present in the live list.
+/// The frozen summary rows that already committed into native scrollback are unaffected.
+/// </para>
+/// <para>
+/// <strong>Thread discipline:</strong> all methods run exclusively on the render loop thread.
+/// No synchronization is required.
+/// </para>
+/// </remarks>
+internal sealed class Collapsible : ILineObject
+{
+    private readonly Line _summary;
+    private readonly IReadOnlyList<Line> _hiddenLines;
+
+    /// <summary>Whether <see cref="Expand"/> has been called (successfully).</summary>
+    internal bool IsExpanded { get; private set; }
+
+    /// <summary>
+    /// Initialises a new <see cref="Collapsible"/> in the collapsed state.
+    /// </summary>
+    /// <param name="summary">The single summary line shown while collapsed.</param>
+    /// <param name="hiddenLines">The lines revealed on expansion.</param>
+    internal Collapsible(Line summary, IReadOnlyList<Line> hiddenLines)
+    {
+        ArgumentNullException.ThrowIfNull(hiddenLines);
+        _summary = summary;
+        _hiddenLines = hiddenLines;
+    }
+
+    /// <summary>
+    /// Marks this collapsible as expanded. Idempotent — a second call has no effect.
+    /// </summary>
+    internal void Expand()
+    {
+        // One-way: false → true only. A second call leaves IsExpanded = true unchanged.
+        IsExpanded = true;
+    }
+
+    /// <summary>
+    /// Returns the visual rows produced by the hidden lines at <paramref name="width"/>,
+    /// without mutating expansion state. Used by the oversized-expansion path.
+    /// </summary>
+    internal IReadOnlyList<Line> GetHiddenRows(int width)
+    {
+        List<Line> result = [];
+        foreach (Line line in _hiddenLines)
+            result.AddRange(LineWrapper.Wrap(line, width));
+        return result;
+    }
+
+    /// <summary>
+    /// Returns the number of visual rows the hidden lines would produce at
+    /// <paramref name="width"/>, without mutating expansion state.
+    /// </summary>
+    internal int MeasureHiddenRows(int width)
+    {
+        int count = 0;
+        foreach (Line line in _hiddenLines)
+            count += LineWrapper.Wrap(line, width).Count;
+        return count;
+    }
+
+    /// <inheritdoc/>
+    public IReadOnlyList<Line> Render(int width)
+    {
+        if (!IsExpanded)
+            return LineWrapper.Wrap(_summary, width);
+
+        // Expanded: summary rows followed by the hidden lines' rows.
+        List<Line> result = [];
+        result.AddRange(LineWrapper.Wrap(_summary, width));
+        foreach (Line line in _hiddenLines)
+            result.AddRange(LineWrapper.Wrap(line, width));
+        return result;
+    }
+}
diff --git a/src/Dcli/Internal/Scrollback/Commands/AppendLineCommand.cs b/src/Dcli/Internal/Scrollback/Commands/AppendLineCommand.cs
new file mode 100644
index 0000000..84cefba
--- /dev/null
+++ b/src/Dcli/Internal/Scrollback/Commands/AppendLineCommand.cs
@@ -0,0 +1,23 @@
+using Dcli.Internal.RenderLoop;
+
+namespace Dcli.Internal.Scrollback.Commands;
+
+/// <summary>
+/// Appends a single styled <see cref="Line"/> to the scrollback live window.
+/// </summary>
+internal sealed class AppendLineCommand : ILoopCommand
+{
+    private readonly ScrollbackModel _scrollback;
+    private readonly Line _line;
+
+    internal AppendLineCommand(ScrollbackModel scrollback, Line line)
+    {
+        ArgumentNullException.ThrowIfNull(scrollback);
+        ArgumentNullException.ThrowIfNull(line);
+        _scrollback = scrollback;
+        _line = line;
+    }
+
+    void ILoopCommand.Apply(RenderModel model) =>
+        _scrollback.Append(new TextBlock(_line), model);
+}
diff --git a/src/Dcli/Internal/Scrollback/Commands/AppendTextCommand.cs b/src/Dcli/Internal/Scrollback/Commands/AppendTextCommand.cs
new file mode 100644
index 0000000..29cff9f
--- /dev/null
+++ b/src/Dcli/Internal/Scrollback/Commands/AppendTextCommand.cs
@@ -0,0 +1,26 @@
+using Dcli.Internal.RenderLoop;
+
+namespace Dcli.Internal.Scrollback.Commands;
+
+/// <summary>
+/// Appends plain text to a live block's accumulation buffer.
+/// </summary>
+internal sealed class AppendTextCommand : ILoopCommand
+{
+    private readonly LiveBlock _block;
+    private readonly string _text;
+
+    internal AppendTextCommand(LiveBlock block, string text)
+    {
+        ArgumentNullException.ThrowIfNull(block);
+        ArgumentNullException.ThrowIfNull(text);
+        _block = block;
+        _text = text;
+    }
+
+    void ILoopCommand.Apply(RenderModel model)
+    {
+        _block.AppendText(_text);
+        model.MarkDirty();
+    }
+}
diff --git a/src/Dcli/Internal/Scrollback/Commands/BeginCollapsibleCommand.cs b/src/Dcli/Internal/Scrollback/Commands/BeginCollapsibleCommand.cs
new file mode 100644
index 0000000..c12b575
--- /dev/null
+++ b/src/Dcli/Internal/Scrollback/Commands/BeginCollapsibleCommand.cs
@@ -0,0 +1,40 @@
+using Dcli.Internal.RenderLoop;
+
+namespace Dcli.Internal.Scrollback.Commands;
+
+/// <summary>
+/// Creates a new <see cref="Collapsible"/>, appends it to the scrollback live window,
+/// and delivers the reference to the caller via a callback.
+/// </summary>
+/// <remarks>
+/// The caller captures the collapsible reference via a callback supplied at construction,
+/// which is invoked synchronously on the loop thread during <see cref="ILoopCommand.Apply"/>.
+/// </remarks>
+internal sealed class BeginCollapsibleCommand : ILoopCommand
+{
+    private readonly ScrollbackModel _scrollback;
+    private readonly Line _summary;
+    private readonly IReadOnlyList<Line> _hiddenLines;
+    private readonly Action<Collapsible> _capture;
+
+    internal BeginCollapsibleCommand(
+        ScrollbackModel scrollback,
+        Line summary,
+        IReadOnlyList<Line> hiddenLines,
+        Action<Collapsible> capture)
+    {
+        ArgumentNullException.ThrowIfNull(scrollback);
+        ArgumentNullException.ThrowIfNull(hiddenLines);
+        ArgumentNullException.ThrowIfNull(capture);
+        _scrollback = scrollback;
+        _summary = summary;
+        _hiddenLines = hiddenLines;
+        _capture = capture;
+    }
+
+    void ILoopCommand.Apply(RenderModel model)
+    {
+        Collapsible collapsible = _scrollback.BeginCollapsible(_summary, _hiddenLines, model);
+        _capture(collapsible);
+    }
+}
diff --git a/src/Dcli/Internal/Scrollback/Commands/BeginLiveCommand.cs b/src/Dcli/Internal/Scrollback/Commands/BeginLiveCommand.cs
new file mode 100644
index 0000000..9a174e7
--- /dev/null
+++ b/src/Dcli/Internal/Scrollback/Commands/BeginLiveCommand.cs
@@ -0,0 +1,31 @@
+using Dcli.Internal.RenderLoop;
+
+namespace Dcli.Internal.Scrollback.Commands;
+
+/// <summary>
+/// Begins a new live block in the scrollback model and captures the resulting
+/// <see cref="LiveBlock"/> so the caller can issue subsequent mutation commands.
+/// </summary>
+/// <remarks>
+/// The caller captures the live block reference via a callback supplied at construction,
+/// which is invoked synchronously on the loop thread during <see cref="ILoopCommand.Apply"/>.
+/// </remarks>
+internal sealed class BeginLiveCommand : ILoopCommand
+{
+    private readonly ScrollbackModel _scrollback;
+    private readonly Action<LiveBlock> _capture;
+
+    internal BeginLiveCommand(ScrollbackModel scrollback, Action<LiveBlock> capture)
+    {
+        ArgumentNullException.ThrowIfNull(scrollback);
+        ArgumentNullException.ThrowIfNull(capture);
+        _scrollback = scrollback;
+        _capture = capture;
+    }
+
+    void ILoopCommand.Apply(RenderModel model)
+    {
+        LiveBlock block = _scrollback.BeginLive(model);
+        _capture(block);
+    }
+}
diff --git a/src/Dcli/Internal/Scrollback/Commands/CommitLiveBlockCommand.cs b/src/Dcli/Internal/Scrollback/Commands/CommitLiveBlockCommand.cs
new file mode 100644
index 0000000..f45a520
--- /dev/null
+++ b/src/Dcli/Internal/Scrollback/Commands/CommitLiveBlockCommand.cs
@@ -0,0 +1,24 @@
+using Dcli.Internal.RenderLoop;
+
+namespace Dcli.Internal.Scrollback.Commands;
+
+/// <summary>
+/// Freezes a live block: marks it committed and triggers a repaint so the final content
+/// is emitted into the live window before the block drains into native scrollback.
+/// </summary>
+internal sealed class CommitLiveBlockCommand : ILoopCommand
+{
+    private readonly ScrollbackModel _scrollback;
+    private readonly LiveBlock _block;
+
+    internal CommitLiveBlockCommand(ScrollbackModel scrollback, LiveBlock block)
+    {
+        ArgumentNullException.ThrowIfNull(scrollback);
+        ArgumentNullException.ThrowIfNull(block);
+        _scrollback = scrollback;
+        _block = block;
+    }
+
+    void ILoopCommand.Apply(RenderModel model) =>
+        _scrollback.CommitLiveBlock(_block, model);
+}
diff --git a/src/Dcli/Internal/Scrollback/Commands/ExpandCollapsibleCommand.cs b/src/Dcli/Internal/Scrollback/Commands/ExpandCollapsibleCommand.cs
new file mode 100644
index 0000000..0e3d0bd
--- /dev/null
+++ b/src/Dcli/Internal/Scrollback/Commands/ExpandCollapsibleCommand.cs
@@ -0,0 +1,30 @@
+using Dcli.Internal.RenderLoop;
+
+namespace Dcli.Internal.Scrollback.Commands;
+
+/// <summary>
+/// Requests expansion of a <see cref="Collapsible"/>. Handles both the normal (fits live
+/// window) and oversized (reprints into flow) cases via
+/// <see cref="ScrollbackModel.ExpandCollapsible"/>.
+/// </summary>
+/// <remarks>
+/// If the collapsible has already committed (dropped from the live list) this command is a
+/// clean no-op: <see cref="ScrollbackModel.ExpandCollapsible"/> skips objects not present in
+/// the live list.
+/// </remarks>
+internal sealed class ExpandCollapsibleCommand : ILoopCommand
+{
+    private readonly ScrollbackModel _scrollback;
+    private readonly Collapsible _collapsible;
+
+    internal ExpandCollapsibleCommand(ScrollbackModel scrollback, Collapsible collapsible)
+    {
+        ArgumentNullException.ThrowIfNull(scrollback);
+        ArgumentNullException.ThrowIfNull(collapsible);
+        _scrollback = scrollback;
+        _collapsible = collapsible;
+    }
+
+    void ILoopCommand.Apply(RenderModel model) =>
+        _scrollback.ExpandCollapsible(_collapsible, model);
+}
diff --git a/src/Dcli/Internal/Scrollback/Commands/SetContentCommand.cs b/src/Dcli/Internal/Scrollback/Commands/SetContentCommand.cs
new file mode 100644
index 0000000..33a9a79
--- /dev/null
+++ b/src/Dcli/Internal/Scrollback/Commands/SetContentCommand.cs
@@ -0,0 +1,26 @@
+using Dcli.Internal.RenderLoop;
+
+namespace Dcli.Internal.Scrollback.Commands;
+
+/// <summary>
+/// Replaces a live block's content wholesale with the given lines.
+/// </summary>
+internal sealed class SetContentCommand : ILoopCommand
+{
+    private readonly LiveBlock _block;
+    private readonly IReadOnlyList<Line> _lines;
+
+    internal SetContentCommand(LiveBlock block, IReadOnlyList<Line> lines)
+    {
+        ArgumentNullException.ThrowIfNull(block);
+        ArgumentNullException.ThrowIfNull(lines);
+        _block = block;
+        _lines = lines;
+    }
+
+    void ILoopCommand.Apply(RenderModel model)
+    {
+        _block.SetContent(_lines);
+        model.MarkDirty();
+    }
+}
diff --git a/src/Dcli/Internal/Scrollback/ILineObject.cs b/src/Dcli/Internal/Scrollback/ILineObject.cs
new file mode 100644
index 0000000..70bd007
--- /dev/null
+++ b/src/Dcli/Internal/Scrollback/ILineObject.cs
@@ -0,0 +1,24 @@
+namespace Dcli.Internal.Scrollback;
+
+/// <summary>
+/// A single element in the flat scrollback list.
+/// </summary>
+/// <remarks>
+/// Implementations: <see cref="TextBlock"/> (this chunk), <c>Collapsible</c> (Chunk B).
+/// <see cref="Render"/> is called only while the object is live (in the live window);
+/// frozen objects were rendered once and may be dropped from memory.
+/// </remarks>
+internal interface ILineObject
+{
+    /// <summary>
+    /// Renders this line-object into visual rows at the given terminal width.
+    /// </summary>
+    /// <param name="width">
+    /// Display column width. Values &lt;= 0 are handled by <see cref="LineWrapper"/> (clamped to 1).
+    /// </param>
+    /// <returns>
+    /// One or more visual rows. Never empty. Each row is a <see cref="Line"/> of
+    /// styled <see cref="Segment"/>s, already fitted to <paramref name="width"/> columns.
+    /// </returns>
+    IReadOnlyList<Line> Render(int width);
+}
diff --git a/src/Dcli/Internal/Scrollback/LiveBlock.cs b/src/Dcli/Internal/Scrollback/LiveBlock.cs
new file mode 100644
index 0000000..2fa1753
--- /dev/null
+++ b/src/Dcli/Internal/Scrollback/LiveBlock.cs
@@ -0,0 +1,91 @@
+using System.Text;
+
+namespace Dcli.Internal.Scrollback;
+
+/// <summary>
+/// A mutable line-object whose content grows via <see cref="AppendText"/> or is replaced
+/// wholesale by <see cref="SetContent"/>, and is frozen by <see cref="Commit"/>.
+/// </summary>
+/// <remarks>
+/// <para>
+/// <strong>Thread discipline:</strong> all methods are called exclusively on the render loop
+/// thread (via <see cref="RenderLoop.ILoopCommand"/> apply). No synchronization is required.
+/// </para>
+/// <para>
+/// While live the block renders its current accumulated text. After <see cref="Commit"/> the
+/// object is removed from the live list; its final rendered rows were already flushed into the
+/// commit queue and the object itself may be dropped.
+/// </para>
+/// <para>
+/// A committed block that has not been replaced still renders its accumulated text; a block
+/// on which <see cref="SetContent"/> was called renders the replacement lines.
+/// </para>
+/// </remarks>
+internal sealed class LiveBlock : ILineObject
+{
+    // When _replacedLines is non-null, Render() uses those instead of _text.
+    private readonly StringBuilder _text = new();
+    private IReadOnlyList<Line>? _replacedLines;
+
+    /// <summary>Whether <see cref="Commit"/> has been called.</summary>
+    internal bool IsCommitted { get; private set; }
+
+    /// <summary>
+    /// Appends <paramref name="text"/> to the live tail.
+    /// Ignored after commit (no-op; guards against out-of-order commands).
+    /// </summary>
+    internal void AppendText(string text)
+    {
+        if (IsCommitted || text.Length == 0)
+            return;
+
+        // SetContent replaces wholesale; subsequent AppendText is not meaningful.
+        // Honour the contract: once replaced, appends are discarded.
+        if (_replacedLines is not null)
+            return;
+
+        _text.Append(text);
+    }
+
+    /// <summary>
+    /// Replaces the entire content with <paramref name="lines"/>.
+    /// After this call <see cref="AppendText"/> is a no-op.
+    /// Ignored after commit.
+    /// </summary>
+    internal void SetContent(IReadOnlyList<Line> lines)
+    {
+        if (IsCommitted)
+            return;
+
+        ArgumentNullException.ThrowIfNull(lines);
+        _replacedLines = lines;
+    }
+
+    /// <summary>
+    /// Marks the block as committed. The caller (<see cref="ScrollbackModel"/>) is
+    /// responsible for removing it from the live list and emitting its rendered rows.
+    /// </summary>
+    internal void Commit() => IsCommitted = true;
+
+    /// <inheritdoc/>
+    public IReadOnlyList<Line> Render(int width)
+    {
+        if (_replacedLines is not null)
+        {
+            if (_replacedLines.Count == 0)
+                return [new Line(Array.Empty<Segment>())];
+
+            List<Line> result = [];
+            foreach (Line logicalLine in _replacedLines)
+                result.AddRange(LineWrapper.Wrap(logicalLine, width));
+            return result;
+        }
+
+        // Render the accumulated text as a single logical line of one plain segment.
+        string txt = _text.ToString();
+        Line line = txt.Length == 0
+            ? new Line(Array.Empty<Segment>())
+            : new Line([new Segment(txt)]);
+        return LineWrapper.Wrap(line, width);
+    }
+}
diff --git a/src/Dcli/Internal/Scrollback/ScrollbackModel.cs b/src/Dcli/Internal/Scrollback/ScrollbackModel.cs
new file mode 100644
index 0000000..6f1aee6
--- /dev/null
+++ b/src/Dcli/Internal/Scrollback/ScrollbackModel.cs
@@ -0,0 +1,238 @@
+using Dcli.Internal.RenderLoop;
+
+namespace Dcli.Internal.Scrollback;
+
+/// <summary>
+/// Owns the flat list of live line-objects and the commit-horizon / overflow logic (§9.2).
+/// </summary>
+/// <remarks>
+/// <para>
+/// <strong>Thread discipline:</strong> all methods run exclusively on the render loop thread.
+/// No synchronization is required.
+/// </para>
+/// <para>
+/// <strong>Live window:</strong> the set of line-objects whose visual rows populate
+/// <see cref="RenderModel.LiveWindowRows"/>. The window is bounded:
+/// <c>maxHeight = Rows − fixedRegionHeight</c>. When rendering the live list would exceed
+/// <c>maxHeight</c>, the topmost objects are committed (their rows are moved to
+/// <see cref="RenderModel.NewlyCommittedRows"/>) until the window fits. Committed objects are
+/// then removed from <see cref="_liveObjects"/> and may be garbage collected.
+/// </para>
+/// <para>
+/// <strong>Per-frame lifecycle for <see cref="RenderModel.NewlyCommittedRows"/>:</strong>
+/// <list type="number">
+///   <item>Commands append objects and mark the model dirty.</item>
+///   <item>Just before painting (<see cref="PrePaint"/>), the live list is rendered at the
+///     current width; overflow objects are committed into <see cref="_pendingCommittedRows"/>.</item>
+///   <item><see cref="PrePaint"/> sets <see cref="RenderModel.NewlyCommittedRows"/> from the
+///     pending buffer and sets <see cref="RenderModel.LiveWindowRows"/> from the remaining
+///     live objects.</item>
+///   <item>After paint, <see cref="ClearCommitted"/> resets
+///     <see cref="RenderModel.NewlyCommittedRows"/> to empty so the rows are never emitted
+///     a second time.</item>
+/// </list>
+/// </para>
+/// </remarks>
+internal sealed class ScrollbackModel
+{
+    // Ordered flat list of live line-objects. Index 0 = oldest (topmost on screen).
+    private readonly List<ILineObject> _liveObjects = [];
+
+    // Rows from objects that committed this frame, waiting to be handed to the painter once.
+    private List<Line>? _pendingCommittedRows;
+
+    /// <summary>
+    /// Appends a line-object to the live tail. Marks the model dirty.
+    /// </summary>
+    internal void Append(ILineObject obj, RenderModel model)
+    {
+        ArgumentNullException.ThrowIfNull(obj);
+        _liveObjects.Add(obj);
+        model.MarkDirty();
+    }
+
+    /// <summary>
+    /// Begins a new live block, appends it to the live tail, and returns it for mutation.
+    /// </summary>
+    internal LiveBlock BeginLive(RenderModel model)
+    {
+        LiveBlock block = new();
+        _liveObjects.Add(block);
+        model.MarkDirty();
+        return block;
+    }
+
+    /// <summary>
+    /// Commits a live block: freezes it and marks the model dirty so the next pre-paint
+    /// pass sweeps it out of the live list into the commit queue.
+    /// </summary>
+    /// <remarks>
+    /// The sweep runs in the same <see cref="PrePaint"/> call that follows, so committed
+    /// rows reach <see cref="RenderModel.NewlyCommittedRows"/> in that same frame — there
+    /// is no one-frame latency.
+    /// </remarks>
+    internal void CommitLiveBlock(LiveBlock block, RenderModel model)
+    {
+        ArgumentNullException.ThrowIfNull(block);
+        block.Commit();
+        // Eagerly allocate the pending buffer so PrePaint never hits the null-branch.
+        _pendingCommittedRows ??= [];
+        model.MarkDirty();
+    }
+
+    /// <summary>
+    /// Creates a new <see cref="Collapsible"/>, appends it to the live tail, and returns it.
+    /// </summary>
+    internal Collapsible BeginCollapsible(Line summary, IReadOnlyList<Line> hiddenLines, RenderModel model)
+    {
+        ArgumentNullException.ThrowIfNull(hiddenLines);
+        Collapsible collapsible = new(summary, hiddenLines);
+        _liveObjects.Add(collapsible);
+        model.MarkDirty();
+        return collapsible;
+    }
+
+    /// <summary>
+    /// Expands a <see cref="Collapsible"/> that is still in the live list.
+    /// </summary>
+    /// <remarks>
+    /// <para>
+    /// <strong>Freeze-collapsed-at-horizon takes precedence:</strong> if the collapsible has
+    /// already committed (dropped from <see cref="_liveObjects"/>), this is a clean no-op —
+    /// the frozen summary rows already in native scrollback are untouched.
+    /// </para>
+    /// <para>
+    /// <strong>Oversized expansion:</strong> if expanding would make the block taller than the
+    /// live-window cap (<c>Rows − fixedRegionHeight</c>), the hidden lines are emitted directly
+    /// into <see cref="_pendingCommittedRows"/> (flowing into native scrollback on the next
+    /// paint) and the block is left collapsed as a summary marker in the live list.
+    /// If the expanded height fits, the block is marked expanded and stays live/re-renderable.
+    /// </para>
+    /// </remarks>
+    internal void ExpandCollapsible(Collapsible collapsible, RenderModel model)
+    {
+        ArgumentNullException.ThrowIfNull(collapsible);
+
+        // Freeze-collapsed-at-horizon: already committed → not in live list → no-op.
+        if (!_liveObjects.Contains(collapsible))
+            return;
+
+        // Already expanded → idempotent no-op.
+        if (collapsible.IsExpanded)
+            return;
+
+        int width = model.Columns;
+        int maxHeight = ComputeMaxHeight(model);
+
+        // Measure expanded height without mutating state: summary rows + hidden rows.
+        int summaryRowCount = collapsible.Render(width).Count; // collapsed render = summary rows
+        int expandedRowCount = summaryRowCount + collapsible.MeasureHiddenRows(width);
+
+        if (expandedRowCount <= maxHeight)
+        {
+            // Normal path: mark expanded; next PrePaint re-renders the full content live.
+            collapsible.Expand();
+            model.MarkDirty();
+        }
+        else
+        {
+            // Oversized path: reprint hidden lines into committed flow; block stays a collapsed
+            // summary marker (do NOT call collapsible.Expand()).
+            _pendingCommittedRows ??= [];
+            _pendingCommittedRows.AddRange(collapsible.GetHiddenRows(width));
+            model.MarkDirty();
+        }
+    }
+
+    /// <summary>
+    /// Called by the loop thread immediately before handing the model to the painter.
+    /// Renders the live list, applies the commit horizon, and populates
+    /// <see cref="RenderModel.LiveWindowRows"/> and <see cref="RenderModel.NewlyCommittedRows"/>.
+    /// </summary>
+    internal void PrePaint(RenderModel model)
+    {
+        int width = model.Columns;
+        int maxHeight = ComputeMaxHeight(model);
+
+        // ── Step 1: sweep committed LiveBlocks from the front ────────────────
+        // A LiveBlock that has been committed is frozen — treat it as ordinary content
+        // that should drain forward. We eagerly drain it here so it hits the commit
+        // queue quickly rather than lingering in the live list across many frames.
+        // Non-LiveBlock objects (TextBlock, Collapsible) are drained only when they overflow.
+        int sweepEnd = 0;
+        while (sweepEnd < _liveObjects.Count && _liveObjects[sweepEnd] is LiveBlock { IsCommitted: true })
+            sweepEnd++;
+
+        // ── Step 2: render all remaining live objects to measure total height ─
+        // Build a parallel array of rendered rows so we don't call Render() twice.
+        List<(ILineObject Obj, IReadOnlyList<Line> Rows)> rendered = [];
+        rendered.Capacity = _liveObjects.Count;
+
+        for (int i = 0; i < _liveObjects.Count; i++)
+            rendered.Add((_liveObjects[i], _liveObjects[i].Render(width)));
+
+        // ── Step 3: commit from the front until the window fits ───────────────
+        // First, eagerly commit all swept committed blocks (index 0 .. sweepEnd-1).
+        // Then, commit additional from the front until height ≤ maxHeight.
+        _pendingCommittedRows ??= [];
+
+        int totalRows = rendered.Sum(r => r.Rows.Count);
+        int commitCount = 0;
+
+        // Drain the pre-swept committed blocks first.
+        while (commitCount < sweepEnd && commitCount < rendered.Count)
+        {
+            _pendingCommittedRows.AddRange(rendered[commitCount].Rows);
+            totalRows -= rendered[commitCount].Rows.Count;
+            commitCount++;
+        }
+
+        // Then drain additional objects until we fit within maxHeight.
+        // maxHeight == 0 means a zero-height live window — all live rows commit.
+        while (totalRows > maxHeight && commitCount < rendered.Count)
+        {
+            _pendingCommittedRows.AddRange(rendered[commitCount].Rows);
+            totalRows -= rendered[commitCount].Rows.Count;
+            commitCount++;
+        }
+
+        // Remove the committed objects from the live list (drop from memory).
+        if (commitCount > 0)
+            _liveObjects.RemoveRange(0, commitCount);
+
+        // ── Step 4: populate paint-state ──────────────────────────────────────
+        model.NewlyCommittedRows = _pendingCommittedRows.Count > 0
+            ? _pendingCommittedRows
+            : (IReadOnlyList<Line>)[];
+
+        // Build live window from remaining rendered rows (already computed above).
+        List<Line> liveRows = [];
+        for (int i = commitCount; i < rendered.Count; i++)
+            liveRows.AddRange(rendered[i].Rows);
+
+        model.LiveWindowRows = liveRows;
+    }
+
+    /// <summary>
+    /// Called by the loop thread after each paint to reset <see cref="RenderModel.NewlyCommittedRows"/>
+    /// so committed rows are never emitted a second time.
+    /// </summary>
+    internal void ClearCommitted(RenderModel model)
+    {
+        _pendingCommittedRows?.Clear();
+        model.NewlyCommittedRows = [];
+    }
+
+    // ── Private helpers ───────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Computes the live window height cap: <c>max(0, Rows − fixedRegionHeight)</c>.
+    /// <c>fixedRegionHeight</c> is the current row count of
+    /// <see cref="RenderModel.FixedRegionRows"/> (§10 populates this; §9 reads it).
+    /// </summary>
+    private static int ComputeMaxHeight(RenderModel model)
+    {
+        int fixedHeight = model.FixedRegionRows.Count;
+        return Math.Max(0, model.Rows - fixedHeight);
+    }
+}
diff --git a/src/Dcli/Internal/Scrollback/TextBlock.cs b/src/Dcli/Internal/Scrollback/TextBlock.cs
new file mode 100644
index 0000000..1368a4e
--- /dev/null
+++ b/src/Dcli/Internal/Scrollback/TextBlock.cs
@@ -0,0 +1,54 @@
+namespace Dcli.Internal.Scrollback;
+
+/// <summary>
+/// An immutable line-object consisting of one or more logical <see cref="Line"/>s.
+/// </summary>
+/// <remarks>
+/// <para>
+/// A <see cref="TextBlock"/> is constructed once and never mutated. Width-aware wrapping
+/// and display-width accounting are fully delegated to <see cref="LineWrapper"/> and
+/// <see cref="DisplayWidth"/> (§3); no wrapping logic lives here.
+/// </para>
+/// <para>
+/// A block with zero logical lines renders as a single empty visual row so the paint-state
+/// always has at least one entry (consistent with <see cref="LineWrapper"/> behaviour for
+/// empty lines).
+/// </para>
+/// </remarks>
+internal sealed class TextBlock : ILineObject
+{
+    private readonly List<Line> _logicalLines;
+
+    /// <summary>
+    /// Initialises a <see cref="TextBlock"/> from a sequence of logical lines.
+    /// </summary>
+    internal TextBlock(IEnumerable<Line> lines)
+    {
+        ArgumentNullException.ThrowIfNull(lines);
+        _logicalLines = lines.ToList();
+    }
+
+    /// <summary>
+    /// Initialises a <see cref="TextBlock"/> from a single logical line.
+    /// </summary>
+    internal TextBlock(Line line)
+    {
+        ArgumentNullException.ThrowIfNull(line);
+        _logicalLines = [line];
+    }
+
+    /// <inheritdoc/>
+    public IReadOnlyList<Line> Render(int width)
+    {
+        if (_logicalLines.Count == 0)
+            return [new Line(Array.Empty<Segment>())];
+
+        List<Line> result = [];
+        foreach (Line logicalLine in _logicalLines)
+        {
+            IReadOnlyList<Line> wrapped = LineWrapper.Wrap(logicalLine, width);
+            result.AddRange(wrapped);
+        }
+        return result;
+    }
+}
diff --git a/src/Dcli/Internal/TerminalCapabilities.cs b/src/Dcli/Internal/TerminalCapabilities.cs
new file mode 100644
index 0000000..9a1c54b
--- /dev/null
+++ b/src/Dcli/Internal/TerminalCapabilities.cs
@@ -0,0 +1,36 @@
+namespace Dcli.Internal;
+
+/// <summary>
+/// Optional terminal capabilities detected from the environment.
+/// All flags default to <see langword="false"/> (conservative — no capability assumed).
+/// </summary>
+internal readonly record struct TerminalCapabilities
+{
+    /// <summary>
+    /// <see langword="true"/> when the terminal advertises 24-bit truecolor via
+    /// <c>COLORTERM=truecolor</c> or <c>COLORTERM=24bit</c>. When <see langword="false"/>,
+    /// <see cref="Dcli.Internal.RenderLoop.SgrTranslator"/> downgrades RGB colors to the
+    /// nearest xterm 256-indexed palette entry.
+    /// </summary>
+    public bool HasTruecolor { get; init; }
+
+    /// <summary>
+    /// <see langword="true"/> when the terminal is known to support the synchronized-output
+    /// DEC private mode (<c>ESC[?2026h…l</c>). Recorded for diagnostics; the fence is
+    /// emitted unconditionally — terminals that do not support the sequence ignore it.
+    /// </summary>
+    public bool HasSynchronizedOutput { get; init; }
+
+    /// <summary>
+    /// <see langword="true"/> when the locale suggests East-Asian CJK content and
+    /// Unicode "ambiguous-width" characters should be treated as two cells wide.
+    /// Recorded for future use; the wcwidth table is unchanged in v1.
+    /// </summary>
+    public bool TreatAmbiguousAsWide { get; init; }
+
+    /// <summary>
+    /// Conservative default: all capabilities absent. Used by headless test rigs and any
+    /// context where the environment has not been probed.
+    /// </summary>
+    internal static readonly TerminalCapabilities Default;
+}
diff --git a/src/Dcli/Internal/TerminalCapabilityDetector.cs b/src/Dcli/Internal/TerminalCapabilityDetector.cs
new file mode 100644
index 0000000..d59a1c9
--- /dev/null
+++ b/src/Dcli/Internal/TerminalCapabilityDetector.cs
@@ -0,0 +1,166 @@
+using System.Runtime.InteropServices;
+
+namespace Dcli.Internal;
+
+/// <summary>
+/// Checks whether the current environment supports VT terminal sequences, and detects
+/// optional capabilities (truecolor, synchronized output, East-Asian ambiguous width).
+/// </summary>
+internal static class TerminalCapabilityDetector
+{
+    /// <summary>
+    /// Throws <see cref="TerminalNotSupportedException"/> if the environment does not meet the
+    /// minimum requirements for dcli to operate.
+    /// </summary>
+    /// <param name="inputs">
+    /// Environment inputs used for detection. Pass <see langword="null"/> to read from the
+    /// real process environment (production path). Pass an explicit value in tests.
+    /// </param>
+    /// <exception cref="TerminalNotSupportedException">
+    /// The process is not attached to a tty, or the terminal does not support VT sequences.
+    /// </exception>
+    internal static void EnsureVtCapable(CapabilityInputs? inputs = null)
+    {
+        CapabilityInputs env = inputs ?? CapabilityInputs.FromEnvironment();
+        Check(env);
+    }
+
+    /// <summary>
+    /// Detects optional terminal capabilities from the environment.
+    /// </summary>
+    /// <param name="inputs">
+    /// Environment inputs. Pass <see langword="null"/> to read from the real process environment
+    /// (production path). Pass an explicit value in tests.
+    /// </param>
+    /// <returns>A <see cref="TerminalCapabilities"/> value reflecting what the terminal advertises.</returns>
+    internal static TerminalCapabilities DetectCapabilities(CapabilityInputs? inputs = null)
+    {
+        CapabilityInputs env = inputs ?? CapabilityInputs.FromEnvironment();
+
+        bool hasTruecolor = env.ColorTermVariable is not null &&
+            (env.ColorTermVariable.Equals("truecolor", StringComparison.OrdinalIgnoreCase) ||
+             env.ColorTermVariable.Equals("24bit", StringComparison.OrdinalIgnoreCase));
+
+        // Terminals that don't recognise ESC[?2026h…l ignore it harmlessly (already emitted
+        // unconditionally by VtFrameRenderer). False negatives here are harmless.
+        string termProg = env.TermProgramVariable ?? string.Empty;
+        string term = env.TermVariable ?? string.Empty;
+        bool hasSyncOutput =
+            termProg.Equals("WezTerm", StringComparison.OrdinalIgnoreCase) ||
+            termProg.Equals("iTerm.app", StringComparison.OrdinalIgnoreCase) ||
+            termProg.Equals("vscode", StringComparison.OrdinalIgnoreCase) ||
+            termProg.Equals("WarpTerminal", StringComparison.OrdinalIgnoreCase) ||
+            termProg.Equals("ghostty", StringComparison.OrdinalIgnoreCase) ||
+            termProg.Equals("kitty", StringComparison.OrdinalIgnoreCase) ||
+            termProg.Equals("foot", StringComparison.OrdinalIgnoreCase) ||
+            term.StartsWith("alacritty", StringComparison.OrdinalIgnoreCase) ||
+            term.Equals("xterm-kitty", StringComparison.OrdinalIgnoreCase) ||
+            env.IsWindows;
+
+        string? lang = env.LangVariable;
+        bool ambiguousWide = lang is not null &&
+            (lang.Contains("zh", StringComparison.OrdinalIgnoreCase) ||
+             lang.Contains("ja", StringComparison.OrdinalIgnoreCase) ||
+             lang.Contains("ko", StringComparison.OrdinalIgnoreCase));
+
+        return new TerminalCapabilities
+        {
+            HasTruecolor = hasTruecolor,
+            HasSynchronizedOutput = hasSyncOutput,
+            TreatAmbiguousAsWide = ambiguousWide,
+        };
+    }
+
+    private static void Check(CapabilityInputs env)
+    {
+        if (!env.IsSupportedPlatform)
+        {
+            throw new TerminalNotSupportedException(
+                "dcli requires Linux, macOS, or Windows. Current platform is not supported.");
+        }
+
+        if (!env.IsStdinTty)
+        {
+            throw new TerminalNotSupportedException(
+                "dcli requires stdin to be an interactive terminal (tty). " +
+                "stdin is currently redirected to a file or pipe.");
+        }
+
+        if (!env.IsStdoutTty)
+        {
+            throw new TerminalNotSupportedException(
+                "dcli requires stdout to be an interactive terminal (tty). " +
+                "stdout is currently redirected to a file or pipe.");
+        }
+
+        string term = env.TermVariable ?? string.Empty;
+        if (string.IsNullOrEmpty(term) || string.Equals(term, "dumb", StringComparison.OrdinalIgnoreCase))
+        {
+            string detail = string.IsNullOrEmpty(term)
+                ? "TERM is not set"
+                : $"TERM={term}";
+            throw new TerminalNotSupportedException(
+                $"dcli requires a VT-capable terminal. {detail} indicates a terminal that does " +
+                "not support VT control sequences.");
+        }
+    }
+}
+
+/// <summary>
+/// Inputs used by <see cref="TerminalCapabilityDetector"/> for VT and capability detection.
+/// Inject an explicit instance in tests to avoid reading the real process environment.
+/// </summary>
+internal sealed class CapabilityInputs
+{
+    /// <summary>The value of the <c>TERM</c> environment variable, or <see langword="null"/>.</summary>
+    public string? TermVariable { get; init; }
+
+    /// <summary>Whether stdin is an interactive tty.</summary>
+    public bool IsStdinTty { get; init; }
+
+    /// <summary>Whether stdout is an interactive tty.</summary>
+    public bool IsStdoutTty { get; init; }
+
+    /// <summary>Whether the current platform is supported (Linux, macOS, or Windows).</summary>
+    public bool IsSupportedPlatform { get; init; }
+
+    /// <summary>The value of the <c>COLORTERM</c> environment variable, or <see langword="null"/>.</summary>
+    public string? ColorTermVariable { get; init; }
+
+    /// <summary>The value of the <c>TERM_PROGRAM</c> environment variable, or <see langword="null"/>.</summary>
+    public string? TermProgramVariable { get; init; }
+
+    /// <summary>
+    /// The locale string to use for East-Asian ambiguous-width detection.
+    /// Set to <c>LANG</c> when available; falls back to <c>LC_ALL</c> when <c>LANG</c> is null.
+    /// </summary>
+    public string? LangVariable { get; init; }
+
+    /// <summary>Whether the process is running on Windows.</summary>
+    public bool IsWindows { get; init; }
+
+    /// <summary>
+    /// Reads capability inputs from the real process environment. Used on the production path.
+    /// </summary>
+    internal static CapabilityInputs FromEnvironment()
+    {
+        bool supported = RuntimeInformation.IsOSPlatform(OSPlatform.Linux)
+                      || RuntimeInformation.IsOSPlatform(OSPlatform.OSX)
+                      || RuntimeInformation.IsOSPlatform(OSPlatform.Windows);
+
+        string? lang = Environment.GetEnvironmentVariable("LANG")
+                    ?? Environment.GetEnvironmentVariable("LC_ALL");
+
+        return new CapabilityInputs
+        {
+            TermVariable = Environment.GetEnvironmentVariable("TERM"),
+            IsStdinTty = !Console.IsInputRedirected,
+            IsStdoutTty = !Console.IsOutputRedirected,
+            IsSupportedPlatform = supported,
+            ColorTermVariable = Environment.GetEnvironmentVariable("COLORTERM"),
+            TermProgramVariable = Environment.GetEnvironmentVariable("TERM_PROGRAM"),
+            LangVariable = lang,
+            IsWindows = RuntimeInformation.IsOSPlatform(OSPlatform.Windows),
+        };
+    }
+}
diff --git a/src/Dcli/Internal/Windows/ConsoleMode.cs b/src/Dcli/Internal/Windows/ConsoleMode.cs
new file mode 100644
index 0000000..7b4a64d
--- /dev/null
+++ b/src/Dcli/Internal/Windows/ConsoleMode.cs
@@ -0,0 +1,53 @@
+// Windows Console Mode P/Invoke.
+//
+// Constants sourced from:
+//   https://learn.microsoft.com/en-us/windows/console/setconsolemode
+//   (Windows Console API documentation, retrieved 2025)
+//
+// Input mode flags (applied to stdin handle):
+//   ENABLE_VIRTUAL_TERMINAL_INPUT (0x0200) — causes the console to emit VT input sequences
+//   ENABLE_LINE_INPUT             (0x0002) — cooked/line-buffered input (clear this)
+//   ENABLE_ECHO_INPUT             (0x0004) — echo typed characters (clear this)
+//   ENABLE_PROCESSED_INPUT        (0x0001) — process CTRL+C as signal (clear this)
+//
+// Output mode flags (applied to stdout handle):
+//   ENABLE_VIRTUAL_TERMINAL_PROCESSING (0x0004) — interpret VT output sequences (set this)
+
+using System.Runtime.InteropServices;
+using System.Runtime.Versioning;
+
+namespace Dcli.Internal.Windows;
+
+[SupportedOSPlatform("windows")]
+internal static partial class ConsoleMode
+{
+    // Input mode flags
+    internal const uint ENABLE_PROCESSED_INPUT = 0x0001U;
+    internal const uint ENABLE_LINE_INPUT = 0x0002U;
+    internal const uint ENABLE_ECHO_INPUT = 0x0004U;
+    internal const uint ENABLE_VIRTUAL_TERMINAL_INPUT = 0x0200U;
+
+    // Output mode flags
+    internal const uint ENABLE_VIRTUAL_TERMINAL_PROCESSING = 0x0004U;
+
+    // Standard handle identifiers
+    internal const int STD_INPUT_HANDLE = -10;
+    internal const int STD_OUTPUT_HANDLE = -11;
+
+    // Invalid handle sentinel
+    internal static readonly nint INVALID_HANDLE_VALUE = new(-1);
+
+    [DefaultDllImportSearchPaths(DllImportSearchPath.System32)]
+    [LibraryImport("kernel32.dll", SetLastError = true)]
+    [return: MarshalAs(UnmanagedType.Bool)]
+    internal static partial bool GetConsoleMode(nint hConsoleHandle, out uint lpMode);
+
+    [DefaultDllImportSearchPaths(DllImportSearchPath.System32)]
+    [LibraryImport("kernel32.dll", SetLastError = true)]
+    [return: MarshalAs(UnmanagedType.Bool)]
+    internal static partial bool SetConsoleMode(nint hConsoleHandle, uint dwMode);
+
+    [DefaultDllImportSearchPaths(DllImportSearchPath.System32)]
+    [LibraryImport("kernel32.dll", SetLastError = true)]
+    internal static partial nint GetStdHandle(int nStdHandle);
+}
diff --git a/src/Dcli/Internal/Windows/WindowsRawModeSession.cs b/src/Dcli/Internal/Windows/WindowsRawModeSession.cs
new file mode 100644
index 0000000..6d897d9
--- /dev/null
+++ b/src/Dcli/Internal/Windows/WindowsRawModeSession.cs
@@ -0,0 +1,92 @@
+using System.Runtime.InteropServices;
+using System.Runtime.Versioning;
+
+namespace Dcli.Internal.Windows;
+
+/// <summary>
+/// Windows raw-mode session. Saves the current console input/output modes, applies raw + VT mode,
+/// and restores on <see cref="Dispose"/> or <see cref="Restore"/>.
+/// Thread-safe and idempotent via lock-free CAS (same as the POSIX implementation).
+/// </summary>
+[SupportedOSPlatform("windows")]
+internal sealed class WindowsRawModeSession : IRawModeSession
+{
+    private int _restoreFlag;
+
+    private readonly nint _stdinHandle;
+    private readonly nint _stdoutHandle;
+    private readonly uint _savedInputMode;
+    private readonly uint _savedOutputMode;
+
+    private WindowsRawModeSession(nint stdinHandle, nint stdoutHandle, uint savedInput, uint savedOutput)
+    {
+        _stdinHandle = stdinHandle;
+        _stdoutHandle = stdoutHandle;
+        _savedInputMode = savedInput;
+        _savedOutputMode = savedOutput;
+    }
+
+    /// <summary>
+    /// Captures the current console modes and enters raw + VT mode.
+    /// </summary>
+    /// <exception cref="InvalidOperationException">
+    /// A console API call failed.
+    /// </exception>
+    [SupportedOSPlatform("windows")]
+    internal static WindowsRawModeSession Enter()
+    {
+        nint stdinHandle = ConsoleMode.GetStdHandle(ConsoleMode.STD_INPUT_HANDLE);
+        nint stdoutHandle = ConsoleMode.GetStdHandle(ConsoleMode.STD_OUTPUT_HANDLE);
+
+        if (stdinHandle == ConsoleMode.INVALID_HANDLE_VALUE || stdoutHandle == ConsoleMode.INVALID_HANDLE_VALUE)
+            throw new InvalidOperationException($"GetStdHandle failed (error={Marshal.GetLastPInvokeError()}).");
+
+        if (!ConsoleMode.GetConsoleMode(stdinHandle, out uint savedInput))
+            throw new InvalidOperationException($"GetConsoleMode (stdin) failed (error={Marshal.GetLastPInvokeError()}).");
+
+        if (!ConsoleMode.GetConsoleMode(stdoutHandle, out uint savedOutput))
+            throw new InvalidOperationException($"GetConsoleMode (stdout) failed (error={Marshal.GetLastPInvokeError()}).");
+
+        // Raw input: clear cooked/echo/signal processing; enable VT input.
+        uint rawInput = savedInput
+                      & ~(ConsoleMode.ENABLE_LINE_INPUT | ConsoleMode.ENABLE_ECHO_INPUT | ConsoleMode.ENABLE_PROCESSED_INPUT)
+                      | ConsoleMode.ENABLE_VIRTUAL_TERMINAL_INPUT;
+
+        if (!ConsoleMode.SetConsoleMode(stdinHandle, rawInput))
+            throw new InvalidOperationException($"SetConsoleMode (stdin raw) failed (error={Marshal.GetLastPInvokeError()}).");
+
+        // VT output: ensure the console interprets VT sequences we emit.
+        uint rawOutput = savedOutput | ConsoleMode.ENABLE_VIRTUAL_TERMINAL_PROCESSING;
+
+        if (!ConsoleMode.SetConsoleMode(stdoutHandle, rawOutput))
+        {
+            // Roll back input change before throwing.
+            ConsoleMode.SetConsoleMode(stdinHandle, savedInput);
+            throw new InvalidOperationException($"SetConsoleMode (stdout VT) failed (error={Marshal.GetLastPInvokeError()}).");
+        }
+
+        return new WindowsRawModeSession(stdinHandle, stdoutHandle, savedInput, savedOutput);
+    }
+
+    /// <inheritdoc/>
+    public void Restore()
+    {
+        if (Interlocked.Exchange(ref _restoreFlag, 1) != 0)
+            return;
+
+        // Best-effort: ignore errors; we may be in ProcessExit.
+        ConsoleMode.SetConsoleMode(_stdinHandle, _savedInputMode);
+        ConsoleMode.SetConsoleMode(_stdoutHandle, _savedOutputMode);
+    }
+
+    /// <inheritdoc/>
+    public void Reapply()
+    {
+        // Re-enter raw mode after resume. Only relevant on POSIX (SIGCONT); on Windows there is no
+        // equivalent suspend/resume mechanism that would cooked the console, so this is a no-op.
+        // It is required by the interface contract and safe to call.
+    }
+
+    /// <inheritdoc/>
+    public void Dispose() => Restore();
+}
diff --git a/src/Dcli/Internal/Windows/WindowsResizeWatcher.cs b/src/Dcli/Internal/Windows/WindowsResizeWatcher.cs
new file mode 100644
index 0000000..d4b064b
--- /dev/null
+++ b/src/Dcli/Internal/Windows/WindowsResizeWatcher.cs
@@ -0,0 +1,22 @@
+using System.Runtime.Versioning;
+using Dcli.Internal.RenderLoop;
+
+namespace Dcli.Internal.Windows;
+
+/// <summary>
+/// Windows implementation of <see cref="IResizeWatcher"/>.
+/// </summary>
+/// <remarks>
+/// Windows runtime delivery of resize is deferred to §14.1 (consistent with the §6.1 Windows
+/// input stub). This compile-only stub keeps the cross-platform build green; the size source
+/// returns 80×24 until §14.1 wires <c>WINDOW_BUFFER_SIZE_EVENT</c> polling.
+/// </remarks>
+[SupportedOSPlatform("windows")]
+internal sealed class WindowsResizeWatcher : IResizeWatcher
+{
+    /// <inheritdoc/>
+    public void Start(Action<int, int> onResize) { }
+
+    /// <inheritdoc/>
+    public void Dispose() { }
+}
diff --git a/src/Dcli/Internal/Windows/WindowsTerminalSizeSource.cs b/src/Dcli/Internal/Windows/WindowsTerminalSizeSource.cs
new file mode 100644
index 0000000..38275db
--- /dev/null
+++ b/src/Dcli/Internal/Windows/WindowsTerminalSizeSource.cs
@@ -0,0 +1,32 @@
+using System.Runtime.Versioning;
+using Dcli.Internal.RenderLoop;
+
+namespace Dcli.Internal.Windows;
+
+/// <summary>
+/// Queries the terminal size via <see cref="Console.WindowWidth"/> / <see cref="Console.WindowHeight"/>.
+/// Returns an 80×24 fallback when the query fails (e.g. when stdout is not a console).
+/// </summary>
+/// <remarks>
+/// §14.1 implements and verifies full Windows runtime behaviour. The fallback ensures the
+/// cross-platform build stays green in environments without an attached console.
+/// </remarks>
+[SupportedOSPlatform("windows")]
+internal sealed class WindowsTerminalSizeSource : ITerminalSizeSource
+{
+    private const int _fallbackColumns = 80;
+    private const int _fallbackRows = 24;
+
+    /// <inheritdoc/>
+    public (int Columns, int Rows) GetSize()
+    {
+        int cols, rows;
+        try { cols = Console.WindowWidth; } catch (IOException) { cols = 0; }
+        try { rows = Console.WindowHeight; } catch (IOException) { rows = 0; }
+
+        if (cols > 0 && rows > 0)
+            return (cols, rows);
+
+        return (_fallbackColumns, _fallbackRows);
+    }
+}
diff --git a/src/Dcli/Line.cs b/src/Dcli/Line.cs
new file mode 100644
index 0000000..05ee8f8
--- /dev/null
+++ b/src/Dcli/Line.cs
@@ -0,0 +1,50 @@
+using System.Collections.ObjectModel;
+
+namespace Dcli;
+
+/// <summary>
+/// An immutable, ordered list of <see cref="Segment"/>s that together form one logical line of
+/// styled text.
+/// <para>
+/// <b>Equality:</b> two <see cref="Line"/> instances are equal when they contain the same segments
+/// in the same order (structural/sequence equality). The default <c>record</c> equality for
+/// <c>IReadOnlyList&lt;Segment&gt;</c> compares by reference; this type overrides
+/// <see cref="Equals(Line?)"/> and <see cref="GetHashCode"/> to provide sequence equality instead,
+/// so that unit tests and golden-frame comparisons behave as expected.
+/// </para>
+/// </summary>
+/// <param name="Segments">The ordered list of segments. Must not be <see langword="null"/>.</param>
+public sealed record Line(IReadOnlyList<Segment> Segments)
+{
+    /// <summary>
+    /// Initializes a new <see cref="Line"/> wrapping a copy of the provided sequence so that
+    /// callers cannot mutate the list after construction.
+    /// </summary>
+    /// <param name="segments">The segments to store.</param>
+    /// <exception cref="ArgumentNullException">
+    /// Thrown when <paramref name="segments"/> is <see langword="null"/>.
+    /// </exception>
+    public Line(IEnumerable<Segment> segments)
+        : this(new ReadOnlyCollection<Segment>(
+            (segments ?? throw new ArgumentNullException(nameof(segments))).ToList()))
+    {
+    }
+
+    /// <summary>
+    /// Returns <see langword="true"/> when <paramref name="other"/> contains exactly the same
+    /// segments in the same order.
+    /// </summary>
+    public bool Equals(Line? other) =>
+        other is not null && Segments.SequenceEqual(other.Segments);
+
+    /// <inheritdoc/>
+    public override int GetHashCode()
+    {
+        HashCode hash = default;
+        foreach (Segment segment in Segments)
+        {
+            hash.Add(segment);
+        }
+        return hash.ToHashCode();
+    }
+}
diff --git a/src/Dcli/LineBuilder.cs b/src/Dcli/LineBuilder.cs
new file mode 100644
index 0000000..29a8e12
--- /dev/null
+++ b/src/Dcli/LineBuilder.cs
@@ -0,0 +1,100 @@
+namespace Dcli;
+
+/// <summary>
+/// Composes a <see cref="Line"/> incrementally from styled text fragments.
+/// Segments are stored in the order they are appended; <see cref="Build"/> returns the finished
+/// <see cref="Line"/>.
+/// <para>
+/// Each method returns <see langword="this"/> so calls can be chained fluently:
+/// <code>
+/// Line line = new LineBuilder().Dim("✻ ").Text("Thinking…").Build();
+/// </code>
+/// </para>
+/// <para>
+/// A builder instance must not be reused after <see cref="Build"/> is called.
+/// </para>
+/// </summary>
+public sealed class LineBuilder
+{
+    private readonly List<Segment> _segments = [];
+
+    /// <summary>
+    /// Appends a segment with the given text and style.
+    /// </summary>
+    /// <param name="text">The literal text to append.</param>
+    /// <param name="style">The style to apply.</param>
+    /// <returns>This builder.</returns>
+    public LineBuilder Append(string text, Style style = default)
+    {
+        _segments.Add(new Segment(text, style));
+        return this;
+    }
+
+    /// <summary>
+    /// Appends a segment with the given text and no styling (inherits terminal defaults).
+    /// </summary>
+    /// <param name="text">The literal text to append.</param>
+    /// <returns>This builder.</returns>
+    public LineBuilder Text(string text) => Append(text, default);
+
+    /// <summary>Appends a segment styled with <see cref="Format.Bold"/>.</summary>
+    /// <param name="text">The literal text to append.</param>
+    /// <returns>This builder.</returns>
+    public LineBuilder Bold(string text) =>
+        Append(text, new Style(Format: Format.Bold));
+
+    /// <summary>Appends a segment styled with <see cref="Format.Italic"/>.</summary>
+    /// <param name="text">The literal text to append.</param>
+    /// <returns>This builder.</returns>
+    public LineBuilder Italic(string text) =>
+        Append(text, new Style(Format: Format.Italic));
+
+    /// <summary>Appends a segment styled with <see cref="Format.Underline"/>.</summary>
+    /// <param name="text">The literal text to append.</param>
+    /// <returns>This builder.</returns>
+    public LineBuilder Underline(string text) =>
+        Append(text, new Style(Format: Format.Underline));
+
+    /// <summary>Appends a segment styled with <see cref="Format.Dim"/>.</summary>
+    /// <param name="text">The literal text to append.</param>
+    /// <returns>This builder.</returns>
+    public LineBuilder Dim(string text) =>
+        Append(text, new Style(Format: Format.Dim));
+
+    /// <summary>Appends a segment styled with <see cref="Format.Reverse"/>.</summary>
+    /// <param name="text">The literal text to append.</param>
+    /// <returns>This builder.</returns>
+    public LineBuilder Reverse(string text) =>
+        Append(text, new Style(Format: Format.Reverse));
+
+    /// <summary>Appends a segment styled with <see cref="Format.Strikethrough"/>.</summary>
+    /// <param name="text">The literal text to append.</param>
+    /// <returns>This builder.</returns>
+    public LineBuilder Strikethrough(string text) =>
+        Append(text, new Style(Format: Format.Strikethrough));
+
+    /// <summary>
+    /// Appends a segment with the given text and a foreground color applied (no other formatting).
+    /// </summary>
+    /// <param name="text">The literal text to append.</param>
+    /// <param name="foreground">The foreground color.</param>
+    /// <returns>This builder.</returns>
+    public LineBuilder Fg(string text, Color foreground) =>
+        Append(text, new Style(Foreground: foreground));
+
+    /// <summary>
+    /// Appends a segment with the given text and a background color applied (no other formatting).
+    /// </summary>
+    /// <param name="text">The literal text to append.</param>
+    /// <param name="background">The background color.</param>
+    /// <returns>This builder.</returns>
+    public LineBuilder Bg(string text, Color background) =>
+        Append(text, new Style(Background: background));
+
+    /// <summary>
+    /// Returns the composed <see cref="Line"/> containing all appended segments in the order they
+    /// were added.
+    /// </summary>
+    /// <returns>The finished <see cref="Line"/>.</returns>
+    public Line Build() => new(_segments);
+}
diff --git a/src/Dcli/LineWrapper.cs b/src/Dcli/LineWrapper.cs
new file mode 100644
index 0000000..2d09517
--- /dev/null
+++ b/src/Dcli/LineWrapper.cs
@@ -0,0 +1,120 @@
+using System.Text;
+
+namespace Dcli;
+
+/// <summary>
+/// Hard cell-wrap of a <see cref="Line"/> into visual rows at a given terminal width.
+/// <para>
+/// This is a pure rendering primitive: it takes a logical <see cref="Line"/> (ordered styled
+/// <see cref="Segment"/>s) and returns a list of visual row <see cref="Line"/>s, each of which
+/// occupies at most <c>width</c> display columns when painted.
+/// </para>
+/// <para>
+/// Rules:
+/// <list type="bullet">
+///   <item>Hard character/cell wrap — NOT word-wrap. Breaks occur at column boundaries only.</item>
+///   <item>A wide (2-column) scalar that would straddle the row boundary moves wholly to the next
+///     row; the trailing single column of the current row is left empty (not padded — padding is
+///     the painter's responsibility).</item>
+///   <item>Segment <see cref="Style"/> is preserved on both halves when a segment is split.</item>
+///   <item>An empty <see cref="Line"/> (no segments, or only empty-text segments) produces
+///     exactly one empty row.</item>
+///   <item><c>width</c> &lt;= 0: treated as width 1 so callers never loop forever.
+///     Wide chars that exceed even width 1 are emitted on their own row rather than dropped.</item>
+///   <item>A single scalar wider than <c>width</c> (e.g., a 2-column emoji at
+///     width 1) is emitted on its own row rather than being dropped.</item>
+/// </list>
+/// </para>
+/// </summary>
+internal static class LineWrapper
+{
+    /// <summary>
+    /// Wraps <paramref name="line"/> into visual rows of at most <paramref name="width"/>
+    /// display columns.
+    /// </summary>
+    /// <param name="line">The logical line to wrap.</param>
+    /// <param name="width">
+    /// Target display width in columns. Values &lt;= 0 are clamped to 1 internally.
+    /// </param>
+    /// <returns>
+    /// One or more <see cref="Line"/> values representing the visual rows. Never empty.
+    /// </returns>
+    internal static IReadOnlyList<Line> Wrap(Line line, int width)
+    {
+        // Clamp width so callers don't need to guard.
+        if (width <= 0)
+            width = 1;
+
+        // Fast path: no segments or all-empty text.
+        bool anyContent = false;
+        foreach (Segment seg in line.Segments)
+        {
+            if (seg.Text.Length > 0)
+            {
+                anyContent = true;
+                break;
+            }
+        }
+        if (!anyContent)
+            return [new Line(Array.Empty<Segment>())];
+
+        List<Line> rows = [];
+        List<Segment> current = [];
+        int col = 0; // current column within the row being built
+
+        void FlushRow()
+        {
+            rows.Add(new Line(current.ToArray()));
+            current.Clear();
+            col = 0;
+        }
+
+        void AppendToRow(string text, Style style)
+        {
+            if (text.Length > 0)
+                current.Add(new Segment(text, style));
+        }
+
+        foreach (Segment segment in line.Segments)
+        {
+            Style style = segment.Style;
+            string text = segment.Text;
+
+            if (text.Length == 0)
+                continue;
+
+            // Walk rune-by-rune through the segment text.
+            // We track the UTF-16 char-index position so we can slice the string.
+            int segStart = 0; // start of the pending fragment within 'text' (char index)
+            int charIdx = 0;  // current char index
+
+            foreach (Rune rune in text.EnumerateRunes())
+            {
+                int runeWidth = DisplayWidth.MeasureWithColumn(rune, col);
+
+                if (col + runeWidth > width && col > 0)
+                {
+                    // Flush pending fragment before the break.
+                    string pending = text[segStart..charIdx];
+                    AppendToRow(pending, style);
+                    FlushRow();
+                    segStart = charIdx;
+                    // col is now 0 after FlushRow.
+                }
+
+                // Advance past this rune.
+                charIdx += rune.Utf16SequenceLength;
+                col += runeWidth;
+            }
+
+            // Flush any remaining text in the segment.
+            string tail = text[segStart..];
+            AppendToRow(tail, style);
+        }
+
+        // Flush the last row (may be empty if everything was exact multiples).
+        rows.Add(new Line(current.ToArray()));
+
+        return rows;
+    }
+}
diff --git a/src/Dcli/README.md b/src/Dcli/README.md
new file mode 100644
index 0000000..6c2288d
--- /dev/null
+++ b/src/Dcli/README.md
@@ -0,0 +1,139 @@
+# dcli
+
+**dcli** is a C#/.NET (`net10.0`) inline terminal-rendering library. Styled output flows
+into the terminal's native scrollback — not an alternate screen — while a small interactive
+region (input editor, status bar, and dropdown overlays) stays pinned at the bottom. The
+design is modelled on the rendering style used by Claude Code and similar AI-driven CLI tools.
+
+dcli is a mechanics-and-rendering library. It owns the raw-mode session, the render loop,
+and the widget layer. The calling application owns data, semantics, and any application
+protocol.
+
+## Requirements
+
+- .NET 10.0 or later
+- A modern VT-capable terminal:
+  - **macOS**: Terminal.app, iTerm2, Ghostty
+  - **Windows**: Windows Terminal (legacy `conhost.exe` is not supported)
+  - **Linux**: xterm, xterm-256color, or any xterm-class terminal emulator
+- The process must be attached to a real tty (not redirected to a file or pipe)
+
+## Quick start
+
+```csharp
+using Dcli;
+
+// Enter raw mode and start the render loop.
+await using Terminal t = await Terminal.StartAsync(new TerminalOptions());
+
+// Append a styled line to the scrollback.
+Line greeting = new LineBuilder()
+    .Bold("dcli")
+    .Text(" — inline terminal rendering")
+    .Build();
+t.Scrollback.Append(greeting);
+
+// Set a status bar message.
+t.Status.SetRows(new LineBuilder().Dim("Press Enter to continue…").Build());
+
+// Ask the user to pick from a list (blocks until the user submits or cancels).
+DialogResult<int> result = await t.SelectAsync(
+    new SelectRequest(
+        Items: ["Option A", "Option B", "Option C"]
+            .Select(s => new LineBuilder().Text(s).Build())
+            .ToList()),
+    cancellationToken: default);
+
+if (result.Outcome == DialogOutcome.Submitted)
+{
+    t.Scrollback.Append(new LineBuilder()
+        .Text($"Selected index: {result.Value}")
+        .Build());
+}
+
+// DisposeAsync stops the loop and restores the terminal.
+```
+
+Terminal restore is guaranteed on every exit path: normal disposal, unhandled exceptions,
+and OS signals (SIGTERM/SIGINT on POSIX, process-exit events on Windows).
+
+## Public surface
+
+### Entry point
+
+| Type | Purpose |
+|------|---------|
+| `Terminal` | Concrete lifecycle handle; obtain via `Terminal.StartAsync(...)`. |
+| `ITerminal` | Interface the application should depend on (allows headless fakes in tests). |
+| `TerminalOptions` | Configuration passed to `StartAsync` (max fixed-region height, frame interval). |
+| `TerminalNotSupportedException` | Thrown by `StartAsync` when the environment is not VT-capable. |
+
+### Sub-surfaces (accessed via `ITerminal`)
+
+| Property | Interface | Purpose |
+|----------|-----------|---------|
+| `Scrollback` | `IScrollback` | Append lines, live blocks, and collapsibles. |
+| `Input` | `IInput` | Programmatic text control of the input editor. |
+| `Status` | `IStatus` | Set the status bar rows at the bottom of the fixed region. |
+| `Autocomplete` | `IAutocomplete` | Show and hide the completion dropdown overlay. |
+
+### Dialogs (methods on `ITerminal`)
+
+| Method | Returns | Description |
+|--------|---------|-------------|
+| `SelectAsync` | `DialogResult<int>` | Single-select list; result is the zero-based selected index. |
+| `MultiSelectAsync` | `DialogResult<int[]>` | Multi-select list; result is the checked indices in ascending order. |
+| `ChoiceAsync` | `DialogResult<int>` | Single-select with an explanatory prompt row. |
+| `InputAsync` | `DialogResult<string>` | Free-text entry dialog, with optional masking for secrets. |
+
+All dialogs return `DialogOutcome.Submitted` on confirmation or `DialogOutcome.Cancelled`
+on Escape or cancellation-token cancellation.
+
+### Events
+
+Drain `ITerminal.Events` (`ChannelReader<TerminalEvent>`) on a background task. Event types:
+
+| Type | Description |
+|------|-------------|
+| `InputSubmitted` | User pressed Enter; carries the submitted text. |
+| `InputChanged` | Input buffer changed; carries the current text. |
+| `KeyPressed` | A key not consumed by the editor was forwarded to the consumer. |
+| `Resized` | Terminal was resized; carries the new column and row counts. |
+
+### Styled-text primitives
+
+| Type | Description |
+|------|-------------|
+| `Line` | Immutable ordered list of `Segment`s forming one logical line. |
+| `Segment` | Immutable run of literal text with a single `Style` applied. |
+| `Style` | Optional foreground/background `Color` plus `Format` flags. |
+| `Color` | Discriminated color: 16 named ANSI, 256-palette index, or 24-bit RGB. |
+| `Format` | `[Flags]` enum: `Bold`, `Italic`, `Underline`, `Dim`, `Reverse`, `Strikethrough`. |
+| `LineBuilder` | Fluent builder for constructing a `Line` from styled fragments. |
+
+### Scrollback handle types
+
+| Type | Description |
+|------|-------------|
+| `ILiveBlock` | Handle for an in-progress mutable scrollback block (`BeginLive`). |
+| `ICollapsible` | Handle for a collapsible block that can be expanded once (`BeginCollapsible`). |
+
+### Input event types
+
+| Type | Description |
+|------|-------------|
+| `KeyEvent` | A decoded key press: a `KeyCode` (Unicode scalar or `NamedKey`) plus `Modifiers`. |
+| `PasteEvent` | A bracketed-paste block decoded from UTF-8. |
+| `ResizeEvent` | A terminal resize (not emitted to the consumer; use the `Resized` terminal event). |
+| `NamedKey` | Enum of non-printable keys: arrows, F-keys, Tab, Enter, Escape, etc. |
+| `Modifiers` | Flags enum: `Ctrl`, `Alt`, `Shift`. |
+| `AutocompleteCandidate` | A completion candidate with display `Line` and `InsertText` to apply on accept. |
+
+## Testing
+
+`Dcli.Testing` — a headless harness that runs the full library without a real terminal —
+is shipping as a separate NuGet package in the next release.
+
+## License
+
+[Mozilla Public License 2.0](https://github.com/daemonicai/dcli/blob/main/LICENSE)
diff --git a/src/Dcli/ScrollbackSurface.cs b/src/Dcli/ScrollbackSurface.cs
new file mode 100644
index 0000000..26512ee
--- /dev/null
+++ b/src/Dcli/ScrollbackSurface.cs
@@ -0,0 +1,292 @@
+using Dcli.Internal.RenderLoop;
+using Dcli.Internal.Scrollback;
+
+namespace Dcli;
+
+// ─────────────────────────────────────────────────────────────────────────────
+// Public handle interfaces
+// ─────────────────────────────────────────────────────────────────────────────
+
+/// <summary>
+/// A handle to a live (mutable) scrollback block that is still visible in the live window.
+/// All methods post fire-and-forget commands; they are applied on the render-loop thread.
+/// </summary>
+/// <remarks>
+/// Obtain a handle via <see cref="ScrollbackSurface.BeginLive"/>.
+/// The underlying <c>LiveBlock</c> object is pre-created on the calling thread and is only
+/// <em>mutated</em> on the loop thread via commands. The handle merely carries the reference
+/// into command constructors. This is the one permitted exception to the "no off-thread model
+/// access" rule: the object is constructed off-thread but written exclusively on the loop thread.
+/// </remarks>
+public interface ILiveBlock
+{
+    /// <summary>
+    /// Appends <paramref name="text"/> to the block's accumulation buffer.
+    /// No-op after <see cref="Commit"/>. No-op if <see cref="SetContent"/> was already called.
+    /// </summary>
+    void AppendText(string text);
+
+    /// <summary>
+    /// Replaces the block's content wholesale with <paramref name="lines"/>.
+    /// After this call <see cref="AppendText"/> is a no-op.
+    /// No-op after <see cref="Commit"/>.
+    /// </summary>
+    void SetContent(IReadOnlyList<Line> lines);
+
+    /// <summary>
+    /// Freezes the block and removes it from the live window on the next paint.
+    /// Its final rendered rows flow into native scrollback.
+    /// </summary>
+    void Commit();
+}
+
+/// <summary>
+/// A handle to a collapsible scrollback block.
+/// All methods post fire-and-forget commands; they are applied on the render-loop thread.
+/// </summary>
+/// <remarks>
+/// Obtain a handle via <see cref="ScrollbackSurface.BeginCollapsible"/>.
+/// The underlying <c>Collapsible</c> object is pre-created on the calling thread with
+/// immutable <c>summary</c> and <c>hiddenLines</c>; only the loop thread mutates its expanded
+/// state. Incremental append to the hidden-line list after construction is a documented gap —
+/// the model stores hidden lines as an immutable snapshot at construction time.
+/// </remarks>
+public interface ICollapsible
+{
+    /// <summary>
+    /// Expands the collapsible to reveal its hidden lines. One-way and idempotent.
+    /// If already expanded or the block has scrolled past the commit horizon, this is a no-op.
+    /// An oversized expansion reprints the hidden lines into native scrollback flow instead of
+    /// expanding in-place.
+    /// </summary>
+    void Expand();
+}
+
+// ─────────────────────────────────────────────────────────────────────────────
+// Façade surface
+// ─────────────────────────────────────────────────────────────────────────────
+
+/// <summary>
+/// Consumer-facing surface for appending content to the scrollback live window.
+/// </summary>
+/// <remarks>
+/// <para>
+/// All methods post fire-and-forget <see cref="Internal.RenderLoop.ILoopCommand"/>s; they
+/// return before the command is applied or a frame is painted.
+/// </para>
+/// <para>
+/// <strong>Thread safety:</strong> all methods are safe to call from any thread; internally
+/// they post to the loop's inbound channel. The render-loop thread is the sole mutator of
+/// scrollback state.
+/// </para>
+/// <para>
+/// <strong>Documented gaps:</strong>
+/// <list type="bullet">
+///   <item><c>AppendRule</c> — needs a width-aware rule line-object that does not yet exist;
+///     deferred to a future change.</item>
+/// </list>
+/// </para>
+/// </remarks>
+public sealed class ScrollbackSurface : IScrollback
+{
+    private readonly LoopEngine _loop;
+
+    internal ScrollbackSurface(LoopEngine loop)
+    {
+        ArgumentNullException.ThrowIfNull(loop);
+        _loop = loop;
+    }
+
+    /// <summary>
+    /// Appends a single styled line to the scrollback live window.
+    /// </summary>
+    public void Append(Line line)
+    {
+        ArgumentNullException.ThrowIfNull(line);
+        _loop.Post(new AppendToScrollbackCommand(line));
+    }
+
+    /// <summary>
+    /// Begins a new live block in the scrollback, returning a handle for incremental mutation.
+    /// </summary>
+    /// <remarks>
+    /// The underlying <c>LiveBlock</c> is pre-created on the calling thread and inserted into
+    /// the live list via an enqueued command. Callers may immediately use the returned handle —
+    /// there is no race window because handle methods also enqueue commands that are applied
+    /// in FIFO order after the insert command.
+    /// </remarks>
+    public ILiveBlock BeginLive()
+    {
+        LiveBlock block = new();
+        _loop.Post(new InsertLiveBlockCommand(block));
+        return new LiveBlockHandle(_loop, block);
+    }
+
+    /// <summary>
+    /// Begins a new collapsible block, returning a handle for one-time expansion.
+    /// </summary>
+    /// <param name="summary">The summary line shown while the block is collapsed.</param>
+    /// <param name="hiddenLines">The lines revealed when the block is expanded.</param>
+    /// <remarks>
+    /// Hidden lines are captured as an immutable snapshot at construction time. Incremental
+    /// append to the hidden-line list after the handle is created is a documented gap for a
+    /// future refinement.
+    /// </remarks>
+    public ICollapsible BeginCollapsible(Line summary, IReadOnlyList<Line> hiddenLines)
+    {
+        ArgumentNullException.ThrowIfNull(summary);
+        ArgumentNullException.ThrowIfNull(hiddenLines);
+        Collapsible collapsible = new(summary, hiddenLines);
+        _loop.Post(new InsertCollapsibleCommand(collapsible));
+        return new CollapsibleHandle(_loop, collapsible);
+    }
+
+    // ── Internal handle implementations ───────────────────────────────────────
+
+    private sealed class LiveBlockHandle : ILiveBlock
+    {
+        private readonly LoopEngine _loop;
+        private readonly LiveBlock _block;
+
+        internal LiveBlockHandle(LoopEngine loop, LiveBlock block)
+        {
+            _loop = loop;
+            _block = block;
+        }
+
+        public void AppendText(string text)
+        {
+            ArgumentNullException.ThrowIfNull(text);
+            _loop.Post(new AppendTextToLiveBlockCommand(_block, text));
+        }
+
+        public void SetContent(IReadOnlyList<Line> lines)
+        {
+            ArgumentNullException.ThrowIfNull(lines);
+            _loop.Post(new SetLiveBlockContentCommand(_block, lines));
+        }
+
+        public void Commit()
+        {
+            _loop.Post(new CommitLiveBlockFacadeCommand(_block));
+        }
+    }
+
+    private sealed class CollapsibleHandle : ICollapsible
+    {
+        private readonly LoopEngine _loop;
+        private readonly Collapsible _collapsible;
+
+        internal CollapsibleHandle(LoopEngine loop, Collapsible collapsible)
+        {
+            _loop = loop;
+            _collapsible = collapsible;
+        }
+
+        public void Expand()
+        {
+            _loop.Post(new ExpandCollapsibleFacadeCommand(_collapsible));
+        }
+    }
+
+    // ── Façade commands (obtain scrollback from model.Scrollback in Apply) ────
+
+    private sealed class AppendToScrollbackCommand : ILoopCommand
+    {
+        private readonly Line _line;
+
+        internal AppendToScrollbackCommand(Line line) => _line = line;
+
+        void ILoopCommand.Apply(RenderModel model)
+        {
+            model.Scrollback.Append(new TextBlock(_line), model);
+            model.MarkDirty();
+        }
+    }
+
+    private sealed class InsertLiveBlockCommand : ILoopCommand
+    {
+        private readonly LiveBlock _block;
+
+        internal InsertLiveBlockCommand(LiveBlock block) => _block = block;
+
+        void ILoopCommand.Apply(RenderModel model)
+        {
+            model.Scrollback.Append(_block, model);
+            model.MarkDirty();
+        }
+    }
+
+    private sealed class InsertCollapsibleCommand : ILoopCommand
+    {
+        private readonly Collapsible _collapsible;
+
+        internal InsertCollapsibleCommand(Collapsible collapsible) => _collapsible = collapsible;
+
+        void ILoopCommand.Apply(RenderModel model)
+        {
+            model.Scrollback.Append(_collapsible, model);
+            model.MarkDirty();
+        }
+    }
+
+    private sealed class AppendTextToLiveBlockCommand : ILoopCommand
+    {
+        private readonly LiveBlock _block;
+        private readonly string _text;
+
+        internal AppendTextToLiveBlockCommand(LiveBlock block, string text)
+        {
+            _block = block;
+            _text = text;
+        }
+
+        void ILoopCommand.Apply(RenderModel model)
+        {
+            _block.AppendText(_text);
+            model.MarkDirty();
+        }
+    }
+
+    private sealed class SetLiveBlockContentCommand : ILoopCommand
+    {
+        private readonly LiveBlock _block;
+        private readonly IReadOnlyList<Line> _lines;
+
+        internal SetLiveBlockContentCommand(LiveBlock block, IReadOnlyList<Line> lines)
+        {
+            _block = block;
+            _lines = lines;
+        }
+
+        void ILoopCommand.Apply(RenderModel model)
+        {
+            _block.SetContent(_lines);
+            model.MarkDirty();
+        }
+    }
+
+    private sealed class CommitLiveBlockFacadeCommand : ILoopCommand
+    {
+        private readonly LiveBlock _block;
+
+        internal CommitLiveBlockFacadeCommand(LiveBlock block) => _block = block;
+
+        void ILoopCommand.Apply(RenderModel model)
+        {
+            model.Scrollback.CommitLiveBlock(_block, model);
+        }
+    }
+
+    private sealed class ExpandCollapsibleFacadeCommand : ILoopCommand
+    {
+        private readonly Collapsible _collapsible;
+
+        internal ExpandCollapsibleFacadeCommand(Collapsible collapsible) => _collapsible = collapsible;
+
+        void ILoopCommand.Apply(RenderModel model)
+        {
+            model.Scrollback.ExpandCollapsible(_collapsible, model);
+        }
+    }
+}
diff --git a/src/Dcli/Segment.cs b/src/Dcli/Segment.cs
new file mode 100644
index 0000000..cf95cb7
--- /dev/null
+++ b/src/Dcli/Segment.cs
@@ -0,0 +1,18 @@
+namespace Dcli;
+
+/// <summary>
+/// An immutable run of text with a single <see cref="Dcli.Style"/> applied uniformly across it.
+/// <para>
+/// Text is stored and returned verbatim — no markup, escape sequences, or control characters are
+/// interpreted. A string such as <c>"[bold]"</c> is literal content, not a formatting directive.
+/// </para>
+/// </summary>
+/// <param name="Text">
+/// The literal text content. May be empty; must not be <see langword="null"/> (enforced by the
+/// nullable reference-type annotation).
+/// </param>
+/// <param name="Style">
+/// The style applied to this segment. Defaults to <c>default(Style)</c> (terminal defaults, no
+/// formatting attributes).
+/// </param>
+public record Segment(string Text, Style Style = default);
diff --git a/src/Dcli/StatusSurface.cs b/src/Dcli/StatusSurface.cs
new file mode 100644
index 0000000..80c0d42
--- /dev/null
+++ b/src/Dcli/StatusSurface.cs
@@ -0,0 +1,66 @@
+using Dcli.Internal.RenderLoop;
+
+namespace Dcli;
+
+/// <summary>
+/// Consumer-facing surface for setting the status bar rows at the bottom of the fixed region.
+/// </summary>
+/// <remarks>
+/// <para>
+/// All methods post fire-and-forget <see cref="Internal.RenderLoop.ILoopCommand"/>s; they
+/// return before the command is applied or a frame is painted.
+/// </para>
+/// <para>
+/// <strong>Sacred status:</strong> the status bar is always fully rendered regardless of height
+/// budget. When the terminal is very short and the budget is exhausted by the status rows, the
+/// input editor is squeezed or hidden; the status is never truncated.
+/// </para>
+/// <para>
+/// <strong>Thread safety:</strong> all methods are safe to call from any thread.
+/// </para>
+/// </remarks>
+public sealed class StatusSurface : IStatus
+{
+    private readonly LoopEngine _loop;
+
+    internal StatusSurface(LoopEngine loop)
+    {
+        ArgumentNullException.ThrowIfNull(loop);
+        _loop = loop;
+    }
+
+    /// <summary>
+    /// Replaces the status bar content with the given rows.
+    /// An empty argument list clears the status bar (no rows rendered).
+    /// </summary>
+    public void SetRows(params Line[] rows)
+    {
+        ArgumentNullException.ThrowIfNull(rows);
+        _loop.Post(new SetStatusCommand(rows));
+    }
+
+    /// <summary>
+    /// Replaces the status bar content with the given rows.
+    /// Passing an empty list clears the status bar.
+    /// </summary>
+    public void SetRows(IReadOnlyList<Line> rows)
+    {
+        ArgumentNullException.ThrowIfNull(rows);
+        _loop.Post(new SetStatusCommand([.. rows]));
+    }
+
+    // ── Command ────────────────────────────────────────────────────────────────
+
+    private sealed class SetStatusCommand : ILoopCommand
+    {
+        private readonly IReadOnlyList<Line> _rows;
+
+        internal SetStatusCommand(IReadOnlyList<Line> rows) => _rows = rows;
+
+        void ILoopCommand.Apply(RenderModel model)
+        {
+            model.FixedRegion.Status.Rows = _rows;
+            model.MarkDirty();
+        }
+    }
+}
diff --git a/src/Dcli/Style.cs b/src/Dcli/Style.cs
new file mode 100644
index 0000000..0cbdd31
--- /dev/null
+++ b/src/Dcli/Style.cs
@@ -0,0 +1,23 @@
+namespace Dcli;
+
+/// <summary>
+/// Styling attributes for a <see cref="Segment"/>: optional foreground color, optional background
+/// color, and zero or more <see cref="Format"/> flags.
+/// <para>
+/// <c>default(Style)</c> (equivalently <c>new Style()</c>) means: inherit terminal default colors
+/// and no formatting attributes — identical to <c>new Style(null, null, Format.None)</c>.
+/// </para>
+/// </summary>
+/// <param name="Foreground">
+/// Foreground color, or <see langword="null"/> to use the terminal default.
+/// </param>
+/// <param name="Background">
+/// Background color, or <see langword="null"/> to use the terminal default.
+/// </param>
+/// <param name="Format">
+/// Text-formatting attributes to apply. Defaults to <see cref="Format.None"/>.
+/// </param>
+public readonly record struct Style(
+    Color? Foreground = null,
+    Color? Background = null,
+    Format Format = Format.None);
diff --git a/src/Dcli/Terminal.cs b/src/Dcli/Terminal.cs
new file mode 100644
index 0000000..9deb76e
--- /dev/null
+++ b/src/Dcli/Terminal.cs
@@ -0,0 +1,512 @@
+using System.Runtime.CompilerServices;
+using System.Threading.Channels;
+using Dcli.Internal;
+using Dcli.Internal.FixedRegion;
+using Dcli.Internal.Input;
+using Dcli.Internal.Posix;
+using Dcli.Internal.RenderLoop;
+using Dcli.Internal.Windows;
+
+namespace Dcli;
+
+/// <summary>
+/// The dcli terminal lifecycle handle.
+/// </summary>
+/// <remarks>
+/// <para>
+/// Obtain an instance via <see cref="StartAsync(TerminalOptions, CancellationToken)"/>. The handle wires together raw mode,
+/// the input reader, and the render loop. Dispose to stop the loop, join all threads, and
+/// restore the terminal.
+/// </para>
+/// <para>
+/// <strong>Restore guarantee:</strong> restoration occurs on <em>every</em> exit path —
+/// normal <see cref="DisposeAsync"/>, an unhandled exception on the loop thread (via the
+/// loop's <c>finally</c> block), and external signals or <c>ProcessExit</c> (via the
+/// <see cref="RestoreCoordinator"/>). All paths converge on the idempotent
+/// <see cref="IRawModeSession.Restore"/>, so no double-restore can occur.
+/// </para>
+/// </remarks>
+public sealed class Terminal : ITerminal
+{
+    private readonly IRawModeSession _session;
+    private readonly RestoreCoordinator _coordinator;
+    private readonly IResizeWatcher _resizeWatcher;
+    private readonly InputReader _inputReader;
+    private readonly LoopEngine _loop;
+
+    // DisposeAsync blocks the calling thread on bounded Thread.Join calls (loop ≤2 s,
+    // reader ≤500 ms) by design — await-using callers should expect thread blocking, not
+    // a truly asynchronous yield. IAsyncDisposable is exposed for the await-using pattern.
+    private int _disposed;
+
+    // Captured loop fault: set by DisposeAsync after joining the loop thread so that
+    // DisposeAsync rethrows and the caller learns the loop crashed.
+    private volatile Exception? _loopFault;
+
+    private Terminal(
+        IRawModeSession session,
+        RestoreCoordinator coordinator,
+        IResizeWatcher resizeWatcher,
+        InputReader inputReader,
+        LoopEngine loop)
+    {
+        _session = session;
+        _coordinator = coordinator;
+        _resizeWatcher = resizeWatcher;
+        _inputReader = inputReader;
+        _loop = loop;
+        Scrollback = new ScrollbackSurface(loop);
+        Input = new InputSurface(loop);
+        Status = new StatusSurface(loop);
+        Autocomplete = new AutocompleteSurface(loop);
+    }
+
+    // ── Internal surface (tests) ────────────────────────────────────────────
+
+    /// <summary>
+    /// The underlying loop engine. Exposed for headless tests that need to post commands
+    /// (e.g. to trigger a paint and observe loop-fault propagation). Not part of the public API.
+    /// </summary>
+    internal LoopEngine Loop => _loop;
+
+    /// <summary>
+    /// The restore coordinator. Exposed for tests that need to simulate signal-driven exit
+    /// (e.g. to verify the full Option C wiring: signal → halt loop → ANSI restore → termios).
+    /// Not part of the public API.
+    /// </summary>
+    internal RestoreCoordinator Coordinator => _coordinator;
+
+    // ── Public surface ──────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Scrollback content surface: append lines, live blocks, and collapsibles.
+    /// </summary>
+    /// <remarks>
+    /// Posts are fire-and-forget; all writes are applied on the render-loop thread.
+    /// </remarks>
+    public IScrollback Scrollback { get; }
+
+    /// <summary>
+    /// Input editor surface: programmatic <see cref="IInput.SetText"/> and
+    /// <see cref="IInput.Clear"/>. Does not emit <see cref="InputChanged"/>.
+    /// </summary>
+    public IInput Input { get; }
+
+    /// <summary>
+    /// Status bar surface: set the sacred status rows at the bottom of the fixed region.
+    /// </summary>
+    public IStatus Status { get; }
+
+    /// <summary>
+    /// Autocomplete overlay surface: show and hide the completion dropdown.
+    /// </summary>
+    public IAutocomplete Autocomplete { get; }
+
+    /// <summary>
+    /// The outbound terminal event stream.
+    /// </summary>
+    /// <remarks>
+    /// Drain this channel on a separate consumer thread or task. A consumer that never reads
+    /// accumulates events but does not stall the render loop.
+    /// </remarks>
+    public ChannelReader<TerminalEvent> Events => _loop.OutboundEvents;
+
+    /// <summary>
+    /// Returns the last-known terminal size from the volatile snapshot. Does not round-trip
+    /// to the render loop thread (Decision 10: snapshot read).
+    /// </summary>
+    public (int Columns, int Rows) GetTerminalSize() => _loop.GetTerminalSize();
+
+    // ── Awaitable dialogs (§12) ─────────────────────────────────────────────
+
+    /// <summary>
+    /// Opens a single-select list dialog and awaits the user's choice.
+    /// </summary>
+    /// <param name="req">The request describing the items and optional title.</param>
+    /// <param name="cancellationToken">
+    /// Cancels the dialog and returns <see cref="DialogOutcome.Cancelled"/>. If already
+    /// cancelled before the dialog opens, returns immediately with <see cref="DialogOutcome.Cancelled"/>.
+    /// </param>
+    /// <returns>
+    /// <see cref="DialogOutcome.Submitted"/> with the zero-based selected index, or
+    /// <see cref="DialogOutcome.Cancelled"/>. When the item list is empty and the user submits,
+    /// <see cref="DialogResult{T}.Value"/> is <c>-1</c>.
+    /// </returns>
+    /// <exception cref="ArgumentNullException"><paramref name="req"/> is <see langword="null"/>.</exception>
+    /// <exception cref="InvalidOperationException">A dialog is already active.</exception>
+    public Task<DialogResult<int>> SelectAsync(
+        SelectRequest req,
+        CancellationToken cancellationToken = default)
+    {
+        ArgumentNullException.ThrowIfNull(req);
+        Dialog dialog = new(multiSelect: false, modal: true, typeToFilter: false, title: req.Title);
+        dialog.List.SetItems(req.Items);
+        return OpenModalAsync<int>(
+            dialog,
+            () => new DialogResult<int>(DialogOutcome.Submitted, dialog.List.SelectedIndex),
+            cancellationToken);
+    }
+
+    /// <summary>
+    /// Opens a multi-select list dialog and awaits the user's selection.
+    /// </summary>
+    /// <param name="req">The request describing the items and optional title.</param>
+    /// <param name="cancellationToken">
+    /// Cancels the dialog and returns <see cref="DialogOutcome.Cancelled"/>.
+    /// </param>
+    /// <returns>
+    /// <see cref="DialogOutcome.Submitted"/> with the checked indices in ascending order, or
+    /// <see cref="DialogOutcome.Cancelled"/> with an empty array.
+    /// </returns>
+    /// <exception cref="ArgumentNullException"><paramref name="req"/> is <see langword="null"/>.</exception>
+    /// <exception cref="InvalidOperationException">A dialog is already active.</exception>
+    public Task<DialogResult<int[]>> MultiSelectAsync(
+        MultiSelectRequest req,
+        CancellationToken cancellationToken = default)
+    {
+        ArgumentNullException.ThrowIfNull(req);
+        Dialog dialog = new(multiSelect: true, modal: true, typeToFilter: false, title: req.Title);
+        dialog.List.SetItems(req.Items);
+        return OpenModalAsync<int[]>(
+            dialog,
+            () => new DialogResult<int[]>(DialogOutcome.Submitted, [.. dialog.List.CheckedIndices]),
+            cancellationToken);
+    }
+
+    /// <summary>
+    /// Opens a single-select choice dialog and awaits the user's choice.
+    /// Semantically equivalent to <see cref="SelectAsync"/> with an optional prompt row.
+    /// </summary>
+    /// <param name="req">The request describing the options and optional prompt.</param>
+    /// <param name="cancellationToken">
+    /// Cancels the dialog and returns <see cref="DialogOutcome.Cancelled"/>.
+    /// </param>
+    /// <returns>
+    /// <see cref="DialogOutcome.Submitted"/> with the zero-based selected index, or
+    /// <see cref="DialogOutcome.Cancelled"/>.
+    /// </returns>
+    /// <exception cref="ArgumentNullException"><paramref name="req"/> is <see langword="null"/>.</exception>
+    /// <exception cref="InvalidOperationException">A dialog is already active.</exception>
+    public Task<DialogResult<int>> ChoiceAsync(
+        ChoiceRequest req,
+        CancellationToken cancellationToken = default)
+    {
+        ArgumentNullException.ThrowIfNull(req);
+        Dialog dialog = new(multiSelect: false, modal: true, typeToFilter: false, title: req.Prompt);
+        dialog.List.SetItems(req.Options);
+        return OpenModalAsync<int>(
+            dialog,
+            () => new DialogResult<int>(DialogOutcome.Submitted, dialog.List.SelectedIndex),
+            cancellationToken);
+    }
+
+    /// <summary>
+    /// Opens a free-text input dialog and awaits the user's entry.
+    /// </summary>
+    /// <param name="req">The request describing the optional prompt, default text, and masking.</param>
+    /// <param name="cancellationToken">
+    /// Cancels the dialog and returns <see cref="DialogOutcome.Cancelled"/>. If already
+    /// cancelled before the dialog opens, returns immediately with <see cref="DialogOutcome.Cancelled"/>.
+    /// </param>
+    /// <returns>
+    /// <see cref="DialogOutcome.Submitted"/> with the entered text (possibly empty), or
+    /// <see cref="DialogOutcome.Cancelled"/> when the user presses Escape or the token fires.
+    /// When <see cref="InputRequest.IsSecret"/> is <see langword="true"/>, the rendered overlay
+    /// shows mask glyphs but <see cref="DialogResult{T}.Value"/> always carries the real text.
+    /// </returns>
+    /// <exception cref="ArgumentNullException"><paramref name="req"/> is <see langword="null"/>.</exception>
+    /// <exception cref="InvalidOperationException">A dialog is already active.</exception>
+    public Task<DialogResult<string>> InputAsync(
+        InputRequest req,
+        CancellationToken cancellationToken = default)
+    {
+        ArgumentNullException.ThrowIfNull(req);
+        InputDialog dialog = new(req.Prompt, req.Default, req.IsSecret);
+        return OpenModalAsync<string>(
+            dialog,
+            () => new DialogResult<string>(DialogOutcome.Submitted, dialog.Text),
+            cancellationToken);
+    }
+
+    /// <summary>
+    /// Core modal-open helper: creates a TCS, posts an <see cref="OpenDialogCommand"/>,
+    /// wires cancellation, and returns the awaitable task.
+    /// </summary>
+    /// <typeparam name="T">The result value type.</typeparam>
+    /// <param name="overlay">The prepared modal overlay.</param>
+    /// <param name="buildResult">
+    /// Parameterless factory called on the loop thread when the overlay is submitted.
+    /// The closure captures the overlay and reads its state (e.g. text, selection index).
+    /// Only called on <see cref="OverlayCloseKind.Submit"/>; Cancel produces a default result.
+    /// </param>
+    /// <param name="cancellationToken">External cancellation token.</param>
+    private Task<DialogResult<T>> OpenModalAsync<T>(
+        IModalOverlay overlay,
+        Func<DialogResult<T>> buildResult,
+        CancellationToken cancellationToken)
+    {
+        // Fast path: already cancelled — never open the dialog.
+        if (cancellationToken.IsCancellationRequested)
+            return Task.FromResult(new DialogResult<T>(DialogOutcome.Cancelled, default!));
+
+        TaskCompletionSource<DialogResult<T>> tcs =
+            new(TaskCreationOptions.RunContinuationsAsynchronously);
+
+        // The registration handle: we need to dispose it once the task completes to avoid leaks.
+        // Use a holder so the lambda can capture and dispose it.
+        CancellationTokenRegistration[] registrationHolder = new CancellationTokenRegistration[1];
+
+        Action completion = () =>
+        {
+            // Running on the loop thread. Read overlay state and complete the TCS.
+            DialogResult<T> result = overlay.CloseRequest == OverlayCloseKind.Submit
+                ? buildResult()
+                : new DialogResult<T>(DialogOutcome.Cancelled, default!);
+            tcs.TrySetResult(result);
+            // Dispose the CT registration to prevent a stale cancel command from posting later.
+            registrationHolder[0].Dispose();
+        };
+
+        Action reject = () =>
+        {
+            // Running on the loop thread. Fault the TCS — caller gets InvalidOperationException.
+            tcs.TrySetException(new InvalidOperationException("A dialog is already active."));
+            registrationHolder[0].Dispose();
+        };
+
+        // Post a cancel command when the token fires. The command checks object identity so
+        // it is a no-op if the overlay has already been dismissed naturally.
+        if (cancellationToken.CanBeCanceled)
+        {
+            Action cancelCompletion = () =>
+            {
+                // Running on the loop thread (from CancelDialogCommand.Apply).
+                tcs.TrySetResult(new DialogResult<T>(DialogOutcome.Cancelled, default!));
+                registrationHolder[0].Dispose();
+            };
+
+            registrationHolder[0] = cancellationToken.Register(
+                () => _loop.Post(new CancelDialogCommand(overlay, cancelCompletion)),
+                useSynchronizationContext: false);
+        }
+
+        _loop.Post(new OpenDialogCommand(overlay, completion, reject));
+        return tcs.Task;
+    }
+
+    // ── Factory ─────────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Enters raw mode and starts the render loop, returning a handle whose
+    /// <see cref="DisposeAsync"/> stops the loop and restores the terminal.
+    /// </summary>
+    /// <param name="options">Configuration; pass <c>new TerminalOptions()</c> for defaults.</param>
+    /// <param name="cancellationToken">Token that cancels the start-up sequence.</param>
+    /// <returns>A live <see cref="Terminal"/> handle.</returns>
+    /// <exception cref="TerminalNotSupportedException">
+    /// The environment does not support VT sequences (non-tty stdout/stdin, <c>TERM=dumb</c>,
+    /// or unsupported platform).
+    /// </exception>
+    public static Task<Terminal> StartAsync(
+        TerminalOptions options,
+        CancellationToken cancellationToken = default)
+    {
+        ArgumentNullException.ThrowIfNull(options);
+        return StartAsync(options, capabilityInputs: null, cancellationToken);
+    }
+
+    /// <summary>
+    /// Overload accepting injected <see cref="CapabilityInputs"/> for testing the capability gate
+    /// without a real tty. Internal — the public overload reads from the real environment.
+    /// </summary>
+    internal static Task<Terminal> StartAsync(
+        TerminalOptions options,
+        CapabilityInputs? capabilityInputs,
+        CancellationToken cancellationToken = default)
+    {
+        ArgumentNullException.ThrowIfNull(options);
+
+        ITerminalSizeSource sizeSource = CreatePlatformSizeSource();
+
+        // Capability gate + raw mode (§4). Throws TerminalNotSupportedException if not VT-capable.
+        IRawModeSession session = RawModeSession.Enter(capabilityInputs);
+
+        // Detect optional capabilities (truecolor, synchronized-output, ambiguous-width) after
+        // the VT gate passes. The gate runs first; capabilities are the "extras" layer.
+        TerminalCapabilities capabilities = TerminalCapabilityDetector.DetectCapabilities(capabilityInputs);
+
+        // Last-resort restore net for signals + ProcessExit (§4). Registered before the loop
+        // starts so no window exists between raw-mode entry and signal coverage.
+        RestoreCoordinator coordinator = RestoreCoordinator.Wire(session);
+
+        IInputByteSource byteSource = CreatePlatformInputByteSource();
+
+        IResizeWatcher resizeWatcher = CreatePlatformResizeWatcher(sizeSource);
+
+        // UTF-8, no BOM, no auto-flush — one explicit Flush() per frame keeps each
+        // synchronized-output fenced frame written as a single kernel write.
+        StreamWriter stdoutWriter = new(
+            Console.OpenStandardOutput(),
+            new System.Text.UTF8Encoding(encoderShouldEmitUTF8Identifier: false),
+            bufferSize: 65536,
+            leaveOpen: false);
+        IOutputSink sink = new VtFrameRenderer(stdoutWriter, capabilities);
+
+        Terminal terminal = StartCore(
+            session,
+            coordinator,
+            resizeWatcher,
+            byteSource,
+            new SystemClock(),
+            sink,
+            sizeSource,
+            TimeSpan.FromMilliseconds(options.MinFrameIntervalMs),
+            options.MaxFixedHeight);
+
+        return Task.FromResult(terminal);
+    }
+
+    /// <summary>
+    /// Internal start path for headless testing: accepts injected edges rather than real OS
+    /// resources. The <paramref name="session"/> is wired into the loop's <c>finally</c> and
+    /// into the <see cref="RestoreCoordinator"/> to cover all exit paths.
+    /// </summary>
+    internal static Terminal StartCore(
+        IRawModeSession session,
+        RestoreCoordinator coordinator,
+        IResizeWatcher resizeWatcher,
+        IInputByteSource byteSource,
+        IClock clock,
+        IOutputSink sink,
+        ITerminalSizeSource sizeSource,
+        TimeSpan minFrameInterval,
+        int? maxFixedHeight = null)
+    {
+        ArgumentNullException.ThrowIfNull(session);
+        ArgumentNullException.ThrowIfNull(coordinator);
+        ArgumentNullException.ThrowIfNull(resizeWatcher);
+        ArgumentNullException.ThrowIfNull(byteSource);
+        ArgumentNullException.ThrowIfNull(clock);
+        ArgumentNullException.ThrowIfNull(sink);
+        ArgumentNullException.ThrowIfNull(sizeSource);
+
+        // Wire the loop: pass the session's Restore as the loop-finally hook so that an
+        // unhandled exception on the loop thread still restores. The coordinator covers
+        // signals/ProcessExit; Dispose covers normal shutdown. All three converge on the
+        // same CAS-idempotent Restore.
+        LoopEngine loop = new(sizeSource, clock, sink, minFrameInterval,
+            restoreOnExit: session.Restore, maxFixedHeight: maxFixedHeight);
+
+        // Wire the loop halt into the coordinator so that signal-driven restore halts the
+        // loop first (causing its finally to emit the ANSI restore on the single-writer thread)
+        // before the coordinator calls the idempotent session restore.
+        coordinator.SetHaltAction(loop.Dispose);
+
+        // Wire resize watcher → loop's inbound channel (SIGWINCH callback posts ResizeEvent).
+        // Capture InputWriter once to avoid capturing the whole loop in the closure.
+        ChannelWriter<InputEvent> inputWriter = loop.InputWriter;
+        resizeWatcher.Start((cols, rows) => inputWriter.TryWrite(new ResizeEvent(cols, rows)));
+
+        // Wire input reader → loop's inbound channel.
+        InputReader inputReader = new(byteSource, loop.InputWriter);
+
+        return new Terminal(session, coordinator, resizeWatcher, inputReader, loop);
+    }
+
+    // ── Dispose ─────────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Stops the render loop and input reader, joins their threads, and restores the terminal.
+    /// Idempotent: safe to call more than once. Rethrows any fatal exception that caused the
+    /// render loop to crash (observable via the faulted <see cref="LoopEngine.LoopTerminated"/>
+    /// task); the terminal is always restored before the exception is rethrown.
+    /// </summary>
+    public ValueTask DisposeAsync()
+    {
+        if (Interlocked.Exchange(ref _disposed, 1) != 0)
+        {
+            // Rethrow a previously-captured loop fault on repeated calls too, so the caller
+            // always sees the crash regardless of which DisposeAsync call they await.
+            Exception? fault = _loopFault;
+            if (fault is not null)
+                return ValueTask.FromException(fault);
+            return ValueTask.CompletedTask;
+        }
+
+        // Stop the resize watcher first so it stops posting once the loop is shutting down.
+        _resizeWatcher.Dispose();
+
+        // Stop the loop first (cancels CTS, completes channel) then stop the reader.
+        // Both Dispose() calls block for a short join timeout.
+        _loop.Dispose();
+        _inputReader.Dispose();
+
+        // Deregister signal/ProcessExit hooks and call Restore (idempotent).
+        _coordinator.Dispose();
+
+        // Dispose the raw-mode session (also calls Restore; idempotent).
+        _session.Dispose();
+
+        // After the loop thread has joined, inspect whether it faulted.
+        // LoopTerminated is already completed at this point (the thread joined above).
+        Task loopTask = _loop.LoopTerminated;
+        if (loopTask.IsFaulted)
+        {
+            Exception fault = loopTask.Exception!.GetBaseException();
+            _loopFault = fault;
+            return ValueTask.FromException(fault);
+        }
+
+        return ValueTask.CompletedTask;
+    }
+
+    // ── Platform wiring helpers ─────────────────────────────────────────────
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    private static ITerminalSizeSource CreatePlatformSizeSource()
+    {
+        if (OperatingSystem.IsLinux() || OperatingSystem.IsMacOS())
+            return new PosixTerminalSizeSource();
+
+        if (OperatingSystem.IsWindows())
+            return new WindowsTerminalSizeSource();
+
+        // Fallback: unknown platform; §4 will have thrown TerminalNotSupportedException before
+        // we reach this, but return a safe default so the compiler is satisfied.
+        return new FixedFallbackSizeSource();
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    private static IResizeWatcher CreatePlatformResizeWatcher(ITerminalSizeSource sizeSource)
+    {
+        if (OperatingSystem.IsLinux() || OperatingSystem.IsMacOS())
+            return new PosixResizeWatcher(sizeSource);
+
+        if (OperatingSystem.IsWindows())
+            return new WindowsResizeWatcher();
+
+        // Unknown platform; §4 will have thrown before this, but satisfy the compiler.
+        return new NoopResizeWatcher();
+    }
+
+    [MethodImpl(MethodImplOptions.NoInlining)]
+    private static IInputByteSource CreatePlatformInputByteSource()
+    {
+        if (OperatingSystem.IsLinux() || OperatingSystem.IsMacOS())
+            return new PosixInputByteSource();
+
+        if (OperatingSystem.IsWindows())
+            return new WindowsInputByteSource();
+
+        throw new PlatformNotSupportedException("dcli only supports Linux, macOS, and Windows.");
+    }
+
+    // ── Private helpers ─────────────────────────────────────────────────────
+
+    /// <summary>Safe 80×24 fallback used when the platform is unknown (§4 rejects it first).</summary>
+    private sealed class FixedFallbackSizeSource : ITerminalSizeSource
+    {
+        public (int Columns, int Rows) GetSize() => (80, 24);
+    }
+
+}
diff --git a/src/Dcli/TerminalEvent.cs b/src/Dcli/TerminalEvent.cs
new file mode 100644
index 0000000..de9984f
--- /dev/null
+++ b/src/Dcli/TerminalEvent.cs
@@ -0,0 +1,40 @@
+namespace Dcli;
+
+/// <summary>
+/// Discriminated union of all events the library emits to the consumer.
+/// Delivered on the outbound <see cref="System.Threading.Channels.Channel{T}"/> that
+/// the consumer drains on its own thread. The loop never executes consumer code.
+/// </summary>
+/// <remarks>
+/// All subtypes are publicly constructible so consumer tests can synthesize events
+/// without a real terminal.
+/// </remarks>
+public abstract record TerminalEvent;
+
+/// <summary>
+/// The user submitted the current input line (e.g. pressed Enter).
+/// The full text of the submitted line is captured at the moment of submission.
+/// </summary>
+/// <param name="Text">The submitted text.</param>
+public sealed record InputSubmitted(string Text) : TerminalEvent;
+
+/// <summary>
+/// The input buffer changed (e.g. a character was typed or deleted).
+/// </summary>
+/// <param name="Text">The current contents of the input buffer after the change.</param>
+public sealed record InputChanged(string Text) : TerminalEvent;
+
+/// <summary>
+/// A key press that the fixed-region input editor did not consume was forwarded to
+/// the consumer. Allows the consumer to handle application-level shortcuts.
+/// </summary>
+/// <param name="Key">The key event as decoded by the VT input parser.</param>
+public sealed record KeyPressed(KeyEvent Key) : TerminalEvent;
+
+/// <summary>
+/// The terminal was resized. The new dimensions reflect the first authoritative size
+/// measurement after the resize signal was received.
+/// </summary>
+/// <param name="Columns">New terminal width in columns.</param>
+/// <param name="Rows">New terminal height in rows.</param>
+public sealed record Resized(int Columns, int Rows) : TerminalEvent;
diff --git a/src/Dcli/TerminalNotSupportedException.cs b/src/Dcli/TerminalNotSupportedException.cs
new file mode 100644
index 0000000..bc722fc
--- /dev/null
+++ b/src/Dcli/TerminalNotSupportedException.cs
@@ -0,0 +1,50 @@
+namespace Dcli;
+
+/// <summary>
+/// Thrown when dcli detects that the current terminal does not support the VT escape sequences
+/// required for operation.
+/// <para>
+/// This exception is thrown by <c>TerminalCapabilityDetector.EnsureVtCapable</c> before
+/// raw mode is entered, so the terminal is always in a known good state when the exception
+/// propagates.
+/// </para>
+/// <para>
+/// Typical causes:
+/// <list type="bullet">
+///   <item>The process is not attached to a tty (stdout is redirected to a file or pipe).</item>
+///   <item><c>TERM=dumb</c> or <c>TERM</c> is empty, indicating a terminal that does not support
+///     VT control sequences.</item>
+///   <item>Running on an unsupported platform (not Linux, macOS, or Windows).</item>
+/// </list>
+/// </para>
+/// </summary>
+public sealed class TerminalNotSupportedException : Exception
+{
+    /// <summary>
+    /// Initialises a new instance of <see cref="TerminalNotSupportedException"/> with no message.
+    /// </summary>
+    public TerminalNotSupportedException()
+    {
+    }
+
+    /// <summary>
+    /// Initialises a new instance of <see cref="TerminalNotSupportedException"/> with a
+    /// descriptive message.
+    /// </summary>
+    /// <param name="message">Human-readable description of why the terminal is not supported.</param>
+    public TerminalNotSupportedException(string message)
+        : base(message)
+    {
+    }
+
+    /// <summary>
+    /// Initialises a new instance of <see cref="TerminalNotSupportedException"/> with a message
+    /// and an inner exception.
+    /// </summary>
+    /// <param name="message">Human-readable description of why the terminal is not supported.</param>
+    /// <param name="innerException">The exception that caused this one, if any.</param>
+    public TerminalNotSupportedException(string message, Exception innerException)
+        : base(message, innerException)
+    {
+    }
+}
diff --git a/src/Dcli/TerminalOptions.cs b/src/Dcli/TerminalOptions.cs
new file mode 100644
index 0000000..f987a13
--- /dev/null
+++ b/src/Dcli/TerminalOptions.cs
@@ -0,0 +1,25 @@
+namespace Dcli;
+
+/// <summary>
+/// Configuration options for <see cref="Terminal.StartAsync(TerminalOptions, CancellationToken)"/>.
+/// </summary>
+public sealed class TerminalOptions
+{
+    /// <summary>
+    /// Maximum height in rows of the fixed interactive region pinned at the bottom of the
+    /// terminal. <see langword="null"/> means "no explicit cap".
+    /// </summary>
+    /// <remarks>
+    /// The fixed region includes the caret line, status bar, and any overlay (dialog /
+    /// autocomplete). When <see langword="null"/>, the render loop applies an internal default
+    /// based on the terminal height (clamped to a minimum of 8 rows).
+    /// </remarks>
+    public int? MaxFixedHeight { get; init; }
+
+    /// <summary>
+    /// Minimum interval between successive rendered frames, in milliseconds.
+    /// Defaults to <c>16</c> (≈ 60 fps ceiling). Increase for lower-frequency rendering;
+    /// decrease only in tests or benchmark scenarios.
+    /// </summary>
+    public int MinFrameIntervalMs { get; init; } = 16;
+}
diff --git a/tests/Dcli.Tests/AutocompleteTests.cs b/tests/Dcli.Tests/AutocompleteTests.cs
new file mode 100644
index 0000000..b434f59
--- /dev/null
+++ b/tests/Dcli.Tests/AutocompleteTests.cs
@@ -0,0 +1,320 @@
+using System.Text;
+using Dcli.Internal.FixedRegion;
+using Xunit;
+
+namespace Dcli.Tests;
+
+/// <summary>
+/// Unit tests for <see cref="Autocomplete"/> — task 11.3.
+/// </summary>
+public sealed class AutocompleteTests
+{
+    // ── Helpers ──────────────────────────────────────────────────────────────────────────────
+
+    private static TextBuffer Buffer(string text = "")
+    {
+        TextBuffer buf = new();
+        if (text.Length > 0)
+            buf.Insert(text);
+        return buf;
+    }
+
+    private static AutocompleteCandidate Candidate(string insertText, string display) =>
+        new(insertText, new LineBuilder().Text(display).Build());
+
+    private static List<AutocompleteCandidate> Candidates(params (string insertText, string display)[] items) =>
+        items.Select(i => Candidate(i.insertText, i.display)).ToList();
+
+    private static KeyEvent Named(NamedKey key) =>
+        new(KeyCode.Named(key), Modifiers.None);
+
+    private static KeyEvent Char(char c) =>
+        new(KeyCode.FromRune(new Rune(c)), Modifiers.None);
+
+    private static bool AllReversed(Line line) =>
+        line.Segments.Count > 0 &&
+        line.Segments.All(s => (s.Style.Format & Format.Reverse) == Format.Reverse);
+
+    private static bool AnySegmentContains(Line line, string text) =>
+        line.Segments.Any(s => s.Text.Contains(text, StringComparison.Ordinal));
+
+    // ── Placement / HidesCursor ───────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void PlacementIsBelowInput()
+    {
+        Autocomplete ac = new(Buffer());
+        Assert.Equal(OverlayPlacement.BelowInput, ac.Placement);
+    }
+
+    [Fact]
+    public void HidesCursorIsFalse()
+    {
+        Autocomplete ac = new(Buffer());
+        Assert.False(ac.HidesCursor);
+    }
+
+    // ── IsDismissed ───────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void InitiallyDismissed()
+    {
+        Autocomplete ac = new(Buffer());
+        Assert.True(ac.IsDismissed);
+    }
+
+    [Fact]
+    public void AfterShowNotDismissed()
+    {
+        Autocomplete ac = new(Buffer());
+        ac.Show(Candidates(("foo", "foo")));
+        Assert.False(ac.IsDismissed);
+    }
+
+    [Fact]
+    public void AfterHideDismissed()
+    {
+        Autocomplete ac = new(Buffer());
+        ac.Show(Candidates(("foo", "foo")));
+        ac.Hide();
+        Assert.True(ac.IsDismissed);
+    }
+
+    [Fact]
+    public void AfterAcceptWithEnterDismissed()
+    {
+        Autocomplete ac = new(Buffer());
+        ac.Show(Candidates(("foo", "foo")));
+        ac.HandleKey(Named(NamedKey.Enter));
+        Assert.True(ac.IsDismissed);
+    }
+
+    [Fact]
+    public void AfterEscapeDismissed()
+    {
+        Autocomplete ac = new(Buffer());
+        ac.Show(Candidates(("foo", "foo")));
+        ac.HandleKey(Named(NamedKey.Escape));
+        Assert.True(ac.IsDismissed);
+    }
+
+    // ── Render ────────────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void ShowRendersCandidateDisplayLines()
+    {
+        Autocomplete ac = new(Buffer());
+        ac.Show(Candidates(("foo", "Foo item"), ("bar", "Bar item")));
+
+        IReadOnlyList<Line> rows = ac.Render(80);
+
+        Assert.Equal(2, rows.Count);
+        Assert.True(AnySegmentContains(rows[0], "Foo item"));
+        Assert.True(AnySegmentContains(rows[1], "Bar item"));
+    }
+
+    [Fact]
+    public void RenderEmptyAfterHide()
+    {
+        Autocomplete ac = new(Buffer());
+        ac.Show(Candidates(("x", "X")));
+        ac.Hide();
+
+        // Render is still delegated to the list; the list was set with items.
+        // The dismissed state is tracked by IsDismissed; Render itself returns
+        // whatever the list returns (composer will not call Render when dismissed).
+        // Verify list still has items but IsDismissed is true.
+        Assert.True(ac.IsDismissed);
+    }
+
+    [Fact]
+    public void ShowWithEmptyCandidateListIsAllowed()
+    {
+        Autocomplete ac = new(Buffer());
+        ac.Show([]);
+        Assert.False(ac.IsDismissed);
+        Assert.Empty(ac.Render(80));
+    }
+
+    // ── Navigation ────────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void DownMovesSelectionToNextRow()
+    {
+        Autocomplete ac = new(Buffer());
+        ac.Show(Candidates(("a", "A"), ("b", "B"), ("c", "C")));
+
+        bool consumed = ac.HandleKey(Named(NamedKey.Down));
+
+        Assert.True(consumed);
+        IReadOnlyList<Line> rows = ac.Render(80);
+        // Row 1 (index 1 = "B") should be highlighted.
+        Assert.True(AllReversed(rows[1]));
+        Assert.False(AllReversed(rows[0]));
+    }
+
+    [Fact]
+    public void UpMovesSelectionToPreviousRow()
+    {
+        Autocomplete ac = new(Buffer());
+        ac.Show(Candidates(("a", "A"), ("b", "B"), ("c", "C")));
+        ac.HandleKey(Named(NamedKey.Down));
+        ac.HandleKey(Named(NamedKey.Down));
+
+        bool consumed = ac.HandleKey(Named(NamedKey.Up));
+
+        Assert.True(consumed);
+        IReadOnlyList<Line> rows = ac.Render(80);
+        // Row 1 (index 1 = "B") should be highlighted after going down twice then up once.
+        Assert.True(AllReversed(rows[1]));
+    }
+
+    [Fact]
+    public void FirstRowIsSelectedAfterShow()
+    {
+        Autocomplete ac = new(Buffer());
+        ac.Show(Candidates(("a", "A"), ("b", "B")));
+
+        IReadOnlyList<Line> rows = ac.Render(80);
+        Assert.True(AllReversed(rows[0]));
+    }
+
+    // ── Accept with Enter ─────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void EnterAppliesInsertTextToBuffer()
+    {
+        TextBuffer buf = Buffer("hello");
+        Autocomplete ac = new(buf);
+        ac.Show(Candidates(("/cmd foo", "/cmd foo")));
+
+        bool consumed = ac.HandleKey(Named(NamedKey.Enter));
+
+        Assert.True(consumed);
+        Assert.Equal("/cmd foo", buf.Text);
+        Assert.Equal("/cmd foo".Length, buf.CaretIndex);
+    }
+
+    [Fact]
+    public void EnterDismissesAfterAccept()
+    {
+        TextBuffer buf = Buffer();
+        Autocomplete ac = new(buf);
+        ac.Show(Candidates(("x", "X")));
+        ac.HandleKey(Named(NamedKey.Enter));
+        Assert.True(ac.IsDismissed);
+    }
+
+    [Fact]
+    public void EnterOnEmptyCandidateListIsNoOp()
+    {
+        TextBuffer buf = Buffer("original");
+        Autocomplete ac = new(buf);
+        ac.Show([]);
+
+        bool consumed = ac.HandleKey(Named(NamedKey.Enter));
+
+        Assert.True(consumed);
+        Assert.Equal("original", buf.Text); // buffer untouched
+        Assert.True(ac.IsDismissed);
+    }
+
+    // ── Accept with Tab ───────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void TabAlsoAcceptsSelectedCandidate()
+    {
+        TextBuffer buf = Buffer("old");
+        Autocomplete ac = new(buf);
+        ac.Show(Candidates(("new text", "New text")));
+
+        bool consumed = ac.HandleKey(Named(NamedKey.Tab));
+
+        Assert.True(consumed);
+        Assert.Equal("new text", buf.Text);
+        Assert.True(ac.IsDismissed);
+    }
+
+    // ── Escape ────────────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void EscapeDismissesWithoutTouchingBuffer()
+    {
+        TextBuffer buf = Buffer("keep this");
+        Autocomplete ac = new(buf);
+        ac.Show(Candidates(("replace", "Replace")));
+
+        bool consumed = ac.HandleKey(Named(NamedKey.Escape));
+
+        Assert.True(consumed);
+        Assert.Equal("keep this", buf.Text);
+        Assert.True(ac.IsDismissed);
+    }
+
+    // ── Fall-through keys ─────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void PrintableKeyReturnsFalseAndDoesNotMutateBuffer()
+    {
+        TextBuffer buf = Buffer("start");
+        Autocomplete ac = new(buf);
+        ac.Show(Candidates(("x", "X")));
+
+        bool consumed = ac.HandleKey(Char('a'));
+
+        Assert.False(consumed);
+        Assert.Equal("start", buf.Text); // buffer untouched
+    }
+
+    [Fact]
+    public void CtrlKeyReturnsFalse()
+    {
+        Autocomplete ac = new(Buffer());
+        ac.Show(Candidates(("x", "X")));
+
+        bool consumed = ac.HandleKey(new KeyEvent(KeyCode.FromRune(new Rune('c')), Modifiers.Ctrl));
+
+        Assert.False(consumed);
+    }
+
+    [Fact]
+    public void RightArrowReturnsFalse()
+    {
+        Autocomplete ac = new(Buffer());
+        ac.Show(Candidates(("x", "X")));
+
+        bool consumed = ac.HandleKey(Named(NamedKey.Right));
+
+        Assert.False(consumed);
+    }
+
+    // ── MaxRows forwarding ────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void MaxRowsForwardsToList()
+    {
+        Autocomplete ac = new(Buffer());
+        ac.Show(Candidates(("a", "A"), ("b", "B"), ("c", "C"), ("d", "D"), ("e", "E")));
+        ac.MaxRows = 3;
+
+        IReadOnlyList<Line> rows = ac.Render(80);
+
+        Assert.Equal(3, rows.Count);
+    }
+
+    // ── Show resets selection ─────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void ShowResetsPreviousSelection()
+    {
+        Autocomplete ac = new(Buffer());
+        ac.Show(Candidates(("a", "A"), ("b", "B"), ("c", "C")));
+        ac.HandleKey(Named(NamedKey.Down));
+        ac.HandleKey(Named(NamedKey.Down)); // selection at index 2
+
+        // New Show call: selection should reset to 0.
+        ac.Show(Candidates(("x", "X"), ("y", "Y")));
+        IReadOnlyList<Line> rows = ac.Render(80);
+        Assert.True(AllReversed(rows[0]));
+    }
+}
diff --git a/tests/Dcli.Tests/Dcli.Tests.csproj b/tests/Dcli.Tests/Dcli.Tests.csproj
new file mode 100644
index 0000000..f1c7035
--- /dev/null
+++ b/tests/Dcli.Tests/Dcli.Tests.csproj
@@ -0,0 +1,28 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+  <PropertyGroup>
+    <IsPackable>false</IsPackable>
+    <IsTestProject>true</IsTestProject>
+    <!-- CS1591: test members don't require XML doc comments. -->
+    <NoWarn>CS1591</NoWarn>
+  </PropertyGroup>
+
+  <ItemGroup>
+    <PackageReference Include="Microsoft.NET.Test.Sdk" Version="17.14.1" />
+    <PackageReference Include="xunit" Version="2.9.3" />
+    <PackageReference Include="xunit.runner.visualstudio" Version="2.8.2">
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+      <PrivateAssets>all</PrivateAssets>
+    </PackageReference>
+    <PackageReference Include="coverlet.collector" Version="6.0.4">
+      <IncludeAssets>runtime; build; native; contentfiles; analyzers; buildtransitive</IncludeAssets>
+      <PrivateAssets>all</PrivateAssets>
+    </PackageReference>
+  </ItemGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\..\src\Dcli\Dcli.csproj" />
+    <ProjectReference Include="..\..\src\Dcli.Testing\Dcli.Testing.csproj" />
+  </ItemGroup>
+
+</Project>
diff --git a/tests/Dcli.Tests/DialogSelectionTests.cs b/tests/Dcli.Tests/DialogSelectionTests.cs
new file mode 100644
index 0000000..7c1a65f
--- /dev/null
+++ b/tests/Dcli.Tests/DialogSelectionTests.cs
@@ -0,0 +1,596 @@
+using System.Text;
+using Dcli.Internal.FixedRegion;
+using Dcli.Internal.RenderLoop;
+using Xunit;
+
+namespace Dcli.Tests;
+
+/// <summary>
+/// Tests for §12 Chunk A: awaitable selection dialogs via <see cref="OpenDialogCommand"/>,
+/// <see cref="CancelDialogCommand"/>, and the loop-engine plumbing.
+/// Tests drive the machinery at the <see cref="LoopEngine"/> level (no real terminal needed).
+/// </summary>
+public sealed class DialogSelectionTests
+{
+    // ── Test infrastructure ───────────────────────────────────────────────────
+
+    private sealed class ConstantSizeSource(int cols, int rows) : ITerminalSizeSource
+    {
+        public (int Columns, int Rows) GetSize() => (cols, rows);
+    }
+
+    // Posts ShowAutocomplete directly via the loop command channel.
+    private sealed class ShowAutocompleteCommand(Autocomplete autocomplete) : ILoopCommand
+    {
+        void ILoopCommand.Apply(RenderModel model) => model.ShowAutocomplete(autocomplete);
+    }
+
+    private sealed class CapturingOutputSink : IOutputSink
+    {
+        internal RenderModel? LastModel { get; private set; }
+        public void Paint(RenderModel model) { LastModel = model; }
+        public void EmitRestoreSequence() { }
+    }
+
+    private sealed class VirtualClock : IClock
+    {
+        private readonly object _lock = new();
+        private TimeSpan _now;
+        private readonly List<(TimeSpan Deadline, TaskCompletionSource Tcs, CancellationTokenSource LinkedCts)> _waiters = [];
+
+        internal VirtualClock(TimeSpan? initial = null) => _now = initial ?? TimeSpan.Zero;
+
+        public TimeSpan Now { get { lock (_lock) { return _now; } } }
+
+        internal void Advance(TimeSpan by)
+        {
+            List<(TaskCompletionSource Tcs, CancellationTokenSource LinkedCts)> toComplete = [];
+            lock (_lock)
+            {
+                _now += by;
+                for (int i = _waiters.Count - 1; i >= 0; i--)
+                {
+                    if (_waiters[i].Deadline <= _now)
+                    {
+                        toComplete.Add((_waiters[i].Tcs, _waiters[i].LinkedCts));
+                        _waiters.RemoveAt(i);
+                    }
+                }
+            }
+            foreach ((TaskCompletionSource tcs, CancellationTokenSource linkedCts) in toComplete)
+            {
+                tcs.TrySetResult();
+                linkedCts.Dispose();
+            }
+        }
+
+        public Task WaitUntilAsync(TimeSpan deadline, CancellationToken cancellationToken)
+        {
+            lock (_lock)
+            {
+                if (deadline <= _now)
+                    return Task.CompletedTask;
+            }
+            CancellationTokenSource linkedCts = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
+            TaskCompletionSource tcs = new(TaskCreationOptions.RunContinuationsAsynchronously);
+            linkedCts.Token.Register(
+                static state => ((TaskCompletionSource)state!).TrySetCanceled(),
+                tcs,
+                useSynchronizationContext: false);
+            lock (_lock)
+            {
+                if (deadline <= _now) { linkedCts.Dispose(); return Task.CompletedTask; }
+                _waiters.Add((deadline, tcs, linkedCts));
+            }
+            return tcs.Task;
+        }
+    }
+
+    private static (LoopEngine Engine, VirtualClock Clock, CapturingOutputSink Sink) CreateEngine(
+        int cols = 80, int rows = 24)
+    {
+        VirtualClock clock = new(TimeSpan.Zero);
+        CapturingOutputSink sink = new();
+        LoopEngine engine = new(
+            new ConstantSizeSource(cols, rows),
+            clock,
+            sink,
+            minFrameInterval: TimeSpan.Zero);
+        return (engine, clock, sink);
+    }
+
+    private static Line PlainLine(string text) => new([new Segment(text)]);
+
+    private static List<Line> Items(params string[] texts) =>
+        texts.Select(PlainLine).ToList();
+
+    // Settle with a 5-second wall-clock guard so tests cannot hang forever.
+    private static Task SettleAsync(LoopEngine engine, VirtualClock clock)
+    {
+        clock.Advance(TimeSpan.FromMilliseconds(10));
+        return engine.SettleAsync().WaitAsync(TimeSpan.FromSeconds(5));
+    }
+
+    // ── Helper: open a select dialog via OpenDialogCommand ───────────────────
+
+    private static Task<DialogResult<int>> PostSelectDialog(
+        LoopEngine engine,
+        List<Line> items,
+        Line? title = null,
+        CancellationToken ct = default)
+    {
+        Dialog dialog = new(multiSelect: false, modal: true, title: title);
+        dialog.List.SetItems(items);
+
+        TaskCompletionSource<DialogResult<int>> tcs =
+            new(TaskCreationOptions.RunContinuationsAsynchronously);
+
+        CancellationTokenRegistration[] regHolder = new CancellationTokenRegistration[1];
+
+        Action completion = () =>
+        {
+            DialogResult<int> result = dialog.CloseRequest == OverlayCloseKind.Submit
+                ? new DialogResult<int>(DialogOutcome.Submitted, dialog.List.SelectedIndex)
+                : new DialogResult<int>(DialogOutcome.Cancelled, -1);
+            tcs.TrySetResult(result);
+            regHolder[0].Dispose();
+        };
+
+        Action reject = () =>
+        {
+            tcs.TrySetException(new InvalidOperationException("A dialog is already active."));
+            regHolder[0].Dispose();
+        };
+
+        if (ct.CanBeCanceled)
+        {
+            Action cancelCompletion = () =>
+            {
+                tcs.TrySetResult(new DialogResult<int>(DialogOutcome.Cancelled, -1));
+                regHolder[0].Dispose();
+            };
+            regHolder[0] = ct.Register(
+                () => engine.Post(new CancelDialogCommand(dialog, cancelCompletion)),
+                useSynchronizationContext: false);
+        }
+
+        engine.Post(new OpenDialogCommand(dialog, completion, reject));
+        return tcs.Task;
+    }
+
+    private static Task<DialogResult<int[]>> PostMultiSelectDialog(
+        LoopEngine engine,
+        List<Line> items,
+        CancellationToken ct = default)
+    {
+        Dialog dialog = new(multiSelect: true, modal: true);
+        dialog.List.SetItems(items);
+
+        TaskCompletionSource<DialogResult<int[]>> tcs =
+            new(TaskCreationOptions.RunContinuationsAsynchronously);
+
+        CancellationTokenRegistration[] regHolder = new CancellationTokenRegistration[1];
+
+        Action completion = () =>
+        {
+            DialogResult<int[]> result = dialog.CloseRequest == OverlayCloseKind.Submit
+                ? new DialogResult<int[]>(DialogOutcome.Submitted, [.. dialog.List.CheckedIndices])
+                : new DialogResult<int[]>(DialogOutcome.Cancelled, []);
+            tcs.TrySetResult(result);
+            regHolder[0].Dispose();
+        };
+
+        Action reject = () =>
+        {
+            tcs.TrySetException(new InvalidOperationException("A dialog is already active."));
+            regHolder[0].Dispose();
+        };
+
+        if (ct.CanBeCanceled)
+        {
+            Action cancelCompletion = () =>
+            {
+                tcs.TrySetResult(new DialogResult<int[]>(DialogOutcome.Cancelled, []));
+                regHolder[0].Dispose();
+            };
+            regHolder[0] = ct.Register(
+                () => engine.Post(new CancelDialogCommand(dialog, cancelCompletion)),
+                useSynchronizationContext: false);
+        }
+
+        engine.Post(new OpenDialogCommand(dialog, completion, reject));
+        return tcs.Task;
+    }
+
+    // ── 12.1 — Select: submit with navigation ────────────────────────────────
+
+    [Fact]
+    public async Task SelectSubmitDownDownEnterReturnsSubmittedIndex2()
+    {
+        (LoopEngine engine, VirtualClock clock, _) = CreateEngine();
+        try
+        {
+            Task<DialogResult<int>> task = PostSelectDialog(engine, Items("A", "B", "C"));
+
+            // ↓ ↓ Enter → navigate to index 2, then submit
+            engine.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Down), Modifiers.None));
+            engine.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Down), Modifiers.None));
+            engine.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Enter), Modifiers.None));
+
+            await SettleAsync(engine, clock);
+            DialogResult<int> result = await task;
+
+            Assert.Equal(DialogOutcome.Submitted, result.Outcome);
+            Assert.Equal(2, result.Value);
+        }
+        finally { engine.Dispose(); }
+    }
+
+    // ── 12.1 — Select: cancel via Escape ──────────────────────────────────────
+
+    [Fact]
+    public async Task SelectCancelEscapeReturnsCancelled()
+    {
+        (LoopEngine engine, VirtualClock clock, _) = CreateEngine();
+        try
+        {
+            Task<DialogResult<int>> task = PostSelectDialog(engine, Items("X", "Y"));
+
+            engine.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Escape), Modifiers.None));
+
+            await SettleAsync(engine, clock);
+            DialogResult<int> result = await task;
+
+            Assert.Equal(DialogOutcome.Cancelled, result.Outcome);
+        }
+        finally { engine.Dispose(); }
+    }
+
+    // ── 12.1 — Multi-select: toggle + submit ─────────────────────────────────
+
+    [Fact]
+    public async Task MultiSelectSubmitSpaceDownSpaceEnterReturnsToggledIndices()
+    {
+        (LoopEngine engine, VirtualClock clock, _) = CreateEngine();
+        try
+        {
+            Task<DialogResult<int[]>> task = PostMultiSelectDialog(engine, Items("Alpha", "Beta", "Gamma"));
+
+            // Space (toggle 0), ↓ (move to 1), Space (toggle 1), Enter (submit)
+            engine.InputWriter.TryWrite(new KeyEvent(KeyCode.FromRune(new Rune(' ')), Modifiers.None));
+            engine.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Down), Modifiers.None));
+            engine.InputWriter.TryWrite(new KeyEvent(KeyCode.FromRune(new Rune(' ')), Modifiers.None));
+            engine.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Enter), Modifiers.None));
+
+            await SettleAsync(engine, clock);
+            DialogResult<int[]> result = await task;
+
+            Assert.Equal(DialogOutcome.Submitted, result.Outcome);
+            Assert.Equal([0, 1], result.Value);
+        }
+        finally { engine.Dispose(); }
+    }
+
+    // ── 12.2 — Cancellation token cancels dialog ─────────────────────────────
+
+    [Fact]
+    public async Task CancellationTokenCancelReturnsDialogCancelled()
+    {
+        (LoopEngine engine, VirtualClock clock, CapturingOutputSink sink) = CreateEngine();
+        try
+        {
+            using CancellationTokenSource dialogCts = new();
+
+            Task<DialogResult<int>> task = PostSelectDialog(engine, Items("P", "Q"), ct: dialogCts.Token);
+
+            // Let the dialog open.
+            await SettleAsync(engine, clock);
+            Assert.IsType<Dialog>(sink.LastModel?.ActiveOverlay);
+
+            // Cancel the token — posts CancelDialogCommand.
+            await dialogCts.CancelAsync();
+            await SettleAsync(engine, clock);
+
+            DialogResult<int> result = await task;
+
+            Assert.Equal(DialogOutcome.Cancelled, result.Outcome);
+            // Overlay must have been cleared.
+            Assert.Null(sink.LastModel?.ActiveOverlay);
+        }
+        finally { engine.Dispose(); }
+    }
+
+    // ── 12.2 — Already-cancelled token → immediate Cancelled ─────────────────
+
+    [Fact]
+    public async Task AlreadyCancelledTokenReturnsCancelledNoOverlay()
+    {
+        (LoopEngine engine, VirtualClock clock, CapturingOutputSink sink) = CreateEngine();
+        try
+        {
+            using CancellationTokenSource cts = new();
+            await cts.CancelAsync();
+
+            // The Terminal.SelectAsync fast-path returns Cancelled immediately when the token is
+            // already cancelled. Replicate the fast-path contract here.
+            DialogResult<int> result = cts.Token.IsCancellationRequested
+                ? new DialogResult<int>(DialogOutcome.Cancelled, default)
+                : throw new InvalidOperationException("Expected already-cancelled");
+
+            Assert.Equal(DialogOutcome.Cancelled, result.Outcome);
+
+            // Engine should have no overlay — nothing was posted.
+            await SettleAsync(engine, clock);
+            Assert.Null(sink.LastModel?.ActiveOverlay);
+        }
+        finally { engine.Dispose(); }
+    }
+
+    // ── 12.2 — Reject second concurrent dialog ───────────────────────────────
+
+    [Fact]
+    public async Task SecondDialogWhileFirstActiveThrowsInvalidOperation()
+    {
+        (LoopEngine engine, VirtualClock clock, CapturingOutputSink sink) = CreateEngine();
+        try
+        {
+            Task<DialogResult<int>> first = PostSelectDialog(engine, Items("1", "2"));
+
+            // Settle so the first dialog's command applies.
+            await SettleAsync(engine, clock);
+            Assert.IsType<Dialog>(sink.LastModel?.ActiveOverlay);
+
+            // Open second dialog — must fault.
+            Task<DialogResult<int>> second = PostSelectDialog(engine, Items("A", "B"));
+            await SettleAsync(engine, clock);
+
+            // Second task must fault with InvalidOperationException.
+            InvalidOperationException ex =
+                await Assert.ThrowsAsync<InvalidOperationException>(() => second);
+            Assert.Contains("already active", ex.Message, StringComparison.OrdinalIgnoreCase);
+
+            // First dialog must still be the active overlay.
+            Assert.IsType<Dialog>(sink.LastModel?.ActiveOverlay);
+
+            // Clean up: dismiss the first dialog.
+            engine.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Escape), Modifiers.None));
+            await SettleAsync(engine, clock);
+            DialogResult<int> firstResult = await first;
+            Assert.Equal(DialogOutcome.Cancelled, firstResult.Outcome);
+        }
+        finally { engine.Dispose(); }
+    }
+
+    // ── Title row: MaxRows reserves one row ───────────────────────────────────
+
+    [Fact]
+    public void DialogWithTitleMaxRowsReservesOneRowForTitle()
+    {
+        Line title = PlainLine("Choose:");
+        Dialog dialog = new(multiSelect: false, modal: true, title: title);
+        dialog.List.SetItems(Items("A", "B", "C"));
+
+        // Set MaxRows = 4. With a title, List.MaxRows = 3 → 3 list rows fit.
+        dialog.MaxRows = 4;
+        IReadOnlyList<Line> rows = dialog.Render(80);
+
+        // Total rows ≤ MaxRows.
+        Assert.True(rows.Count <= 4, $"Rows count {rows.Count} exceeds MaxRows 4");
+        // Title is first row.
+        Assert.Same(title, rows[0]);
+        // All 3 list items fit → total 4.
+        Assert.Equal(4, rows.Count);
+    }
+
+    [Fact]
+    public void DialogWithTitleRenderPrependsTitleRow()
+    {
+        Line title = PlainLine("My Title");
+        Dialog dialog = new(multiSelect: false, modal: true, title: title);
+        dialog.List.SetItems(Items("X", "Y"));
+        dialog.MaxRows = 10; // no truncation
+
+        IReadOnlyList<Line> rows = dialog.Render(80);
+
+        // First row is the title, followed by list items.
+        Assert.Equal(3, rows.Count); // title + 2 items
+        Assert.Same(title, rows[0]);
+        Assert.Contains(rows[1].Segments, s => s.Text.Contains('X', StringComparison.Ordinal));
+        Assert.Contains(rows[2].Segments, s => s.Text.Contains('Y', StringComparison.Ordinal));
+    }
+
+    [Fact]
+    public void DialogWithTitleMaxRowsNeverExceedsBudget()
+    {
+        Line title = PlainLine("Title");
+        Dialog dialog = new(multiSelect: false, modal: true, title: title);
+        dialog.List.SetItems(Items("1", "2", "3", "4", "5"));
+        dialog.MaxRows = 3; // budget: title(1) + list(max 2)
+
+        IReadOnlyList<Line> rows = dialog.Render(80);
+
+        Assert.True(rows.Count <= 3, $"Rows count {rows.Count} exceeds budget 3");
+        Assert.Same(title, rows[0]);
+    }
+
+    // ── Dialog without title: MaxRows unchanged behaviour ────────────────────
+
+    [Fact]
+    public void DialogNoTitleMaxRowsRoundTrips()
+    {
+        Dialog dialog = new(multiSelect: false, modal: true);
+        dialog.MaxRows = 5;
+        Assert.Equal(5, dialog.MaxRows);
+    }
+
+    // ── DialogOutcome.Back exists but is not produced ─────────────────────────
+
+    [Fact]
+    public void DialogOutcomeBackEnumValueExists()
+    {
+        // Back must exist for API/wizard compatibility even though v1 dialogs don't produce it.
+        Assert.True(Enum.IsDefined(DialogOutcome.Back));
+    }
+
+    // ── DialogResult is publicly constructible (§12.6 needs this) ───────────
+
+    [Fact]
+    public void DialogResultIsPubliclyConstructible()
+    {
+        DialogResult<int> r = new(DialogOutcome.Submitted, 42);
+        Assert.Equal(DialogOutcome.Submitted, r.Outcome);
+        Assert.Equal(42, r.Value);
+
+        DialogResult<int[]> r2 = new(DialogOutcome.Cancelled, []);
+        Assert.Equal(DialogOutcome.Cancelled, r2.Outcome);
+    }
+
+    // ── Overlay cleared after dismiss ─────────────────────────────────────────
+
+    [Fact]
+    public async Task DialogAfterSubmitOverlayIsCleared()
+    {
+        (LoopEngine engine, VirtualClock clock, CapturingOutputSink sink) = CreateEngine();
+        try
+        {
+            Task<DialogResult<int>> task = PostSelectDialog(engine, Items("A", "B", "C"));
+
+            engine.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Enter), Modifiers.None));
+            await SettleAsync(engine, clock);
+
+            Assert.Null(sink.LastModel?.ActiveOverlay);
+            DialogResult<int> result = await task;
+            Assert.Equal(DialogOutcome.Submitted, result.Outcome);
+        }
+        finally { engine.Dispose(); }
+    }
+
+    // ── Cancel command is no-op after natural dismiss ─────────────────────────
+
+    [Fact]
+    public async Task CancelCommandAfterNaturalDismissIsNoOp()
+    {
+        (LoopEngine engine, VirtualClock clock, CapturingOutputSink sink) = CreateEngine();
+        try
+        {
+            using CancellationTokenSource dialogCts = new();
+
+            Task<DialogResult<int>> task = PostSelectDialog(engine, Items("A"), ct: dialogCts.Token);
+
+            // Dismiss naturally.
+            engine.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Enter), Modifiers.None));
+            await SettleAsync(engine, clock);
+            DialogResult<int> result = await task;
+            Assert.Equal(DialogOutcome.Submitted, result.Outcome);
+
+            // Now fire the cancellation token — cancel command should be a no-op.
+            await dialogCts.CancelAsync();
+            await SettleAsync(engine, clock);
+
+            // Overlay is still null (not re-opened).
+            Assert.Null(sink.LastModel?.ActiveOverlay);
+        }
+        finally { engine.Dispose(); }
+    }
+
+    // ── B1: Dialog suppresses an active Autocomplete (does NOT reject) ────────
+
+    [Fact]
+    public async Task OpenDialogWhileAutocompleteActiveSupressesAutocomplete()
+    {
+        (LoopEngine engine, VirtualClock clock, CapturingOutputSink sink) = CreateEngine();
+        try
+        {
+            // Show an autocomplete first.
+            TextBuffer buf = new();
+            Autocomplete ac = new(buf);
+            ac.Show([new AutocompleteCandidate("item1", PlainLine("item1"))]);
+            engine.Post(new ShowAutocompleteCommand(ac));
+            await SettleAsync(engine, clock);
+            Assert.IsType<Autocomplete>(sink.LastModel?.ActiveOverlay);
+
+            // Now open a dialog — must suppress the autocomplete, not reject.
+            Task<DialogResult<int>> task = PostSelectDialog(engine, Items("A", "B"));
+            await SettleAsync(engine, clock);
+
+            // ActiveOverlay must be the new dialog.
+            Assert.IsType<Dialog>(sink.LastModel?.ActiveOverlay);
+
+            // The dialog task must not be faulted — the autocomplete was suppressed, not rejected.
+            Assert.False(task.IsCompleted, "Dialog task should still be pending (not faulted)");
+
+            // Dismiss the dialog cleanly.
+            engine.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Escape), Modifiers.None));
+            await SettleAsync(engine, clock);
+            DialogResult<int> result = await task;
+            Assert.Equal(DialogOutcome.Cancelled, result.Outcome);
+        }
+        finally { engine.Dispose(); }
+    }
+
+    // ── B1: Second concurrent Dialog is still rejected; first survives ────────
+
+    [Fact]
+    public async Task SecondDialogWhileFirstDialogActiveIsRejected()
+    {
+        (LoopEngine engine, VirtualClock clock, CapturingOutputSink sink) = CreateEngine();
+        try
+        {
+            Task<DialogResult<int>> first = PostSelectDialog(engine, Items("1", "2"));
+            await SettleAsync(engine, clock);
+            Assert.IsType<Dialog>(sink.LastModel?.ActiveOverlay);
+
+            // Second dialog — must be rejected because the overlay is already a Dialog.
+            Task<DialogResult<int>> second = PostSelectDialog(engine, Items("A", "B"));
+            await SettleAsync(engine, clock);
+
+            InvalidOperationException ex =
+                await Assert.ThrowsAsync<InvalidOperationException>(() => second);
+            Assert.Contains("already active", ex.Message, StringComparison.OrdinalIgnoreCase);
+
+            // First dialog must still be the active overlay.
+            Assert.IsType<Dialog>(sink.LastModel?.ActiveOverlay);
+
+            // Dismiss the first dialog.
+            engine.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Escape), Modifiers.None));
+            await SettleAsync(engine, clock);
+            DialogResult<int> firstResult = await first;
+            Assert.Equal(DialogOutcome.Cancelled, firstResult.Outcome);
+        }
+        finally { engine.Dispose(); }
+    }
+
+    // ── B2: Titled dialog Render never exceeds MaxRows budget ─────────────────
+
+    [Fact]
+    public void TitledDialogMaxRows1RenderReturnsTitleOnlyRow()
+    {
+        // At MaxRows=1 the title consumes the entire budget; zero list rows must appear.
+        Line title = PlainLine("Title");
+        Dialog dialog = new(multiSelect: false, modal: true, title: title);
+        dialog.List.SetItems(Items("A", "B", "C"));
+        dialog.MaxRows = 1;
+
+        IReadOnlyList<Line> rows = dialog.Render(80);
+
+        Assert.True(rows.Count <= 1, $"Rows count {rows.Count} exceeds MaxRows 1");
+        Line only = Assert.Single(rows);
+        Assert.Same(title, only);
+    }
+
+    [Fact]
+    public void TitledDialogMaxRows2RenderReturnsTitlePlusOneListRow()
+    {
+        // At MaxRows=2: title(1) + list(1) = 2 rows total.
+        Line title = PlainLine("Title");
+        Dialog dialog = new(multiSelect: false, modal: true, title: title);
+        dialog.List.SetItems(Items("A", "B", "C"));
+        dialog.MaxRows = 2;
+
+        IReadOnlyList<Line> rows = dialog.Render(80);
+
+        Assert.True(rows.Count <= 2, $"Rows count {rows.Count} exceeds MaxRows 2");
+        Assert.Equal(2, rows.Count);
+        Assert.Same(title, rows[0]);
+    }
+}
diff --git a/tests/Dcli.Tests/DialogTests.cs b/tests/Dcli.Tests/DialogTests.cs
new file mode 100644
index 0000000..3a1dcc7
--- /dev/null
+++ b/tests/Dcli.Tests/DialogTests.cs
@@ -0,0 +1,328 @@
+using System.Text;
+using Dcli.Internal.FixedRegion;
+using Xunit;
+
+namespace Dcli.Tests;
+
+/// <summary>
+/// Unit tests for <see cref="Dialog"/> — task 11.4.
+/// </summary>
+public sealed class DialogTests
+{
+    // ── Helpers ──────────────────────────────────────────────────────────────────────────────
+
+    private static KeyEvent Named(NamedKey key) =>
+        new(KeyCode.Named(key), Modifiers.None);
+
+    private static KeyEvent Char(char c) =>
+        new(KeyCode.FromRune(new Rune(c)), Modifiers.None);
+
+    private static void AddItems(Dialog dialog, params string[] texts)
+    {
+        dialog.List.SetItems(texts.Select(t => new LineBuilder().Text(t).Build()).ToList());
+    }
+
+    private static bool AllReversed(Line line) =>
+        line.Segments.Count > 0 &&
+        line.Segments.All(s => (s.Style.Format & Format.Reverse) == Format.Reverse);
+
+    private static bool AnySegmentContains(Line line, string text) =>
+        line.Segments.Any(s => s.Text.Contains(text, StringComparison.Ordinal));
+
+    // ── Placement / HidesCursor ───────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void PlacementIsAboveInput()
+    {
+        Dialog d = new();
+        Assert.Equal(OverlayPlacement.AboveInput, d.Placement);
+    }
+
+    [Fact]
+    public void HidesCursorTrueWhenModal()
+    {
+        Dialog d = new(modal: true);
+        Assert.True(d.HidesCursor);
+    }
+
+    [Fact]
+    public void HidesCursorFalseWhenNonModal()
+    {
+        Dialog d = new(modal: false);
+        Assert.False(d.HidesCursor);
+    }
+
+    // ── IsDismissed ───────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void InitiallyNotDismissed()
+    {
+        Dialog d = new();
+        Assert.False(d.IsDismissed);
+        Assert.Null(d.CloseRequest);
+    }
+
+    [Fact]
+    public void EnterSetsDismissedWithSubmit()
+    {
+        Dialog d = new();
+        d.HandleKey(Named(NamedKey.Enter));
+        Assert.True(d.IsDismissed);
+        Assert.Equal(OverlayCloseKind.Submit, d.CloseRequest);
+    }
+
+    [Fact]
+    public void EscapeSetsDismissedWithCancel()
+    {
+        Dialog d = new();
+        d.HandleKey(Named(NamedKey.Escape));
+        Assert.True(d.IsDismissed);
+        Assert.Equal(OverlayCloseKind.Cancel, d.CloseRequest);
+    }
+
+    // ── Navigation ────────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void DownMovesSelectionDown()
+    {
+        Dialog d = new();
+        AddItems(d, "alpha", "beta", "gamma");
+
+        bool consumed = d.HandleKey(Named(NamedKey.Down));
+
+        Assert.True(consumed);
+        IReadOnlyList<Line> rows = d.Render(80);
+        Assert.True(AllReversed(rows[1]));
+        Assert.False(AllReversed(rows[0]));
+    }
+
+    [Fact]
+    public void UpMovesSelectionUp()
+    {
+        Dialog d = new();
+        AddItems(d, "alpha", "beta", "gamma");
+        d.HandleKey(Named(NamedKey.Down));
+        d.HandleKey(Named(NamedKey.Down));
+
+        bool consumed = d.HandleKey(Named(NamedKey.Up));
+
+        Assert.True(consumed);
+        IReadOnlyList<Line> rows = d.Render(80);
+        Assert.True(AllReversed(rows[1]));
+    }
+
+    // ── Multi-select: space toggle ────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void SpaceTogglesCurrentItemInMultiSelectDialog()
+    {
+        Dialog d = new(multiSelect: true);
+        AddItems(d, "one", "two", "three");
+
+        bool consumed = d.HandleKey(Char(' '));
+
+        Assert.True(consumed);
+        Assert.Contains(0, d.List.CheckedIndices);
+    }
+
+    [Fact]
+    public void SpaceTogglesOffAfterToggleOn()
+    {
+        Dialog d = new(multiSelect: true);
+        AddItems(d, "one", "two");
+
+        d.HandleKey(Char(' ')); // check 0
+        d.HandleKey(Char(' ')); // uncheck 0
+
+        Assert.DoesNotContain(0, d.List.CheckedIndices);
+    }
+
+    [Fact]
+    public void SpaceNavigationAndToggle()
+    {
+        Dialog d = new(multiSelect: true);
+        AddItems(d, "alpha", "beta", "gamma");
+
+        d.HandleKey(Named(NamedKey.Down)); // move to 1
+        d.HandleKey(Char(' '));            // toggle 1
+        d.HandleKey(Named(NamedKey.Down)); // move to 2
+        d.HandleKey(Char(' '));            // toggle 2
+
+        Assert.Contains(1, d.List.CheckedIndices);
+        Assert.Contains(2, d.List.CheckedIndices);
+        Assert.DoesNotContain(0, d.List.CheckedIndices);
+    }
+
+    [Fact]
+    public void SpaceInNonMultiSelectModalDialogIsConsumedWithNoToggle()
+    {
+        // Non-multiselect, modal, no type-to-filter: space hits the modal catch-all.
+        Dialog d = new(multiSelect: false, modal: true, typeToFilter: false);
+        AddItems(d, "one", "two");
+
+        bool consumed = d.HandleKey(Char(' '));
+
+        Assert.True(consumed);
+        Assert.Empty(d.List.CheckedIndices);
+    }
+
+    // ── Enter / Escape ────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void EnterReturnsConsumedTrue()
+    {
+        Dialog d = new();
+        bool consumed = d.HandleKey(Named(NamedKey.Enter));
+        Assert.True(consumed);
+    }
+
+    [Fact]
+    public void EscapeReturnsConsumedTrue()
+    {
+        Dialog d = new();
+        bool consumed = d.HandleKey(Named(NamedKey.Escape));
+        Assert.True(consumed);
+    }
+
+    // ── Modal catch-all ───────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void ModalDialogConsumesUnhandledPrintableKey()
+    {
+        // modal=true, typeToFilter=false → printable should hit catch-all and return true.
+        Dialog d = new(modal: true, typeToFilter: false);
+        bool consumed = d.HandleKey(Char('z'));
+        Assert.True(consumed);
+    }
+
+    [Fact]
+    public void ModalDialogConsumesArbitraryNamedKey()
+    {
+        Dialog d = new(modal: true);
+        bool consumed = d.HandleKey(Named(NamedKey.F5));
+        Assert.True(consumed);
+    }
+
+    [Fact]
+    public void NonModalDialogFallsThroughForUnhandledKey()
+    {
+        Dialog d = new(modal: false);
+        bool consumed = d.HandleKey(Char('z'));
+        Assert.False(consumed);
+    }
+
+    // ── Type-to-filter ────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void TypeToFilterAppendsCharacter()
+    {
+        Dialog d = new(typeToFilter: true);
+
+        d.HandleKey(Char('h'));
+        d.HandleKey(Char('i'));
+
+        Assert.Equal("hi", d.FilterText);
+    }
+
+    [Fact]
+    public void TypeToFilterPrintableKeyConsumed()
+    {
+        Dialog d = new(typeToFilter: true);
+        bool consumed = d.HandleKey(Char('x'));
+        Assert.True(consumed);
+    }
+
+    [Fact]
+    public void TypeToFilterBackspaceTrimLastChar()
+    {
+        Dialog d = new(typeToFilter: true);
+        d.HandleKey(Char('a'));
+        d.HandleKey(Char('b'));
+        d.HandleKey(Char('c'));
+
+        bool consumed = d.HandleKey(Named(NamedKey.Backspace));
+
+        Assert.True(consumed);
+        Assert.Equal("ab", d.FilterText);
+    }
+
+    [Fact]
+    public void TypeToFilterBackspaceOnEmptyIsNoOp()
+    {
+        Dialog d = new(typeToFilter: true);
+        // Should not throw on empty FilterText.
+        bool consumed = d.HandleKey(Named(NamedKey.Backspace));
+        Assert.True(consumed);
+        Assert.Equal(string.Empty, d.FilterText);
+    }
+
+    [Fact]
+    public void TypeToFilterBackspaceHandlesMultiCharClusters()
+    {
+        // A surrogate pair (e.g. 😀 U+1F600) should be removed as one cluster.
+        Dialog d = new(typeToFilter: true);
+        // Append 😀 by constructing a KeyEvent with its Rune.
+        Rune emoji = new(0x1F600);
+        d.HandleKey(new KeyEvent(KeyCode.FromRune(emoji), Modifiers.None));
+        d.HandleKey(Char('!'));
+
+        Assert.Equal("\U0001F600!", d.FilterText);
+
+        d.HandleKey(Named(NamedKey.Backspace)); // remove '!'
+        Assert.Equal("\U0001F600", d.FilterText);
+
+        d.HandleKey(Named(NamedKey.Backspace)); // remove emoji
+        Assert.Equal(string.Empty, d.FilterText);
+    }
+
+    [Fact]
+    public void NonTypeToFilterModalDialogDoesNotChangeFilterText()
+    {
+        Dialog d = new(modal: true, typeToFilter: false);
+        d.HandleKey(Char('a'));
+        Assert.Equal(string.Empty, d.FilterText);
+    }
+
+    [Fact]
+    public void TypeToFilterSpaceTakesMultiSelectPrecedence()
+    {
+        // When both multiSelect and typeToFilter are true, space toggles (rule 4)
+        // rather than appending to FilterText (rule 5).
+        Dialog d = new(multiSelect: true, modal: true, typeToFilter: true);
+        AddItems(d, "item");
+
+        d.HandleKey(Char(' '));
+
+        Assert.Contains(0, d.List.CheckedIndices); // space toggled, not appended to filter
+        Assert.Equal(string.Empty, d.FilterText);
+    }
+
+    // ── Render ────────────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void RenderDelegatesToList()
+    {
+        Dialog d = new();
+        AddItems(d, "apple", "banana");
+
+        IReadOnlyList<Line> rows = d.Render(80);
+
+        Assert.Equal(2, rows.Count);
+        Assert.True(AnySegmentContains(rows[0], "apple"));
+        Assert.True(AnySegmentContains(rows[1], "banana"));
+    }
+
+    // ── MaxRows forwarding ────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void MaxRowsForwardsToList()
+    {
+        Dialog d = new();
+        AddItems(d, "a", "b", "c", "d", "e");
+        d.MaxRows = 3;
+
+        IReadOnlyList<Line> rows = d.Render(80);
+
+        Assert.Equal(3, rows.Count);
+    }
+}
diff --git a/tests/Dcli.Tests/DisplayWidthTests.cs b/tests/Dcli.Tests/DisplayWidthTests.cs
new file mode 100644
index 0000000..636291b
--- /dev/null
+++ b/tests/Dcli.Tests/DisplayWidthTests.cs
@@ -0,0 +1,200 @@
+using System.Text;
+using Xunit;
+
+namespace Dcli.Tests;
+
+/// <summary>
+/// Behavioural tests for Section 3: display-width measurement and line wrapping.
+/// Each test maps to a spec scenario described in spec.md / the Section-3 brief.
+/// </summary>
+public sealed class DisplayWidthTests
+{
+    // -----------------------------------------------------------------------------------------
+    // 3.1  DisplayWidth.Measure(Rune) — per-scalar width
+    // -----------------------------------------------------------------------------------------
+
+    [Fact]
+    public void AsciiLetterIsWidth1()
+    {
+        Assert.Equal(1, DisplayWidth.Measure(new Rune('A')));
+        Assert.Equal(1, DisplayWidth.Measure(new Rune('z')));
+        Assert.Equal(1, DisplayWidth.Measure(new Rune('0')));
+    }
+
+    [Fact]
+    public void CjkIdeographIsWidth2()
+    {
+        // U+4E2D (中) is a CJK Unified Ideograph.
+        Assert.Equal(2, DisplayWidth.Measure(new Rune(0x4E2D)));
+        // U+6587 (文)
+        Assert.Equal(2, DisplayWidth.Measure(new Rune(0x6587)));
+    }
+
+    [Fact]
+    public void HiraganaIsWidth2()
+    {
+        // U+3042 (あ) — Hiragana block is Wide.
+        Assert.Equal(2, DisplayWidth.Measure(new Rune(0x3042)));
+    }
+
+    [Fact]
+    public void HangulSyllableIsWidth2()
+    {
+        // U+AC00 (가) — first Hangul syllable.
+        Assert.Equal(2, DisplayWidth.Measure(new Rune(0xAC00)));
+    }
+
+    [Fact]
+    public void FullwidthLatinIsWidth2()
+    {
+        // U+FF21 (Ａ) — Fullwidth Latin Capital A.
+        Assert.Equal(2, DisplayWidth.Measure(new Rune(0xFF21)));
+    }
+
+    [Fact]
+    public void WideEmojiScalarIsWidth2()
+    {
+        // U+1F600 (😀) — Grinning Face emoji, wide.
+        Assert.Equal(2, DisplayWidth.Measure(new Rune(0x1F600)));
+    }
+
+    [Fact]
+    public void CombiningMarkIsWidth0()
+    {
+        // U+0301 (COMBINING ACUTE ACCENT) — NonSpacingMark.
+        Assert.Equal(0, DisplayWidth.Measure(new Rune(0x0301)));
+        // U+0308 (COMBINING DIAERESIS)
+        Assert.Equal(0, DisplayWidth.Measure(new Rune(0x0308)));
+    }
+
+    [Fact]
+    public void ZeroWidthSpaceIsWidth0()
+    {
+        // U+200B ZERO WIDTH SPACE
+        Assert.Equal(0, DisplayWidth.Measure(new Rune(0x200B)));
+    }
+
+    [Fact]
+    public void ZeroWidthJoinerIsWidth0()
+    {
+        // U+200D ZERO WIDTH JOINER
+        Assert.Equal(0, DisplayWidth.Measure(new Rune(0x200D)));
+    }
+
+    [Fact]
+    public void ZeroWidthNonJoinerIsWidth0()
+    {
+        // U+200C ZERO WIDTH NON-JOINER
+        Assert.Equal(0, DisplayWidth.Measure(new Rune(0x200C)));
+    }
+
+    [Fact]
+    public void BomIsWidth0()
+    {
+        // U+FEFF BOM / ZERO WIDTH NO-BREAK SPACE — Format category.
+        Assert.Equal(0, DisplayWidth.Measure(new Rune(0xFEFF)));
+    }
+
+    [Fact]
+    public void SoftHyphenIsWidth0()
+    {
+        // U+00AD SOFT HYPHEN — explicitly zero-width.
+        Assert.Equal(0, DisplayWidth.Measure(new Rune(0x00AD)));
+    }
+
+    [Fact]
+    public void ControlCharsBelowSpaceAreWidth0()
+    {
+        // ESC, NUL, BEL, etc. — all control chars < 0x20 except TAB.
+        Assert.Equal(0, DisplayWidth.Measure(new Rune(0x00)));  // NUL
+        Assert.Equal(0, DisplayWidth.Measure(new Rune(0x1B)));  // ESC
+        Assert.Equal(0, DisplayWidth.Measure(new Rune(0x07)));  // BEL
+        Assert.Equal(0, DisplayWidth.Measure(new Rune(0x7F)));  // DEL
+    }
+
+    [Fact]
+    public void TabFromColumn0IsWidth8()
+    {
+        // Tab at column 0 advances to column 8.
+        Assert.Equal(DisplayWidth.TabWidth, DisplayWidth.MeasureWithColumn(new Rune('\t'), 0));
+    }
+
+    [Fact]
+    public void TabFromColumn4IsWidth4()
+    {
+        // Column 4 → next stop at 8 → width = 4.
+        Assert.Equal(4, DisplayWidth.MeasureWithColumn(new Rune('\t'), 4));
+    }
+
+    [Fact]
+    public void TabFromColumn8IsWidth8()
+    {
+        // Column 8 is itself a stop → next stop at 16 → width = 8.
+        Assert.Equal(8, DisplayWidth.MeasureWithColumn(new Rune('\t'), 8));
+    }
+
+    [Fact]
+    public void TabFromColumn7IsWidth1()
+    {
+        // Column 7 → next stop at 8 → width = 1.
+        Assert.Equal(1, DisplayWidth.MeasureWithColumn(new Rune('\t'), 7));
+    }
+
+    // -----------------------------------------------------------------------------------------
+    // 3.1  DisplayWidth.Measure(string)
+    // -----------------------------------------------------------------------------------------
+
+    [Fact]
+    public void MeasureStringPureAsciiEqualsLength()
+    {
+        Assert.Equal(5, DisplayWidth.Measure("hello"));
+    }
+
+    [Fact]
+    public void MeasureStringWithCjkCountsDoubleColumns()
+    {
+        // "中文" = 2 + 2 = 4
+        Assert.Equal(4, DisplayWidth.Measure("中文"));
+    }
+
+    [Fact]
+    public void MeasureStringBaseCharPlusCombiningMarkEqualsBase()
+    {
+        // "é" = é (e + combining acute) → 1 + 0 = 1
+        Assert.Equal(1, DisplayWidth.Measure("é"));
+    }
+
+    [Fact]
+    public void MeasureStringZwjSequenceWidthIsBaseScalarsOnly()
+    {
+        // "A‍B" = A + ZWJ + B → 1 + 0 + 1 = 2 (per-scalar, no grapheme clustering)
+        Assert.Equal(2, DisplayWidth.Measure("A‍B"));
+    }
+
+    [Fact]
+    public void MeasureStringTabAtStartIsFullTabWidth()
+    {
+        // "\thello" — tab expands to 8, then 5 ASCII = 13.
+        Assert.Equal(13, DisplayWidth.Measure("\thello"));
+    }
+
+    [Fact]
+    public void MeasureEmptyStringIsZero()
+    {
+        Assert.Equal(0, DisplayWidth.Measure(""));
+    }
+
+    [Fact]
+    public void MeasureStringMixedAsciiAndCjk()
+    {
+        // "AB中C" = 1+1+2+1 = 5
+        Assert.Equal(5, DisplayWidth.Measure("AB中C"));
+    }
+
+    [Fact]
+    public void MeasureStringWideSurrogatePairEmoji()
+    {
+        // U+1F600 😀 is a surrogate pair in UTF-16: should be width 2.
+        Assert.Equal(2, DisplayWidth.Measure("\U0001F600"));
+    }
+}
diff --git a/tests/Dcli.Tests/FacadeTests.cs b/tests/Dcli.Tests/FacadeTests.cs
new file mode 100644
index 0000000..f2a0a21
--- /dev/null
+++ b/tests/Dcli.Tests/FacadeTests.cs
@@ -0,0 +1,602 @@
+using System.Text;
+using Dcli.Internal;
+using Dcli.Internal.FixedRegion;
+using Dcli.Internal.Input;
+using Dcli.Internal.RenderLoop;
+using Xunit;
+using DcliTerminal = Dcli.Terminal;
+
+namespace Dcli.Tests;
+
+/// <summary>
+/// End-to-end tests for §12 Chunk C: façade surfaces (Scrollback, Input, Status, Autocomplete)
+/// and event emission (InputSubmitted, InputChanged).
+/// All tests use injected edges — no real tty, no real raw mode.
+/// </summary>
+public sealed class FacadeTests
+{
+    // ── Test infrastructure ───────────────────────────────────────────────────
+
+    private sealed class CapturingOutputSink : IOutputSink
+    {
+        private volatile RenderModel? _lastModel;
+
+        internal RenderModel? LastModel => _lastModel;
+
+        public void Paint(RenderModel model)
+        {
+            _lastModel = model;
+        }
+
+        public void EmitRestoreSequence() { }
+    }
+
+    private sealed class ImmediateTimeoutByteSource : IInputByteSource
+    {
+        public int Read(Span<byte> buffer) => 0;
+    }
+
+    private sealed class FixedSizeSource : ITerminalSizeSource
+    {
+        public (int Columns, int Rows) GetSize() => (80, 24);
+    }
+
+    private sealed class VirtualClock : IClock
+    {
+        private readonly object _lock = new();
+        private TimeSpan _now;
+        private readonly List<(TimeSpan Deadline, TaskCompletionSource Tcs, CancellationTokenSource LinkedCts)> _waiters = [];
+
+        internal VirtualClock(TimeSpan? initial = null) => _now = initial ?? TimeSpan.Zero;
+
+        public TimeSpan Now { get { lock (_lock) { return _now; } } }
+
+        internal void Advance(TimeSpan by)
+        {
+            List<(TaskCompletionSource Tcs, CancellationTokenSource LinkedCts)> toComplete = [];
+            lock (_lock)
+            {
+                _now += by;
+                for (int i = _waiters.Count - 1; i >= 0; i--)
+                {
+                    if (_waiters[i].Deadline <= _now)
+                    {
+                        toComplete.Add((_waiters[i].Tcs, _waiters[i].LinkedCts));
+                        _waiters.RemoveAt(i);
+                    }
+                }
+            }
+            foreach ((TaskCompletionSource tcs, CancellationTokenSource linkedCts) in toComplete)
+            {
+                tcs.TrySetResult();
+                linkedCts.Dispose();
+            }
+        }
+
+        public Task WaitUntilAsync(TimeSpan deadline, CancellationToken cancellationToken)
+        {
+            lock (_lock)
+            {
+                if (deadline <= _now)
+                    return Task.CompletedTask;
+            }
+            CancellationTokenSource linkedCts = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
+            TaskCompletionSource tcs = new(TaskCreationOptions.RunContinuationsAsynchronously);
+            linkedCts.Token.Register(
+                static state => ((TaskCompletionSource)state!).TrySetCanceled(),
+                tcs,
+                useSynchronizationContext: false);
+            lock (_lock)
+            {
+                if (deadline <= _now) { linkedCts.Dispose(); return Task.CompletedTask; }
+                _waiters.Add((deadline, tcs, linkedCts));
+            }
+            return tcs.Task;
+        }
+    }
+
+    // ── Factory ───────────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Wires all injected edges through Terminal.StartCore. session and coordinator are
+    /// transferred to term and disposed by term.DisposeAsync; they are returned in the tuple so
+    /// the CA2000 analyzer sees they escape the factory and are not leaked locally.
+    /// </summary>
+    private static (DcliTerminal Term, CapturingOutputSink Sink, VirtualClock Clock,
+        RecordingRawModeSession Session) CreateTerminal()
+    {
+        RecordingRawModeSession session = new();
+        RestoreCoordinator coordinator = RestoreCoordinator.Wire(session);
+        CapturingOutputSink sink = new();
+        VirtualClock clock = new(TimeSpan.Zero);
+
+        // NoopResizeWatcher is a no-op disposable; ownership transfers to term on success.
+        using NoopResizeWatcher resizeWatcher = new();
+        DcliTerminal term = DcliTerminal.StartCore(
+            session,
+            coordinator,
+            resizeWatcher,
+            new ImmediateTimeoutByteSource(),
+            clock,
+            sink,
+            new FixedSizeSource(),
+            minFrameInterval: TimeSpan.Zero);
+
+        // coordinator and resizeWatcher transferred to term; DisposeAsync disposes them.
+        // The `using` also calls Dispose() on scope exit, but NoopResizeWatcher.Dispose() is idempotent.
+        return (term, sink, clock, session);
+    }
+
+    // Settle with a 5-second wall-clock guard so tests cannot hang.
+    private static Task SettleAsync(DcliTerminal term, VirtualClock clock)
+    {
+        clock.Advance(TimeSpan.FromMilliseconds(10));
+        return term.Loop.SettleAsync().WaitAsync(TimeSpan.FromSeconds(5));
+    }
+
+    private static Line PlainLine(string text) => new([new Segment(text)]);
+
+    // ── Drain helpers for outbound event channel ──────────────────────────────
+
+    /// <summary>
+    /// Drains all currently available events from the outbound channel.
+    /// Returns immediately — does not block waiting for new events.
+    /// </summary>
+    private static List<TerminalEvent> DrainEvents(DcliTerminal term)
+    {
+        List<TerminalEvent> events = [];
+        while (term.Events.TryRead(out TerminalEvent? ev))
+            events.Add(ev);
+        return events;
+    }
+
+    /// <summary>
+    /// Waits for at least one event to arrive on the outbound channel, with a timeout guard.
+    /// Returns all events drained after the wait.
+    /// </summary>
+    private static async Task<List<TerminalEvent>> WaitForEventsAsync(
+        DcliTerminal term,
+        int minCount = 1)
+    {
+        using CancellationTokenSource cts = new(TimeSpan.FromSeconds(5));
+        List<TerminalEvent> events = [];
+
+        while (events.Count < minCount)
+        {
+            if (term.Events.TryRead(out TerminalEvent? ev))
+            {
+                events.Add(ev);
+            }
+            else
+            {
+                // Wait for a new item to arrive, then loop.
+                await term.Events.WaitToReadAsync(cts.Token);
+            }
+        }
+
+        // Drain any additional items that arrived at the same time.
+        while (term.Events.TryRead(out TerminalEvent? extra))
+            events.Add(extra);
+
+        return events;
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // 12.4 — InputChanged emitted on user key press
+    // ─────────────────────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Typing a printable key emits InputChanged with the new buffer text.
+    /// This is the key input-change → candidates round-trip test from 12.5.
+    /// </summary>
+    [Fact]
+    public async Task TypingPrintableKeyEmitsInputChanged()
+    {
+        (DcliTerminal term, CapturingOutputSink sink, VirtualClock clock, _) = CreateTerminal();
+        await using (term)
+        {
+            // Post a printable 'x' key.
+            term.Loop.InputWriter.TryWrite(new KeyEvent(KeyCode.FromRune(new Rune('x')), Modifiers.None));
+
+            await SettleAsync(term, clock);
+
+            List<TerminalEvent> events = await WaitForEventsAsync(term);
+
+            InputChanged? changed = events.OfType<InputChanged>().FirstOrDefault();
+            Assert.NotNull(changed);
+            Assert.Equal("x", changed.Text);
+
+            // Also verify the editor state via the sink's captured model.
+            Assert.NotNull(sink.LastModel);
+            Assert.Equal("x", sink.LastModel.FixedRegion.Editor.Text);
+        }
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // 12.5 — Input-change → autocomplete candidates round-trip
+    // ─────────────────────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// After receiving InputChanged, consumer calls Autocomplete.Show; the overlay appears.
+    /// </summary>
+    [Fact]
+    public async Task InputChangedThenShowAutocompleteDisplaysOverlay()
+    {
+        (DcliTerminal term, CapturingOutputSink sink, VirtualClock clock, _) = CreateTerminal();
+        await using (term)
+        {
+            // Type 'x' to generate InputChanged.
+            term.Loop.InputWriter.TryWrite(new KeyEvent(KeyCode.FromRune(new Rune('x')), Modifiers.None));
+
+            await SettleAsync(term, clock);
+
+            List<TerminalEvent> events = await WaitForEventsAsync(term);
+            Assert.Contains(events, e => e is InputChanged { Text: "x" });
+
+            // Consumer responds with autocomplete candidates.
+            Line display = PlainLine("xyz - a candidate");
+            term.Autocomplete.Show([new AutocompleteCandidate("xyz", display)]);
+
+            await SettleAsync(term, clock);
+
+            // The active overlay must be an autocomplete showing that candidate.
+            Assert.NotNull(sink.LastModel);
+            Autocomplete? overlay = sink.LastModel.ActiveOverlay as Autocomplete;
+            Assert.NotNull(overlay);
+            // Verify the candidate is reflected in the rendered rows.
+            IReadOnlyList<Line> rows = overlay.Render(80);
+            Assert.True(rows.Count > 0, "Autocomplete overlay must render at least one row.");
+        }
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // 12.4 — InputSubmitted on Enter
+    // ─────────────────────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Typing text then pressing Enter emits InputSubmitted with the text, clears the buffer,
+    /// and the text is recallable from history via Up.
+    /// </summary>
+    [Fact]
+    public async Task EnterAfterTypingEmitsInputSubmittedClearsBufferAddsToHistory()
+    {
+        (DcliTerminal term, CapturingOutputSink sink, VirtualClock clock, _) = CreateTerminal();
+        await using (term)
+        {
+            // Type 'h', 'i'.
+            term.Loop.InputWriter.TryWrite(new KeyEvent(KeyCode.FromRune(new Rune('h')), Modifiers.None));
+            term.Loop.InputWriter.TryWrite(new KeyEvent(KeyCode.FromRune(new Rune('i')), Modifiers.None));
+
+            await SettleAsync(term, clock);
+            // Consume the InputChanged events.
+            DrainEvents(term);
+
+            // Press Enter.
+            term.Loop.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Enter), Modifiers.None));
+
+            await SettleAsync(term, clock);
+
+            List<TerminalEvent> events = await WaitForEventsAsync(term);
+
+            // (a) InputSubmitted with "hi".
+            InputSubmitted? submitted = events.OfType<InputSubmitted>().FirstOrDefault();
+            Assert.NotNull(submitted);
+            Assert.Equal("hi", submitted.Text);
+
+            // (b) KeyPressed(Enter) must NOT appear — Enter is consumed by submit.
+            Assert.DoesNotContain(events, e => e is KeyPressed { Key.Code.NamedValue: NamedKey.Enter });
+
+            // (c) Buffer was cleared after submit.
+            Assert.NotNull(sink.LastModel);
+            Assert.Equal(string.Empty, sink.LastModel.FixedRegion.Editor.Text);
+
+            // (d) "hi" is recallable from history: press Up.
+            term.Loop.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Up), Modifiers.None));
+            await SettleAsync(term, clock);
+
+            // After Up, the editor should contain "hi".
+            Assert.Equal("hi", sink.LastModel.FixedRegion.Editor.Text);
+        }
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // 12.4 — Empty Enter emits InputSubmitted("") but nothing added to history
+    // ─────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task EmptyEnterEmitsInputSubmittedEmptyAndAddsNothingToHistory()
+    {
+        (DcliTerminal term, CapturingOutputSink sink, VirtualClock clock, _) = CreateTerminal();
+        await using (term)
+        {
+            // Press Enter with empty buffer.
+            term.Loop.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Enter), Modifiers.None));
+
+            await SettleAsync(term, clock);
+            List<TerminalEvent> events = await WaitForEventsAsync(term);
+
+            InputSubmitted? submitted = events.OfType<InputSubmitted>().FirstOrDefault();
+            Assert.NotNull(submitted);
+            Assert.Equal(string.Empty, submitted.Text);
+
+            // History should still be empty — pressing Up should not change the buffer.
+            term.Loop.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Up), Modifiers.None));
+            await SettleAsync(term, clock);
+
+            // Buffer remains empty (Up is a no-op with empty history).
+            Assert.NotNull(sink.LastModel);
+            Assert.Equal(string.Empty, sink.LastModel.FixedRegion.Editor.Text);
+        }
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // 12.4 — No InputChanged for pure caret moves (Left/Right)
+    // ─────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task PureCaretMoveDoesNotEmitInputChanged()
+    {
+        (DcliTerminal term, _, VirtualClock clock, _) = CreateTerminal();
+        await using (term)
+        {
+            // Type something first so Left/Right has text to move over.
+            term.Loop.InputWriter.TryWrite(new KeyEvent(KeyCode.FromRune(new Rune('a')), Modifiers.None));
+            await SettleAsync(term, clock);
+            DrainEvents(term); // discard the InputChanged for 'a'
+
+            // Move caret left — should not emit InputChanged.
+            term.Loop.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Left), Modifiers.None));
+            await SettleAsync(term, clock);
+
+            List<TerminalEvent> leftEvents = DrainEvents(term);
+            Assert.DoesNotContain(leftEvents, e => e is InputChanged);
+
+            // Move caret right — should not emit InputChanged.
+            term.Loop.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Right), Modifiers.None));
+            await SettleAsync(term, clock);
+
+            List<TerminalEvent> rightEvents = DrainEvents(term);
+            Assert.DoesNotContain(rightEvents, e => e is InputChanged);
+        }
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // 12.3 — Input.SetText and Input.Clear do NOT emit InputChanged
+    // ─────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task ProgrammaticSetTextDoesNotEmitInputChanged()
+    {
+        (DcliTerminal term, CapturingOutputSink sink, VirtualClock clock, _) = CreateTerminal();
+        await using (term)
+        {
+            term.Input.SetText("hello");
+            await SettleAsync(term, clock);
+
+            List<TerminalEvent> events = DrainEvents(term);
+
+            // No InputChanged.
+            Assert.DoesNotContain(events, e => e is InputChanged);
+
+            // But the buffer was updated.
+            Assert.NotNull(sink.LastModel);
+            Assert.Equal("hello", sink.LastModel.FixedRegion.Editor.Text);
+        }
+    }
+
+    [Fact]
+    public async Task ProgrammaticClearDoesNotEmitInputChanged()
+    {
+        (DcliTerminal term, CapturingOutputSink sink, VirtualClock clock, _) = CreateTerminal();
+        await using (term)
+        {
+            term.Input.SetText("hello");
+            await SettleAsync(term, clock);
+            DrainEvents(term);
+
+            term.Input.Clear();
+            await SettleAsync(term, clock);
+
+            List<TerminalEvent> events = DrainEvents(term);
+            Assert.DoesNotContain(events, e => e is InputChanged);
+
+            Assert.NotNull(sink.LastModel);
+            Assert.Equal(string.Empty, sink.LastModel.FixedRegion.Editor.Text);
+        }
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // 12.3 — Scrollback.Append adds a live row
+    // ─────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task ScrollbackAppendAddsLiveRow()
+    {
+        (DcliTerminal term, CapturingOutputSink sink, VirtualClock clock, _) = CreateTerminal();
+        await using (term)
+        {
+            Line hello = PlainLine("hello scrollback");
+            term.Scrollback.Append(hello);
+            await SettleAsync(term, clock);
+
+            Assert.NotNull(sink.LastModel);
+            IReadOnlyList<Line> liveRows = sink.LastModel.LiveWindowRows;
+            Assert.True(liveRows.Count > 0, "Expected at least one live row after Append.");
+            // The hello line's text should appear in the live window.
+            bool found = liveRows.Any(row => row.Segments.Any(s =>
+                s.Text.Contains("hello scrollback", StringComparison.Ordinal)));
+            Assert.True(found, "Expected 'hello scrollback' in live window rows.");
+        }
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // 12.3 — BeginLive + AppendText + Commit streams then freezes
+    // ─────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task LiveBlockAppendTextThenCommitStreamsThenFreezes()
+    {
+        (DcliTerminal term, CapturingOutputSink sink, VirtualClock clock, _) = CreateTerminal();
+        await using (term)
+        {
+            ILiveBlock block = term.Scrollback.BeginLive();
+            block.AppendText("streaming...");
+            await SettleAsync(term, clock);
+
+            // While live: text should appear in LiveWindowRows.
+            Assert.NotNull(sink.LastModel);
+            bool liveBefore = sink.LastModel.LiveWindowRows.Any(row =>
+                row.Segments.Any(s => s.Text.Contains("streaming", StringComparison.Ordinal)));
+            Assert.True(liveBefore, "Expected live block text in LiveWindowRows before commit.");
+
+            // Commit the block.
+            block.Commit();
+            await SettleAsync(term, clock);
+
+            // After commit, the block drains from the live list into NewlyCommittedRows (or has
+            // already been emitted). The live window should no longer contain it (it committed).
+            // Since NewlyCommittedRows is cleared after each paint, we only check that the
+            // live window shrank (block was removed from live list).
+            bool liveAfter = sink.LastModel.LiveWindowRows.Any(row =>
+                row.Segments.Any(s => s.Text.Contains("streaming", StringComparison.Ordinal)));
+            Assert.False(liveAfter, "Committed block must not remain in LiveWindowRows.");
+        }
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // 12.3 — Status.Set sets status rows
+    // ─────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task StatusSetUpdatesStatusRows()
+    {
+        (DcliTerminal term, CapturingOutputSink sink, VirtualClock clock, _) = CreateTerminal();
+        await using (term)
+        {
+            Line statusLine = PlainLine("ready");
+            term.Status.SetRows(statusLine);
+            await SettleAsync(term, clock);
+
+            Assert.NotNull(sink.LastModel);
+            IReadOnlyList<Line> statusRows = sink.LastModel.FixedRegion.Status.Rows;
+            Assert.Single(statusRows);
+            Assert.Same(statusLine, statusRows[0]);
+        }
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // 12.3 — Autocomplete.Hide clears the overlay
+    // ─────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task AutocompleteHideClearsOverlay()
+    {
+        (DcliTerminal term, CapturingOutputSink sink, VirtualClock clock, _) = CreateTerminal();
+        await using (term)
+        {
+            // Show an autocomplete overlay.
+            term.Autocomplete.Show([new AutocompleteCandidate("item1", PlainLine("item1"))]);
+            await SettleAsync(term, clock);
+
+            Assert.NotNull(sink.LastModel);
+            Assert.IsType<Autocomplete>(sink.LastModel.ActiveOverlay);
+
+            // Hide it.
+            term.Autocomplete.Hide();
+            await SettleAsync(term, clock);
+
+            Assert.Null(sink.LastModel.ActiveOverlay);
+        }
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // 12.3 — Autocomplete.Hide is a no-op when a dialog is active
+    // ─────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task AutocompleteHideIsNoOpWhenDialogActive()
+    {
+        (DcliTerminal term, CapturingOutputSink sink, VirtualClock clock, _) = CreateTerminal();
+        await using (term)
+        {
+            // Open a dialog — this takes priority over autocomplete.
+            Task<DialogResult<int>> dialogTask = term.SelectAsync(
+                new SelectRequest([PlainLine("opt1"), PlainLine("opt2")]));
+
+            clock.Advance(TimeSpan.FromMilliseconds(10));
+            await term.Loop.SettleAsync().WaitAsync(TimeSpan.FromSeconds(5));
+
+            Assert.IsType<Dialog>(sink.LastModel?.ActiveOverlay);
+
+            // Calling Hide should not clear the dialog.
+            term.Autocomplete.Hide();
+            clock.Advance(TimeSpan.FromMilliseconds(10));
+            await term.Loop.SettleAsync().WaitAsync(TimeSpan.FromSeconds(5));
+
+            // Dialog must still be active.
+            Assert.IsType<Dialog>(sink.LastModel?.ActiveOverlay);
+
+            // Clean up: dismiss the dialog.
+            term.Loop.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Escape), Modifiers.None));
+            clock.Advance(TimeSpan.FromMilliseconds(10));
+            await term.Loop.SettleAsync().WaitAsync(TimeSpan.FromSeconds(5));
+            await dialogTask; // must be cancelled
+        }
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // 12.3 — Autocomplete.Show is no-op when modal dialog is active
+    // ─────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task AutocompleteShowIsNoOpWhenDialogActive()
+    {
+        (DcliTerminal term, CapturingOutputSink sink, VirtualClock clock, _) = CreateTerminal();
+        await using (term)
+        {
+            // Open a dialog.
+            Task<DialogResult<int>> dialogTask = term.SelectAsync(
+                new SelectRequest([PlainLine("opt1")]));
+
+            clock.Advance(TimeSpan.FromMilliseconds(10));
+            await term.Loop.SettleAsync().WaitAsync(TimeSpan.FromSeconds(5));
+
+            Assert.IsType<Dialog>(sink.LastModel?.ActiveOverlay);
+
+            // Try to show autocomplete — should be a no-op.
+            term.Autocomplete.Show([new AutocompleteCandidate("x", PlainLine("x"))]);
+            clock.Advance(TimeSpan.FromMilliseconds(10));
+            await term.Loop.SettleAsync().WaitAsync(TimeSpan.FromSeconds(5));
+
+            // Dialog must still be the active overlay (not replaced by autocomplete).
+            Assert.IsType<Dialog>(sink.LastModel?.ActiveOverlay);
+
+            // Clean up.
+            term.Loop.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Escape), Modifiers.None));
+            clock.Advance(TimeSpan.FromMilliseconds(10));
+            await term.Loop.SettleAsync().WaitAsync(TimeSpan.FromSeconds(5));
+            await dialogTask;
+        }
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // 12.4 — Ctrl+Enter falls through as KeyPressed, not InputSubmitted
+    // ─────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task CtrlEnterFallsThroughAsKeyPressedNotSubmitted()
+    {
+        (DcliTerminal term, _, VirtualClock clock, _) = CreateTerminal();
+        await using (term)
+        {
+            term.Loop.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Enter), Modifiers.Ctrl));
+            await SettleAsync(term, clock);
+
+            List<TerminalEvent> events = await WaitForEventsAsync(term);
+
+            // Must emit KeyPressed, not InputSubmitted.
+            Assert.Contains(events, e => e is KeyPressed { Key.Code.NamedValue: NamedKey.Enter });
+            Assert.DoesNotContain(events, e => e is InputSubmitted);
+        }
+    }
+}
diff --git a/tests/Dcli.Tests/FakeTerminalTests.cs b/tests/Dcli.Tests/FakeTerminalTests.cs
new file mode 100644
index 0000000..d97dba0
--- /dev/null
+++ b/tests/Dcli.Tests/FakeTerminalTests.cs
@@ -0,0 +1,495 @@
+using System.Threading.Channels;
+using Xunit;
+
+namespace Dcli.Tests;
+
+// ─────────────────────────────────────────────────────────────────────────────
+// Hand-written fakes — this entire file is intentionally self-contained so
+// that it reads as documentation for how a dmon controller would test against
+// dcli without a real terminal, raw-mode session, or static state to reset.
+// ─────────────────────────────────────────────────────────────────────────────
+
+#region Fake sub-surfaces
+
+/// <summary>Records every Append/BeginLive/BeginCollapsible call for assertion.</summary>
+internal sealed class FakeScrollback : IScrollback
+{
+    internal List<Line> Appended { get; } = [];
+    internal int BeginLiveCount { get; private set; }
+    internal List<(Line Summary, IReadOnlyList<Line> Hidden)> Collapsibles { get; } = [];
+
+    public void Append(Line line)
+    {
+        ArgumentNullException.ThrowIfNull(line);
+        Appended.Add(line);
+    }
+
+    public ILiveBlock BeginLive()
+    {
+        BeginLiveCount++;
+        return new NoOpLiveBlock();
+    }
+
+    public ICollapsible BeginCollapsible(Line summary, IReadOnlyList<Line> hiddenLines)
+    {
+        ArgumentNullException.ThrowIfNull(summary);
+        ArgumentNullException.ThrowIfNull(hiddenLines);
+        Collapsibles.Add((summary, hiddenLines));
+        return new NoOpCollapsible();
+    }
+
+    private sealed class NoOpLiveBlock : ILiveBlock
+    {
+        public void AppendText(string text) { }
+        public void SetContent(IReadOnlyList<Line> lines) { }
+        public void Commit() { }
+    }
+
+    private sealed class NoOpCollapsible : ICollapsible
+    {
+        public void Expand() { }
+    }
+}
+
+/// <summary>Records SetText and Clear calls.</summary>
+internal sealed class FakeInput : IInput
+{
+    internal List<string> SetTextCalls { get; } = [];
+    internal int ClearCount { get; private set; }
+
+    public void SetText(string text)
+    {
+        ArgumentNullException.ThrowIfNull(text);
+        SetTextCalls.Add(text);
+    }
+
+    public void Clear() => ClearCount++;
+}
+
+/// <summary>Records SetRows calls.</summary>
+internal sealed class FakeStatus : IStatus
+{
+    internal List<IReadOnlyList<Line>> SetCalls { get; } = [];
+
+    public void SetRows(params Line[] rows)
+    {
+        ArgumentNullException.ThrowIfNull(rows);
+        SetCalls.Add(rows);
+    }
+
+    public void SetRows(IReadOnlyList<Line> rows)
+    {
+        ArgumentNullException.ThrowIfNull(rows);
+        SetCalls.Add(rows);
+    }
+}
+
+/// <summary>Records Show/Hide calls.</summary>
+internal sealed class FakeAutocomplete : IAutocomplete
+{
+    internal List<IReadOnlyList<AutocompleteCandidate>> ShowCalls { get; } = [];
+    internal int HideCount { get; private set; }
+
+    public void Show(IReadOnlyList<AutocompleteCandidate> candidates)
+    {
+        ArgumentNullException.ThrowIfNull(candidates);
+        ShowCalls.Add(candidates);
+    }
+
+    public void Hide() => HideCount++;
+}
+
+#endregion
+
+#region FakeTerminal
+
+/// <summary>
+/// A hand-written fake <see cref="ITerminal"/> that records command-side calls and returns
+/// scripted dialog results, for use in consumer-side tests without a real terminal.
+/// </summary>
+internal sealed class FakeTerminal : ITerminal
+{
+    private readonly Channel<TerminalEvent> _events =
+        Channel.CreateUnbounded<TerminalEvent>(new UnboundedChannelOptions { SingleWriter = false, SingleReader = false });
+
+    // ── Sub-surfaces ──────────────────────────────────────────────────────────
+
+    internal FakeScrollback FakeScrollback { get; } = new();
+    internal FakeInput FakeInput { get; } = new();
+    internal FakeStatus FakeStatus { get; } = new();
+    internal FakeAutocomplete FakeAutocomplete { get; } = new();
+
+    // ── Dialog recordings ─────────────────────────────────────────────────────
+
+    internal List<SelectRequest> SelectRequests { get; } = [];
+    internal List<MultiSelectRequest> MultiSelectRequests { get; } = [];
+    internal List<ChoiceRequest> ChoiceRequests { get; } = [];
+    internal List<InputRequest> InputRequests { get; } = [];
+
+    // ── Scripted dialog results ───────────────────────────────────────────────
+
+    /// <summary>Scripted result returned by the next <see cref="SelectAsync"/> call.</summary>
+    internal DialogResult<int> NextSelectResult { get; set; } =
+        new(DialogOutcome.Cancelled, default);
+
+    /// <summary>Scripted result returned by the next <see cref="MultiSelectAsync"/> call.</summary>
+    internal DialogResult<int[]> NextMultiSelectResult { get; set; } =
+        new(DialogOutcome.Cancelled, []);
+
+    /// <summary>Scripted result returned by the next <see cref="ChoiceAsync"/> call.</summary>
+    internal DialogResult<int> NextChoiceResult { get; set; } =
+        new(DialogOutcome.Cancelled, default);
+
+    /// <summary>Scripted result returned by the next <see cref="InputAsync"/> call.</summary>
+    internal DialogResult<string> NextInputResult { get; set; } =
+        new(DialogOutcome.Cancelled, string.Empty);
+
+    // ── ITerminal implementation ──────────────────────────────────────────────
+
+    public IScrollback Scrollback => FakeScrollback;
+    public IInput Input => FakeInput;
+    public IStatus Status => FakeStatus;
+    public IAutocomplete Autocomplete => FakeAutocomplete;
+
+    public ChannelReader<TerminalEvent> Events => _events.Reader;
+
+    public (int Columns, int Rows) GetTerminalSize() => (80, 24);
+
+    public Task<DialogResult<int>> SelectAsync(
+        SelectRequest req,
+        CancellationToken cancellationToken = default)
+    {
+        ArgumentNullException.ThrowIfNull(req);
+        SelectRequests.Add(req);
+        return Task.FromResult(NextSelectResult);
+    }
+
+    public Task<DialogResult<int[]>> MultiSelectAsync(
+        MultiSelectRequest req,
+        CancellationToken cancellationToken = default)
+    {
+        ArgumentNullException.ThrowIfNull(req);
+        MultiSelectRequests.Add(req);
+        return Task.FromResult(NextMultiSelectResult);
+    }
+
+    public Task<DialogResult<int>> ChoiceAsync(
+        ChoiceRequest req,
+        CancellationToken cancellationToken = default)
+    {
+        ArgumentNullException.ThrowIfNull(req);
+        ChoiceRequests.Add(req);
+        return Task.FromResult(NextChoiceResult);
+    }
+
+    public Task<DialogResult<string>> InputAsync(
+        InputRequest req,
+        CancellationToken cancellationToken = default)
+    {
+        ArgumentNullException.ThrowIfNull(req);
+        InputRequests.Add(req);
+        return Task.FromResult(NextInputResult);
+    }
+
+    /// <summary>
+    /// Pushes a synthesized <see cref="TerminalEvent"/> into the <see cref="Events"/> channel
+    /// so that consumer code draining the channel can observe and react to it.
+    /// </summary>
+    internal void PushEvent(TerminalEvent evt) => _events.Writer.TryWrite(evt);
+
+    public ValueTask DisposeAsync()
+    {
+        _events.Writer.TryComplete();
+        return ValueTask.CompletedTask;
+    }
+}
+
+#endregion
+
+#region Consumer-style code under test
+
+/// <summary>
+/// Represents a consumer-style controller that depends only on <see cref="ITerminal"/>.
+/// Demonstrates the pattern a dmon controller would use.
+/// </summary>
+internal sealed class SampleConsumer
+{
+    private readonly ITerminal _terminal;
+
+    internal SampleConsumer(ITerminal terminal)
+    {
+        ArgumentNullException.ThrowIfNull(terminal);
+        _terminal = terminal;
+    }
+
+    /// <summary>
+    /// Opens a select dialog; on Submitted appends the selected index as a status line
+    /// and clears the input editor. On Cancelled appends a "cancelled" line.
+    /// </summary>
+    internal async Task RunSelectAndReactAsync(SelectRequest req)
+    {
+        DialogResult<int> result = await _terminal.SelectAsync(req);
+        if (result.Outcome == DialogOutcome.Submitted)
+        {
+            Segment seg = new($"selected:{result.Value}", new Style());
+            _terminal.Scrollback.Append(new Line([seg]));
+            _terminal.Input.Clear();
+        }
+        else
+        {
+            Segment seg = new("cancelled", new Style());
+            _terminal.Status.SetRows(new Line([seg]));
+        }
+    }
+
+    /// <summary>
+    /// Reads one event from the channel and reacts: on <see cref="KeyPressed"/> appends
+    /// the key description to scrollback.
+    /// </summary>
+    internal async Task DrainOneEventAsync()
+    {
+        TerminalEvent evt = await _terminal.Events.ReadAsync();
+        if (evt is KeyPressed kp)
+        {
+            Segment seg = new($"key:{kp.Key.Code}", new Style());
+            _terminal.Scrollback.Append(new Line([seg]));
+        }
+    }
+}
+
+#endregion
+
+// ─────────────────────────────────────────────────────────────────────────────
+// Tests
+// ─────────────────────────────────────────────────────────────────────────────
+
+/// <summary>
+/// Tests for §12 task 12.7: proves tier-A substitutability of the <see cref="ITerminal"/>
+/// façade. All tests run with no real terminal, loop, or parser.
+/// </summary>
+public sealed class FakeTerminalTests
+{
+    // ── Helpers ───────────────────────────────────────────────────────────────
+
+    private static Line PlainLine(string text) => new([new Segment(text, new Style())]);
+
+    // ── (a) No static state — fake compiles and runs standalone ──────────────
+
+    [Fact]
+    public async Task FakeTerminalCanBeCreatedAndDisposedWithNoStaticState()
+    {
+        // Two independent fakes must not share any state.
+        FakeTerminal fake1 = new();
+        FakeTerminal fake2 = new();
+
+        fake1.FakeScrollback.Append(PlainLine("hello"));
+
+        await fake1.DisposeAsync();
+        await fake2.DisposeAsync();
+
+        Assert.Single(fake1.FakeScrollback.Appended);
+        Assert.Empty(fake2.FakeScrollback.Appended);
+    }
+
+    // ── (b) Public event/result types are directly constructible ─────────────
+
+    [Fact]
+    public void KeyEventIsDirectlyConstructible()
+    {
+        // Tier-A requirement: a test can build a KeyEvent for an arrow key without the parser.
+        KeyEvent keyEvt = new(KeyCode.Named(NamedKey.Down), Modifiers.None);
+        Assert.Equal(NamedKey.Down, keyEvt.Code.NamedValue);
+        Assert.Equal(Modifiers.None, keyEvt.Modifiers);
+    }
+
+    [Fact]
+    public void DialogResultIsDirectlyConstructible()
+    {
+        // Tier-A requirement: scripted dialog result without driving the loop.
+        DialogResult<int> result = new(DialogOutcome.Submitted, 2);
+        Assert.Equal(DialogOutcome.Submitted, result.Outcome);
+        Assert.Equal(2, result.Value);
+    }
+
+    [Fact]
+    public void TerminalEventsAreDirectlyConstructible()
+    {
+        KeyEvent key = new(KeyCode.Named(NamedKey.Up), Modifiers.None);
+        KeyPressed kp = new(key);
+        InputSubmitted sub = new("hello");
+        InputChanged chg = new("he");
+        Resized rsz = new(120, 30);
+
+        Assert.Equal("hello", sub.Text);
+        Assert.Equal("he", chg.Text);
+        Assert.Equal(120, rsz.Columns);
+        Assert.Equal(30, rsz.Rows);
+        Assert.Equal(key, kp.Key);
+    }
+
+    // ── (c) Command-side calls are recorded ───────────────────────────────────
+
+    [Fact]
+    public async Task SubmittedSelectRecordsRequestAndAppendedLine()
+    {
+        FakeTerminal fake = new();
+        await using (fake)
+        {
+            Line item = PlainLine("option A");
+            SelectRequest req = new([item]);
+            fake.NextSelectResult = new DialogResult<int>(DialogOutcome.Submitted, 0);
+
+            SampleConsumer consumer = new(fake);
+            await consumer.RunSelectAndReactAsync(req);
+
+            // The request was recorded.
+            Assert.Single(fake.SelectRequests);
+            Assert.Same(req, fake.SelectRequests[0]);
+
+            // On Submitted: Scrollback.Append was called, Input.Clear was called.
+            Assert.Single(fake.FakeScrollback.Appended);
+            Assert.Contains("selected:0", fake.FakeScrollback.Appended[0].Segments[0].Text, StringComparison.Ordinal);
+            Assert.Equal(1, fake.FakeInput.ClearCount);
+
+            // Status was NOT called on Submitted path.
+            Assert.Empty(fake.FakeStatus.SetCalls);
+        }
+    }
+
+    [Fact]
+    public async Task CancelledSelectSetsStatusAndDoesNotAppend()
+    {
+        FakeTerminal fake = new();
+        await using (fake)
+        {
+            SelectRequest req = new([PlainLine("x")]);
+            fake.NextSelectResult = new DialogResult<int>(DialogOutcome.Cancelled, default);
+
+            SampleConsumer consumer = new(fake);
+            await consumer.RunSelectAndReactAsync(req);
+
+            // On Cancelled: Status.Set was called.
+            Assert.Single(fake.FakeStatus.SetCalls);
+            Assert.Contains("cancelled", fake.FakeStatus.SetCalls[0][0].Segments[0].Text, StringComparison.Ordinal);
+
+            // Scrollback.Append was NOT called on Cancelled path.
+            Assert.Empty(fake.FakeScrollback.Appended);
+            Assert.Equal(0, fake.FakeInput.ClearCount);
+        }
+    }
+
+    // ── (d) Synthesized event drives consumer reaction ────────────────────────
+
+    [Fact]
+    public async Task SynthesizedArrowKeyEventDrivesConsumerReaction()
+    {
+        FakeTerminal fake = new();
+        await using (fake)
+        {
+            // Consumer-style code: depends only on ITerminal.
+            SampleConsumer consumer = new(fake);
+
+            // Synthesize a Down-arrow KeyPressed event (no real parser needed).
+            KeyEvent keyEvt = new(KeyCode.Named(NamedKey.Down), Modifiers.None);
+            fake.PushEvent(new KeyPressed(keyEvt));
+
+            // Consumer drains one event and reacts.
+            await consumer.DrainOneEventAsync();
+
+            // The consumer appended a "key:…" line to scrollback.
+            Assert.Single(fake.FakeScrollback.Appended);
+            Assert.Contains("key:", fake.FakeScrollback.Appended[0].Segments[0].Text, StringComparison.Ordinal);
+        }
+    }
+
+    // ── (e) Other surface calls are recorded ─────────────────────────────────
+
+    [Fact]
+    public async Task ScrollbackBeginLiveAndBeginCollapsibleAreRecorded()
+    {
+        FakeTerminal fake = new();
+        await using (fake)
+        {
+            ILiveBlock block = fake.Scrollback.BeginLive();
+            Assert.NotNull(block);
+            Assert.Equal(1, fake.FakeScrollback.BeginLiveCount);
+
+            Line summary = PlainLine("summary");
+            Line hidden = PlainLine("detail");
+            ICollapsible coll = fake.Scrollback.BeginCollapsible(summary, [hidden]);
+            Assert.NotNull(coll);
+            Assert.Single(fake.FakeScrollback.Collapsibles);
+            Assert.Same(summary, fake.FakeScrollback.Collapsibles[0].Summary);
+        }
+    }
+
+    [Fact]
+    public async Task InputSetTextIsRecorded()
+    {
+        FakeTerminal fake = new();
+        await using (fake)
+        {
+            fake.Input.SetText("hello");
+            fake.Input.SetText("world");
+
+            Assert.Equal(["hello", "world"], fake.FakeInput.SetTextCalls);
+        }
+    }
+
+    [Fact]
+    public async Task AutocompleteShowAndHideAreRecorded()
+    {
+        FakeTerminal fake = new();
+        await using (fake)
+        {
+            AutocompleteCandidate candidate = new("val", PlainLine("display"));
+            fake.Autocomplete.Show([candidate]);
+            fake.Autocomplete.Hide();
+
+            Assert.Single(fake.FakeAutocomplete.ShowCalls);
+            Assert.Equal(1, fake.FakeAutocomplete.HideCount);
+        }
+    }
+
+    [Fact]
+    public async Task MultiSelectChoiceAndInputDialogsRecordRequests()
+    {
+        FakeTerminal fake = new();
+        await using (fake)
+        {
+            // MultiSelect
+            MultiSelectRequest msReq = new([PlainLine("a"), PlainLine("b")]);
+            fake.NextMultiSelectResult = new DialogResult<int[]>(DialogOutcome.Submitted, [0, 1]);
+            DialogResult<int[]> msResult = await fake.MultiSelectAsync(msReq);
+            Assert.Equal(DialogOutcome.Submitted, msResult.Outcome);
+            Assert.Equal([0, 1], msResult.Value);
+            Assert.Single(fake.MultiSelectRequests);
+
+            // Choice
+            ChoiceRequest chReq = new([PlainLine("yes"), PlainLine("no")]);
+            fake.NextChoiceResult = new DialogResult<int>(DialogOutcome.Submitted, 1);
+            DialogResult<int> chResult = await fake.ChoiceAsync(chReq);
+            Assert.Equal(1, chResult.Value);
+            Assert.Single(fake.ChoiceRequests);
+
+            // Input
+            InputRequest inReq = new(Prompt: PlainLine("Name?"));
+            fake.NextInputResult = new DialogResult<string>(DialogOutcome.Submitted, "Alice");
+            DialogResult<string> inResult = await fake.InputAsync(inReq);
+            Assert.Equal("Alice", inResult.Value);
+            Assert.Single(fake.InputRequests);
+        }
+    }
+
+    [Fact]
+    public async Task GetTerminalSizeReturnsConfiguredValue()
+    {
+        FakeTerminal fake = new();
+        await using (fake)
+        {
+            (int cols, int rows) = fake.GetTerminalSize();
+            Assert.Equal(80, cols);
+            Assert.Equal(24, rows);
+        }
+    }
+}
diff --git a/tests/Dcli.Tests/FixedRegionTests.cs b/tests/Dcli.Tests/FixedRegionTests.cs
new file mode 100644
index 0000000..ac1f4af
--- /dev/null
+++ b/tests/Dcli.Tests/FixedRegionTests.cs
@@ -0,0 +1,630 @@
+using System.Text;
+using Dcli.Internal.FixedRegion;
+using Dcli.Internal.RenderLoop;
+using Dcli.Internal.Scrollback;
+using Xunit;
+
+namespace Dcli.Tests;
+
+/// <summary>
+/// Tests for tasks 10.3–10.5: StatusLine, FixedRegionComposer cap formula,
+/// stack/budget behaviour, caret in combined frame, key routing, and the
+/// wrap-boundary caret convention pin (Chunk-A reviewer nit).
+/// </summary>
+public sealed class FixedRegionTests
+{
+    // ── Helpers ──────────────────────────────────────────────────────────────
+
+    private static RenderModel MakeModel(int cols = 80, int rows = 24, int? maxFixedHeight = null)
+    {
+        ConstantSizeSource size = new(cols, rows);
+        return new RenderModel(size, maxFixedHeight);
+    }
+
+    private sealed class ConstantSizeSource : ITerminalSizeSource
+    {
+        private readonly int _cols;
+        private readonly int _rows;
+
+        internal ConstantSizeSource(int cols, int rows)
+        {
+            _cols = cols;
+            _rows = rows;
+        }
+
+        public (int Columns, int Rows) GetSize() => (_cols, _rows);
+    }
+
+    private static Line PlainLine(string text) =>
+        new([new Segment(text)]);
+
+    // ── 10.5 — Cap formula ───────────────────────────────────────────────────
+
+    [Fact]
+    public void CapDefault24Rows()
+    {
+        // No configured MaxFixedHeight on 24-row terminal → 50% of 24 = 12.
+        int cap = FixedRegionComposer.ComputeCap(rows: 24, appSet: null);
+        Assert.Equal(12, cap);
+    }
+
+    [Fact]
+    public void CapMinFloor10Rows()
+    {
+        // 50% of 10 = 5 < 8 (floor), rows ≥ 8 → cap = 8.
+        int cap = FixedRegionComposer.ComputeCap(rows: 10, appSet: null);
+        Assert.Equal(8, cap);
+    }
+
+    [Fact]
+    public void CapTinyTerminal5Rows()
+    {
+        // rows < 8 → cap = rows (floor yields to terminal height).
+        int cap = FixedRegionComposer.ComputeCap(rows: 5, appSet: null);
+        Assert.Equal(5, cap);
+    }
+
+    [Fact]
+    public void CapAppSetClampedIntoRange()
+    {
+        // appSet=4 < 8 on 24-row terminal → clamped to min 8.
+        Assert.Equal(8, FixedRegionComposer.ComputeCap(rows: 24, appSet: 4));
+
+        // appSet=30 > 24 rows → clamped to rows.
+        Assert.Equal(24, FixedRegionComposer.ComputeCap(rows: 24, appSet: 30));
+
+        // appSet=10 in [8, 24] → passes through unchanged.
+        Assert.Equal(10, FixedRegionComposer.ComputeCap(rows: 24, appSet: 10));
+    }
+
+    [Fact]
+    public void CapTinyTerminalAppSetIgnored()
+    {
+        // rows=5 < 8 → cap = rows regardless of appSet.
+        Assert.Equal(5, FixedRegionComposer.ComputeCap(rows: 5, appSet: 3));
+        Assert.Equal(5, FixedRegionComposer.ComputeCap(rows: 5, appSet: 10));
+    }
+
+    // ── 10.5 — Stack / budget behaviour ─────────────────────────────────────
+
+    [Fact]
+    public void StatusAlwaysShown()
+    {
+        // Even when cap is tight, status rows appear in the fixed region.
+        // cap = rows/2 = 4 (rows=8). Status=2, input=1 row → fixedRows = 3 (input + status).
+        RenderModel model = MakeModel(cols: 80, rows: 8);
+        StatusLine status = new() { Rows = [PlainLine("status-A"), PlainLine("status-B")] };
+        TextBuffer editor = new();
+        editor.Insert("hi");
+        FixedRegionComposer composer = new(editor, status);
+
+        composer.Compose(model);
+
+        // Fixed region should contain both status rows.
+        IReadOnlyList<Line> fixed_ = model.FixedRegionRows;
+        Assert.Contains(fixed_, r => r.Segments.Count > 0 && r.Segments[0].Text == "status-A");
+        Assert.Contains(fixed_, r => r.Segments.Count > 0 && r.Segments[0].Text == "status-B");
+    }
+
+    [Fact]
+    public void FixedRegionConsumsOnlyNeededRowsWhenUnderCap()
+    {
+        // cap = 12 (24-row terminal). Input="A" (1 row), status=1 row → fixed=2 rows, not 12.
+        RenderModel model = MakeModel(cols: 80, rows: 24);
+        StatusLine status = new() { Rows = [PlainLine("status")] };
+        TextBuffer editor = new();
+        editor.Insert("A");
+        FixedRegionComposer composer = new(editor, status);
+
+        composer.Compose(model);
+
+        Assert.Equal(2, model.FixedRegionRows.Count);
+    }
+
+    [Fact]
+    public void InputScrollsInternallyWhenExceedsCap()
+    {
+        // cap = 8 (floor, 10-row terminal, 50%=5<8). Status=3 rows → inputAllot=5.
+        // Input with many lines → Render(width, 5) returns at most 5 rows.
+        RenderModel model = MakeModel(cols: 80, rows: 10);
+        StatusLine status = new()
+        {
+            Rows = [PlainLine("s1"), PlainLine("s2"), PlainLine("s3")]
+        };
+        TextBuffer editor = new();
+        // Insert 8 logical lines (9 visual rows with width=80).
+        editor.Insert("line1\nline2\nline3\nline4\nline5\nline6\nline7\nline8");
+        FixedRegionComposer composer = new(editor, status);
+
+        composer.Compose(model);
+
+        // Fixed region: inputAllot=5 (cap 8 - 3 status) → at most 5 input rows + 3 status = 8.
+        Assert.True(model.FixedRegionRows.Count <= 8,
+            $"Fixed region should not exceed cap; got {model.FixedRegionRows.Count}");
+        // Status rows still present at the bottom.
+        Assert.Equal("s1", model.FixedRegionRows[^3].Segments[0].Text);
+        Assert.Equal("s2", model.FixedRegionRows[^2].Segments[0].Text);
+        Assert.Equal("s3", model.FixedRegionRows[^1].Segments[0].Text);
+        // Caret is visible (editor was given allotted height and scrolled internally).
+        Assert.True(model.IsCursorVisible);
+        Assert.NotNull(model.EditorCaretLocal);
+    }
+
+    [Fact]
+    public void FixedRegionComposedBeforeScrollbackSoLiveWindowIsCorrect()
+    {
+        // The fixed region must be set before scrollback PrePaint so the live-window budget
+        // (rows − fixedRegionRows.Count) is correct.
+        // Simulate the paint step: compose fixed region → scrollback PrePaint → check budget.
+        RenderModel model = MakeModel(cols: 80, rows: 10);
+        StatusLine status = new() { Rows = [PlainLine("st")] };
+        TextBuffer editor = new();
+        editor.Insert("hello");
+        FixedRegionComposer composer = new(editor, status);
+
+        // Append some scrollback content.
+        ScrollbackModel scrollback = model.Scrollback;
+        scrollback.Append(new TextBlock(PlainLine("content1")), model);
+        scrollback.Append(new TextBlock(PlainLine("content2")), model);
+
+        // Step 1: compose fixed region (sets FixedRegionRows = 2 rows: input + status).
+        composer.Compose(model);
+        int fixedCount = model.FixedRegionRows.Count; // should be 2
+
+        // Step 2: scrollback PrePaint (uses rows − fixedCount as live-window cap).
+        scrollback.PrePaint(model);
+
+        // Live window should be bounded by rows − fixedCount = 10 − 2 = 8.
+        Assert.Equal(fixedCount, model.FixedRegionRows.Count);
+        Assert.True(model.LiveWindowRows.Count <= model.Rows - fixedCount,
+            $"Live window {model.LiveWindowRows.Count} exceeds budget {model.Rows - fixedCount}");
+    }
+
+    // ── 10.5 — Caret in combined frame ───────────────────────────────────────
+
+    [Fact]
+    public async Task CaretPositionAccountsForLiveWindowOffset()
+    {
+        // Drive via LoopEngine to test the full paint step (compose → PrePaint → caret offset).
+        ConstantSizeSource size = new(80, 24);
+        CapturingOutputSink sink = new();
+        VirtualClock clock = new();
+        using LoopEngine engine = new(size, clock, sink, minFrameInterval: TimeSpan.Zero);
+
+        // Append scrollback content to create a live window with rows.
+        engine.Post(new AppendScrollbackCommand(PlainLine("scrollback-line")));
+
+        // Type a character into the editor (editing key → consumed, buffer mutated).
+        engine.InputWriter.TryWrite(new KeyEvent(KeyCode.FromRune(new Rune('A')), Modifiers.None));
+
+        clock.Advance(TimeSpan.FromMilliseconds(100));
+        using CancellationTokenSource cts = new(TimeSpan.FromSeconds(5));
+        await engine.SettleAsync(cts.Token);
+
+        // CaretPosition.Row should be ≥ LiveWindowRows.Count (the caret is in the fixed region).
+        Assert.NotNull(sink.LastModel?.CaretPosition);
+        (int caretRow, _) = sink.LastModel!.CaretPosition!.Value;
+        int liveRows = sink.LastModel.LiveWindowRows.Count;
+        Assert.True(caretRow >= liveRows,
+            $"Caret row {caretRow} should be >= live window row count {liveRows}");
+        Assert.True(sink.LastModel.IsCursorVisible);
+    }
+
+    // ── 10.5 — Key routing ───────────────────────────────────────────────────
+
+    [Fact]
+    public async Task PrintableKeyInsertsIntoEditor()
+    {
+        ConstantSizeSource size = new(80, 24);
+        CapturingOutputSink sink = new();
+        VirtualClock clock = new();
+        using LoopEngine engine = new(size, clock, sink, minFrameInterval: TimeSpan.Zero);
+
+        engine.InputWriter.TryWrite(new KeyEvent(KeyCode.FromRune(new Rune('H')), Modifiers.None));
+        engine.InputWriter.TryWrite(new KeyEvent(KeyCode.FromRune(new Rune('i')), Modifiers.None));
+
+        clock.Advance(TimeSpan.FromMilliseconds(100));
+        using CancellationTokenSource cts = new(TimeSpan.FromSeconds(5));
+        await engine.SettleAsync(cts.Token);
+
+        // Printable keys are consumed — no KeyPressed events. InputChanged is emitted for each
+        // edit that changes the buffer ('H' → "H", 'i' → "Hi" = 2 events).
+        Assert.Equal(2, engine.OutboundEvents.Count);
+        Assert.All(Enumerable.Range(0, 2), _ =>
+        {
+            Assert.True(engine.OutboundEvents.TryRead(out TerminalEvent? ev));
+            Assert.IsType<InputChanged>(ev);
+        });
+
+        // The fixed region should reflect the typed text.
+        Assert.NotNull(sink.LastModel);
+        // Editor's rendered content should include "Hi".
+        string fixedText = string.Concat(
+            sink.LastModel!.FixedRegionRows.SelectMany(r => r.Segments).Select(s => s.Text));
+        Assert.Contains("Hi", fixedText, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public async Task ArrowKeyMovesCaretWithoutEmittingEvent()
+    {
+        ConstantSizeSource size = new(80, 24);
+        CapturingOutputSink sink = new();
+        VirtualClock clock = new();
+        using LoopEngine engine = new(size, clock, sink, minFrameInterval: TimeSpan.Zero);
+
+        engine.InputWriter.TryWrite(new KeyEvent(KeyCode.FromRune(new Rune('A')), Modifiers.None));
+        engine.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Left), Modifiers.None));
+
+        clock.Advance(TimeSpan.FromMilliseconds(100));
+        using CancellationTokenSource cts = new(TimeSpan.FromSeconds(5));
+        await engine.SettleAsync(cts.Token);
+
+        // 'A' is consumed and emits InputChanged (text changed); Left moves the caret only and
+        // does not emit. Net result: exactly one InputChanged event on the outbound channel.
+        Assert.Equal(1, engine.OutboundEvents.Count);
+        Assert.True(engine.OutboundEvents.TryRead(out TerminalEvent? arrowEv));
+        Assert.IsType<InputChanged>(arrowEv);
+    }
+
+    [Fact]
+    public async Task EnterKeyFallsThroughToOutboundChannel()
+    {
+        ConstantSizeSource size = new(80, 24);
+        CapturingOutputSink sink = new();
+        VirtualClock clock = new();
+        using LoopEngine engine = new(size, clock, sink, minFrameInterval: TimeSpan.Zero);
+
+        engine.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Enter), Modifiers.None));
+
+        clock.Advance(TimeSpan.FromMilliseconds(100));
+        using CancellationTokenSource cts = new(TimeSpan.FromSeconds(5));
+        await engine.SettleAsync(cts.Token);
+
+        // Enter with no modifier and no overlay: emits InputSubmitted (not KeyPressed).
+        Assert.True(engine.OutboundEvents.TryRead(out TerminalEvent? ev));
+        InputSubmitted submitted = Assert.IsType<InputSubmitted>(ev);
+        Assert.Equal(string.Empty, submitted.Text); // buffer was empty
+    }
+
+    [Fact]
+    public async Task TabKeyFallsThroughToOutboundChannel()
+    {
+        ConstantSizeSource size = new(80, 24);
+        CapturingOutputSink sink = new();
+        VirtualClock clock = new();
+        using LoopEngine engine = new(size, clock, sink, minFrameInterval: TimeSpan.Zero);
+
+        engine.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Tab), Modifiers.None));
+
+        clock.Advance(TimeSpan.FromMilliseconds(100));
+        using CancellationTokenSource cts = new(TimeSpan.FromSeconds(5));
+        await engine.SettleAsync(cts.Token);
+
+        Assert.True(engine.OutboundEvents.TryRead(out TerminalEvent? ev));
+        Assert.IsType<KeyPressed>(ev);
+    }
+
+    [Fact]
+    public async Task CtrlKeyFallsThrough()
+    {
+        ConstantSizeSource size = new(80, 24);
+        CapturingOutputSink sink = new();
+        VirtualClock clock = new();
+        using LoopEngine engine = new(size, clock, sink, minFrameInterval: TimeSpan.Zero);
+
+        // Ctrl+C — should fall through, not be inserted.
+        engine.InputWriter.TryWrite(new KeyEvent(
+            KeyCode.FromRune(new Rune('c')), Modifiers.Ctrl));
+
+        clock.Advance(TimeSpan.FromMilliseconds(100));
+        using CancellationTokenSource cts = new(TimeSpan.FromSeconds(5));
+        await engine.SettleAsync(cts.Token);
+
+        Assert.True(engine.OutboundEvents.TryRead(out _));
+    }
+
+    // ── Wrap-boundary caret convention pin (Chunk-A reviewer nit) ────────────
+
+    [Fact]
+    public void CaretAtNonNewlineWrapBoundaryReportsNextVisualRowCol0()
+    {
+        // "AAAA" at width=2 wraps: row0="AA" [0..2), row1="AA" [2..4).
+        // CaretIndex=2 is exactly at the wrap boundary (start of row1).
+        // Convention: the caret belongs to the NEXT row (row index 1), col 0.
+        TextBuffer buf = new();
+        buf.Insert("AAAA");
+        buf.SetCaretIndexForTest(2); // exactly at non-newline wrap boundary
+
+        RenderResult result = buf.Render(width: 2);
+        (int row, int col) = result.CaretPosition;
+
+        Assert.Equal(1, row); // belongs to next row, not the earlier row
+        Assert.Equal(0, col); // col 0 — start of that row
+    }
+
+    [Fact]
+    public void CaretAtNewlineBoundaryBelongsToCurrentRow()
+    {
+        // "AB\nCD" at width=80. Row0="AB" ends with newline at char index 2.
+        // CaretIndex=2 is at the '\n' — caret belongs to row0 (before the newline).
+        TextBuffer buf = new();
+        buf.Insert("AB\nCD");
+        buf.SetCaretIndexForTest(2); // at the '\n'
+
+        RenderResult result = buf.Render(width: 80);
+        (int row, _) = result.CaretPosition;
+
+        Assert.Equal(0, row); // belongs to row 0 (before the newline)
+    }
+
+    // ── Degenerate: status ≥ cap ─────────────────────────────────────────────
+
+    [Fact]
+    public void DegenerateStatusAloneGeCapCursorHidden()
+    {
+        // 5-row terminal: cap = 5. Status has 5 rows → statusCount >= cap → caret hidden.
+        RenderModel model = MakeModel(cols: 80, rows: 5);
+        StatusLine status = new()
+        {
+            Rows = Enumerable.Range(1, 5).Select(i => PlainLine($"s{i}")).ToList()
+        };
+        TextBuffer editor = new();
+        editor.Insert("input");
+        FixedRegionComposer composer = new(editor, status);
+
+        composer.Compose(model);
+
+        // Status shown in full (or clamped to terminal rows).
+        Assert.True(model.FixedRegionRows.Count <= 5);
+        Assert.False(model.IsCursorVisible, "Cursor must be hidden when status >= cap");
+        Assert.Null(model.EditorCaretLocal);
+    }
+
+    // ── Nit 1 — EditorCaretLocal reset on degenerate transition ─────────────
+
+    [Fact]
+    public void DegenerateTransitionResetsEditorCaretLocal()
+    {
+        // Normal compose on the model so EditorCaretLocal is set non-null.
+        RenderModel model = MakeModel(cols: 80, rows: 24);
+        StatusLine statusNormal = new() { Rows = [PlainLine("status")] };
+        TextBuffer editor = new();
+        editor.Insert("hello");
+        FixedRegionComposer composerNormal = new(editor, statusNormal);
+        composerNormal.Compose(model);
+
+        // Sanity: after a normal compose EditorCaretLocal must be non-null.
+        Assert.NotNull(model.EditorCaretLocal);
+
+        // Now compose a degenerate frame (status >= cap) on the SAME model.
+        // 5-row terminal: cap = 5 (rows < 8). Status with 5 rows fills the cap.
+        model.UpdateSize(80, 5);
+        StatusLine statusDegenerate = new()
+        {
+            Rows = Enumerable.Range(1, 5).Select(i => PlainLine($"s{i}")).ToList()
+        };
+        FixedRegionComposer composerDegenerate = new(editor, statusDegenerate);
+        composerDegenerate.Compose(model);
+
+        // All three caret-related fields must be null/false — no contradiction.
+        Assert.Null(model.EditorCaretLocal);
+        Assert.Null(model.CaretPosition);
+        Assert.False(model.IsCursorVisible, "Cursor must be hidden in degenerate frame");
+    }
+
+    // ── Nit 2 — Up/Down history-vs-move routing decision ────────────────────
+
+    [Fact]
+    public async Task UpOnFirstVisualRowRecallsHistory()
+    {
+        // History-nav convention (LoopEngine.cs:~343-347):
+        // Up recalls the previous (older) history entry when the caret is on the FIRST visual row.
+        ConstantSizeSource size = new(80, 24);
+        CapturingOutputSink sink = new();
+        VirtualClock clock = new();
+        using LoopEngine engine = new(size, clock, sink, minFrameInterval: TimeSpan.Zero);
+
+        // Populate history with one entry, then clear the buffer so the editor is empty.
+        engine.Post(new AddToHistoryCommand("previous-entry"));
+
+        // Caret is on row 0 (first visual row) — single-line empty buffer.
+        engine.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Up), Modifiers.None));
+
+        clock.Advance(TimeSpan.FromMilliseconds(100));
+        using CancellationTokenSource cts = new(TimeSpan.FromSeconds(5));
+        await engine.SettleAsync(cts.Token);
+
+        // Up is consumed. It changes the text (history recall), so InputChanged is emitted.
+        // No KeyPressed on the outbound channel.
+        Assert.Equal(1, engine.OutboundEvents.Count);
+        Assert.True(engine.OutboundEvents.TryRead(out TerminalEvent? upEv));
+        Assert.IsType<InputChanged>(upEv);
+
+        // The fixed region should now reflect the recalled history entry.
+        Assert.NotNull(sink.LastModel);
+        string fixedText = string.Concat(
+            sink.LastModel!.FixedRegionRows.SelectMany(r => r.Segments).Select(s => s.Text));
+        Assert.Contains("previous-entry", fixedText, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public async Task UpOnMiddleVisualRowMovesCaret()
+    {
+        // Up on a non-first visual row moves the caret up within the buffer — no history recall.
+        ConstantSizeSource size = new(80, 24);
+        CapturingOutputSink sink = new();
+        VirtualClock clock = new();
+        using LoopEngine engine = new(size, clock, sink, minFrameInterval: TimeSpan.Zero);
+
+        // Populate history (so history recall would be possible if incorrectly triggered).
+        engine.Post(new AddToHistoryCommand("should-not-appear"));
+
+        // Type two logical lines — caret ends up on the second visual row (row index 1).
+        engine.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Enter), Modifiers.None));
+        // Enter falls through (not consumed by editor), so type two lines via multiline insert.
+        engine.Post(new SetEditorTextCommand("line1\nline2"));
+
+        // Settle to get the model into a known state before the Up key.
+        clock.Advance(TimeSpan.FromMilliseconds(100));
+        using CancellationTokenSource cts1 = new(TimeSpan.FromSeconds(5));
+        await engine.SettleAsync(cts1.Token);
+
+        // Drain any fall-through events from the Enter key.
+        while (engine.OutboundEvents.TryRead(out _)) { }
+
+        // Caret is on the last visual row (row 1 of 2). Press Up → moves caret to row 0.
+        engine.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Up), Modifiers.None));
+
+        clock.Advance(TimeSpan.FromMilliseconds(100));
+        using CancellationTokenSource cts2 = new(TimeSpan.FromSeconds(5));
+        await engine.SettleAsync(cts2.Token);
+
+        // Up was consumed (no KeyPressed emitted for Up).
+        // Any remaining outbound events must NOT be Up-key events.
+        while (engine.OutboundEvents.TryRead(out TerminalEvent? ev))
+        {
+            if (ev is KeyPressed kp)
+                Assert.NotEqual(NamedKey.Up, kp.Key.Code.NamedValue);
+        }
+
+        // Buffer text must remain unchanged — no history recall occurred.
+        Assert.NotNull(sink.LastModel);
+        string fixedText = string.Concat(
+            sink.LastModel!.FixedRegionRows.SelectMany(r => r.Segments).Select(s => s.Text));
+        Assert.DoesNotContain("should-not-appear", fixedText, StringComparison.Ordinal);
+        Assert.Contains("line1", fixedText, StringComparison.Ordinal);
+        Assert.Contains("line2", fixedText, StringComparison.Ordinal);
+    }
+
+    // ── Loop engine integration (LoopEngine owns editor, status, composer) ───
+
+    [Fact]
+    public async Task LoopEngineFixedRegionComposedEveryPaint()
+    {
+        ConstantSizeSource size = new(80, 24);
+        CapturingOutputSink sink = new();
+        VirtualClock clock = new();
+        using LoopEngine engine = new(size, clock, sink, minFrameInterval: TimeSpan.Zero);
+
+        engine.Post(new DirtyCommand());
+        clock.Advance(TimeSpan.FromMilliseconds(100));
+        using CancellationTokenSource cts = new(TimeSpan.FromSeconds(5));
+        await engine.SettleAsync(cts.Token);
+
+        // After a paint, the fixed region must be non-null (composed with at least 1 input row).
+        Assert.NotNull(sink.LastModel);
+        Assert.NotEmpty(sink.LastModel!.FixedRegionRows);
+    }
+
+    // ── Test infrastructure ───────────────────────────────────────────────────
+
+    /// <summary>
+    /// Captures the last <see cref="RenderModel"/> passed to <see cref="Paint"/>.
+    /// </summary>
+    private sealed class CapturingOutputSink : IOutputSink
+    {
+        internal RenderModel? LastModel { get; private set; }
+
+        public void Paint(RenderModel model)
+        {
+            LastModel = model;
+        }
+
+        public void EmitRestoreSequence() { }
+    }
+
+    private sealed class VirtualClock : IClock
+    {
+        private readonly object _lock = new();
+        private TimeSpan _now;
+        private readonly List<(TimeSpan Deadline, TaskCompletionSource Tcs, CancellationTokenSource LinkedCts)> _waiters = [];
+
+        internal VirtualClock(TimeSpan? initial = null) => _now = initial ?? TimeSpan.Zero;
+
+        public TimeSpan Now { get { lock (_lock) { return _now; } } }
+
+        internal void Advance(TimeSpan by)
+        {
+            List<(TaskCompletionSource Tcs, CancellationTokenSource LinkedCts)> toComplete = [];
+            lock (_lock)
+            {
+                _now += by;
+                for (int i = _waiters.Count - 1; i >= 0; i--)
+                {
+                    if (_waiters[i].Deadline <= _now)
+                    {
+                        toComplete.Add((_waiters[i].Tcs, _waiters[i].LinkedCts));
+                        _waiters.RemoveAt(i);
+                    }
+                }
+            }
+            foreach ((TaskCompletionSource tcs, CancellationTokenSource linkedCts) in toComplete)
+            {
+                tcs.TrySetResult();
+                linkedCts.Dispose();
+            }
+        }
+
+        public Task WaitUntilAsync(TimeSpan deadline, CancellationToken cancellationToken)
+        {
+            lock (_lock)
+            {
+                if (deadline <= _now)
+                    return Task.CompletedTask;
+            }
+            CancellationTokenSource linkedCts = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
+            TaskCompletionSource tcs = new(TaskCreationOptions.RunContinuationsAsynchronously);
+            linkedCts.Token.Register(
+                static state => ((TaskCompletionSource)state!).TrySetCanceled(),
+                tcs,
+                useSynchronizationContext: false);
+            lock (_lock)
+            {
+                if (deadline <= _now) { linkedCts.Dispose(); return Task.CompletedTask; }
+                _waiters.Add((deadline, tcs, linkedCts));
+            }
+            return tcs.Task;
+        }
+    }
+
+    /// <summary>Helper command: appends a line to scrollback.</summary>
+    private sealed class AppendScrollbackCommand(Line line) : ILoopCommand
+    {
+        void ILoopCommand.Apply(RenderModel model)
+        {
+            model.Scrollback.Append(new TextBlock(line), model);
+        }
+    }
+
+    /// <summary>Helper command: just marks the model dirty.</summary>
+    private sealed class DirtyCommand : ILoopCommand
+    {
+        void ILoopCommand.Apply(RenderModel model) => model.MarkDirty();
+    }
+
+    /// <summary>
+    /// Helper command: calls <see cref="TextBuffer.AddToHistory"/> on the editor so routing
+    /// tests can prime the history list without going through submit/Enter (§12).
+    /// </summary>
+    private sealed class AddToHistoryCommand(string entry) : ILoopCommand
+    {
+        void ILoopCommand.Apply(RenderModel model)
+        {
+            model.FixedRegion.Editor.AddToHistory(entry);
+            model.MarkDirty();
+        }
+    }
+
+    /// <summary>
+    /// Helper command: replaces the editor's buffer with <paramref name="text"/> and moves the
+    /// caret to the end, so tests can position the caret on a known visual row.
+    /// </summary>
+    private sealed class SetEditorTextCommand(string text) : ILoopCommand
+    {
+        void ILoopCommand.Apply(RenderModel model)
+        {
+            model.FixedRegion.Editor.SetText(text);
+            model.MarkDirty();
+        }
+    }
+}
diff --git a/tests/Dcli.Tests/InputDialogTests.cs b/tests/Dcli.Tests/InputDialogTests.cs
new file mode 100644
index 0000000..552a55f
--- /dev/null
+++ b/tests/Dcli.Tests/InputDialogTests.cs
@@ -0,0 +1,554 @@
+using System.Text;
+using Dcli.Internal.FixedRegion;
+using Dcli.Internal.RenderLoop;
+using Xunit;
+
+namespace Dcli.Tests;
+
+/// <summary>
+/// Tests for §12 Chunk B: <see cref="InputDialog"/>, <see cref="InputRequest"/>,
+/// <see cref="IModalOverlay"/> generalisation, and seam #1 (cursor in overlay).
+/// Tests drive the machinery at the <see cref="LoopEngine"/> level (no real terminal needed).
+/// </summary>
+public sealed class InputDialogTests
+{
+    // ── Test infrastructure (mirrored from DialogSelectionTests) ──────────────
+
+    private sealed class ConstantSizeSource(int cols, int rows) : ITerminalSizeSource
+    {
+        public (int Columns, int Rows) GetSize() => (cols, rows);
+    }
+
+    private sealed class CapturingOutputSink : IOutputSink
+    {
+        internal RenderModel? LastModel { get; private set; }
+        public void Paint(RenderModel model) { LastModel = model; }
+        public void EmitRestoreSequence() { }
+    }
+
+    private sealed class VirtualClock : IClock
+    {
+        private readonly object _lock = new();
+        private TimeSpan _now;
+        private readonly List<(TimeSpan Deadline, TaskCompletionSource Tcs, CancellationTokenSource LinkedCts)> _waiters = [];
+
+        internal VirtualClock(TimeSpan? initial = null) => _now = initial ?? TimeSpan.Zero;
+
+        public TimeSpan Now { get { lock (_lock) { return _now; } } }
+
+        internal void Advance(TimeSpan by)
+        {
+            List<(TaskCompletionSource Tcs, CancellationTokenSource LinkedCts)> toComplete = [];
+            lock (_lock)
+            {
+                _now += by;
+                for (int i = _waiters.Count - 1; i >= 0; i--)
+                {
+                    if (_waiters[i].Deadline <= _now)
+                    {
+                        toComplete.Add((_waiters[i].Tcs, _waiters[i].LinkedCts));
+                        _waiters.RemoveAt(i);
+                    }
+                }
+            }
+            foreach ((TaskCompletionSource tcs, CancellationTokenSource linkedCts) in toComplete)
+            {
+                tcs.TrySetResult();
+                linkedCts.Dispose();
+            }
+        }
+
+        public Task WaitUntilAsync(TimeSpan deadline, CancellationToken cancellationToken)
+        {
+            lock (_lock)
+            {
+                if (deadline <= _now)
+                    return Task.CompletedTask;
+            }
+            CancellationTokenSource linkedCts = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
+            TaskCompletionSource tcs = new(TaskCreationOptions.RunContinuationsAsynchronously);
+            linkedCts.Token.Register(
+                static state => ((TaskCompletionSource)state!).TrySetCanceled(),
+                tcs,
+                useSynchronizationContext: false);
+            lock (_lock)
+            {
+                if (deadline <= _now) { linkedCts.Dispose(); return Task.CompletedTask; }
+                _waiters.Add((deadline, tcs, linkedCts));
+            }
+            return tcs.Task;
+        }
+    }
+
+    private static (LoopEngine Engine, VirtualClock Clock, CapturingOutputSink Sink) CreateEngine(
+        int cols = 80, int rows = 24)
+    {
+        VirtualClock clock = new(TimeSpan.Zero);
+        CapturingOutputSink sink = new();
+        LoopEngine engine = new(
+            new ConstantSizeSource(cols, rows),
+            clock,
+            sink,
+            minFrameInterval: TimeSpan.Zero);
+        return (engine, clock, sink);
+    }
+
+    private static Line PlainLine(string text) => new([new Segment(text)]);
+
+    private static Task SettleAsync(LoopEngine engine, VirtualClock clock)
+    {
+        clock.Advance(TimeSpan.FromMilliseconds(10));
+        return engine.SettleAsync().WaitAsync(TimeSpan.FromSeconds(5));
+    }
+
+    // ── Helper: open an InputDialog via OpenDialogCommand ────────────────────
+
+    private static Task<DialogResult<string>> PostInputDialog(
+        LoopEngine engine,
+        Line? prompt = null,
+        string? @default = null,
+        bool isSecret = false,
+        CancellationToken ct = default)
+    {
+        InputDialog dialog = new(prompt, @default, isSecret);
+
+        TaskCompletionSource<DialogResult<string>> tcs =
+            new(TaskCreationOptions.RunContinuationsAsynchronously);
+
+        CancellationTokenRegistration[] regHolder = new CancellationTokenRegistration[1];
+
+        Action completion = () =>
+        {
+            DialogResult<string> result = dialog.CloseRequest == OverlayCloseKind.Submit
+                ? new DialogResult<string>(DialogOutcome.Submitted, dialog.Text)
+                : new DialogResult<string>(DialogOutcome.Cancelled, default!);
+            tcs.TrySetResult(result);
+            regHolder[0].Dispose();
+        };
+
+        Action reject = () =>
+        {
+            tcs.TrySetException(new InvalidOperationException("A dialog is already active."));
+            regHolder[0].Dispose();
+        };
+
+        if (ct.CanBeCanceled)
+        {
+            Action cancelCompletion = () =>
+            {
+                tcs.TrySetResult(new DialogResult<string>(DialogOutcome.Cancelled, default!));
+                regHolder[0].Dispose();
+            };
+            regHolder[0] = ct.Register(
+                () => engine.Post(new CancelDialogCommand(dialog, cancelCompletion)),
+                useSynchronizationContext: false);
+        }
+
+        engine.Post(new OpenDialogCommand(dialog, completion, reject));
+        return tcs.Task;
+    }
+
+    // ── Type text + Enter → Submitted ────────────────────────────────────────
+
+    [Fact]
+    public async Task TypeHelloThenEnterReturnsSubmittedHello()
+    {
+        (LoopEngine engine, VirtualClock clock, _) = CreateEngine();
+        try
+        {
+            Task<DialogResult<string>> task = PostInputDialog(engine);
+
+            engine.InputWriter.TryWrite(new KeyEvent(KeyCode.FromRune(new Rune('h')), Modifiers.None));
+            engine.InputWriter.TryWrite(new KeyEvent(KeyCode.FromRune(new Rune('e')), Modifiers.None));
+            engine.InputWriter.TryWrite(new KeyEvent(KeyCode.FromRune(new Rune('l')), Modifiers.None));
+            engine.InputWriter.TryWrite(new KeyEvent(KeyCode.FromRune(new Rune('l')), Modifiers.None));
+            engine.InputWriter.TryWrite(new KeyEvent(KeyCode.FromRune(new Rune('o')), Modifiers.None));
+            engine.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Enter), Modifiers.None));
+
+            await SettleAsync(engine, clock);
+            DialogResult<string> result = await task;
+
+            Assert.Equal(DialogOutcome.Submitted, result.Outcome);
+            Assert.Equal("hello", result.Value);
+        }
+        finally { engine.Dispose(); }
+    }
+
+    // ── Seeded Default + Enter → Submitted with default text ─────────────────
+
+    [Fact]
+    public async Task SeededDefaultPlusEnterReturnsSubmittedDefault()
+    {
+        (LoopEngine engine, VirtualClock clock, _) = CreateEngine();
+        try
+        {
+            Task<DialogResult<string>> task = PostInputDialog(engine, @default: "preset");
+            engine.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Enter), Modifiers.None));
+
+            await SettleAsync(engine, clock);
+            DialogResult<string> result = await task;
+
+            Assert.Equal(DialogOutcome.Submitted, result.Outcome);
+            Assert.Equal("preset", result.Value);
+        }
+        finally { engine.Dispose(); }
+    }
+
+    // ── IsSecret: rendered rows masked, result is real text ──────────────────
+
+    [Fact]
+    public async Task IsSecretMasksRenderedRowsButResultIsRealText()
+    {
+        (LoopEngine engine, VirtualClock clock, CapturingOutputSink sink) = CreateEngine();
+        try
+        {
+            Task<DialogResult<string>> task = PostInputDialog(engine, isSecret: true);
+
+            engine.InputWriter.TryWrite(new KeyEvent(KeyCode.FromRune(new Rune('p')), Modifiers.None));
+            engine.InputWriter.TryWrite(new KeyEvent(KeyCode.FromRune(new Rune('w')), Modifiers.None));
+
+            // Settle so the frame is painted and the sink captures a model.
+            await SettleAsync(engine, clock);
+
+            // The rendered overlay rows must contain the mask glyph, not the plaintext.
+            RenderModel? model = sink.LastModel;
+            Assert.NotNull(model);
+            IReadOnlyList<Line> fixedRows = model.FixedRegionRows;
+            string renderedText = string.Concat(fixedRows.SelectMany(r => r.Segments.Select(s => s.Text)));
+            Assert.Contains("•", renderedText, StringComparison.Ordinal);
+            Assert.DoesNotContain("pw", renderedText, StringComparison.Ordinal);
+
+            // Submit and verify the real text comes through.
+            engine.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Enter), Modifiers.None));
+            await SettleAsync(engine, clock);
+            DialogResult<string> result = await task;
+
+            Assert.Equal(DialogOutcome.Submitted, result.Outcome);
+            Assert.Equal("pw", result.Value);
+        }
+        finally { engine.Dispose(); }
+    }
+
+    // ── Escape → Cancelled; overlay cleared ──────────────────────────────────
+
+    [Fact]
+    public async Task EscapeReturnsCancelledAndOverlayCleared()
+    {
+        (LoopEngine engine, VirtualClock clock, CapturingOutputSink sink) = CreateEngine();
+        try
+        {
+            Task<DialogResult<string>> task = PostInputDialog(engine);
+            engine.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Escape), Modifiers.None));
+
+            await SettleAsync(engine, clock);
+            DialogResult<string> result = await task;
+
+            Assert.Equal(DialogOutcome.Cancelled, result.Outcome);
+            Assert.Null(sink.LastModel?.ActiveOverlay);
+        }
+        finally { engine.Dispose(); }
+    }
+
+    // ── CancellationToken cancel → Cancelled; overlay cleared ─────────────────
+
+    [Fact]
+    public async Task CancellationTokenCancelReturnsCancelledAndOverlayCleared()
+    {
+        (LoopEngine engine, VirtualClock clock, CapturingOutputSink sink) = CreateEngine();
+        try
+        {
+            using CancellationTokenSource cts = new();
+
+            Task<DialogResult<string>> task = PostInputDialog(engine, ct: cts.Token);
+
+            // Let the dialog open.
+            await SettleAsync(engine, clock);
+            Assert.IsType<InputDialog>(sink.LastModel?.ActiveOverlay);
+
+            // Cancel the token.
+            await cts.CancelAsync();
+            await SettleAsync(engine, clock);
+
+            DialogResult<string> result = await task;
+
+            Assert.Equal(DialogOutcome.Cancelled, result.Outcome);
+            Assert.Null(sink.LastModel?.ActiveOverlay);
+        }
+        finally { engine.Dispose(); }
+    }
+
+    // ── Backspace edits the buffer ────────────────────────────────────────────
+
+    [Fact]
+    public async Task BackspaceDeletesLastCharacter()
+    {
+        (LoopEngine engine, VirtualClock clock, _) = CreateEngine();
+        try
+        {
+            Task<DialogResult<string>> task = PostInputDialog(engine);
+
+            // Type "abc", Backspace → buffer should be "ab"
+            engine.InputWriter.TryWrite(new KeyEvent(KeyCode.FromRune(new Rune('a')), Modifiers.None));
+            engine.InputWriter.TryWrite(new KeyEvent(KeyCode.FromRune(new Rune('b')), Modifiers.None));
+            engine.InputWriter.TryWrite(new KeyEvent(KeyCode.FromRune(new Rune('c')), Modifiers.None));
+            engine.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Backspace), Modifiers.None));
+            engine.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Enter), Modifiers.None));
+
+            await SettleAsync(engine, clock);
+            DialogResult<string> result = await task;
+
+            Assert.Equal(DialogOutcome.Submitted, result.Outcome);
+            Assert.Equal("ab", result.Value);
+        }
+        finally { engine.Dispose(); }
+    }
+
+    // ── Seam #1: cursor visible at overlay caret; not hidden ─────────────────
+
+    [Fact]
+    public async Task InputDialogCursorIsVisibleInOverlayRegion()
+    {
+        (LoopEngine engine, VirtualClock clock, CapturingOutputSink sink) = CreateEngine(cols: 80, rows: 24);
+        try
+        {
+            Task<DialogResult<string>> task = PostInputDialog(engine);
+
+            // Settle so a frame paints.
+            await SettleAsync(engine, clock);
+
+            RenderModel? model = sink.LastModel;
+            Assert.NotNull(model);
+
+            // The cursor must be visible — InputDialog owns it, not the main editor.
+            Assert.True(model.IsCursorVisible, "Expected cursor visible for InputDialog");
+
+            // The caret must be in the overlay region (above the input rows).
+            // With no text, the overlay occupies the topmost rows of the fixed region,
+            // so CaretPosition.Row should be 0 (first row).
+            Assert.NotNull(model.CaretPosition);
+            Assert.Equal(0, model.CaretPosition!.Value.Row);
+
+            // Clean up.
+            engine.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Escape), Modifiers.None));
+            await SettleAsync(engine, clock);
+            await task;
+        }
+        finally { engine.Dispose(); }
+    }
+
+    // ── Seam #1 contrast: select Dialog hides cursor ─────────────────────────
+
+    [Fact]
+    public async Task SelectDialogHidesCursorContrastWithInputDialog()
+    {
+        (LoopEngine engine, VirtualClock clock, CapturingOutputSink sink) = CreateEngine();
+        try
+        {
+            // Open a select dialog (HidesCursor=true) and verify cursor is hidden.
+            Dialog selectDialog = new(multiSelect: false, modal: true);
+            selectDialog.List.SetItems([PlainLine("A"), PlainLine("B")]);
+
+            TaskCompletionSource<int> tcs = new(TaskCreationOptions.RunContinuationsAsynchronously);
+            engine.Post(new OpenDialogCommand(
+                selectDialog,
+                () => tcs.TrySetResult(selectDialog.CloseRequest == OverlayCloseKind.Submit ? 1 : 0),
+                () => tcs.TrySetResult(-1)));
+
+            await SettleAsync(engine, clock);
+
+            RenderModel? model = sink.LastModel;
+            Assert.NotNull(model);
+            Assert.False(model.IsCursorVisible, "Select dialog must hide the cursor");
+
+            // Dismiss.
+            engine.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Escape), Modifiers.None));
+            await SettleAsync(engine, clock);
+            await tcs.Task;
+        }
+        finally { engine.Dispose(); }
+    }
+
+    // ── Reject: InputDialog while select Dialog active ────────────────────────
+
+    [Fact]
+    public async Task OpenInputDialogWhileSelectDialogActiveThrowsInvalidOperation()
+    {
+        (LoopEngine engine, VirtualClock clock, CapturingOutputSink sink) = CreateEngine();
+        try
+        {
+            // Open a select dialog first.
+            Dialog selectDialog = new(multiSelect: false, modal: true);
+            selectDialog.List.SetItems([PlainLine("A")]);
+
+            TaskCompletionSource<int> firstTcs = new(TaskCreationOptions.RunContinuationsAsynchronously);
+            engine.Post(new OpenDialogCommand(
+                selectDialog,
+                () => firstTcs.TrySetResult(0),
+                () => firstTcs.TrySetException(new InvalidOperationException("rejected"))));
+
+            await SettleAsync(engine, clock);
+            Assert.IsType<Dialog>(sink.LastModel?.ActiveOverlay);
+
+            // Now try to open an InputDialog — must fault.
+            Task<DialogResult<string>> inputTask = PostInputDialog(engine);
+            await SettleAsync(engine, clock);
+
+            InvalidOperationException ex =
+                await Assert.ThrowsAsync<InvalidOperationException>(() => inputTask);
+            Assert.Contains("already active", ex.Message, StringComparison.OrdinalIgnoreCase);
+
+            // The first overlay must still be active.
+            Assert.IsType<Dialog>(sink.LastModel?.ActiveOverlay);
+
+            // Clean up.
+            engine.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Escape), Modifiers.None));
+            await SettleAsync(engine, clock);
+            await firstTcs.Task;
+        }
+        finally { engine.Dispose(); }
+    }
+
+    // ── Reject: select Dialog while InputDialog active ────────────────────────
+
+    [Fact]
+    public async Task OpenSelectDialogWhileInputDialogActiveThrowsInvalidOperation()
+    {
+        (LoopEngine engine, VirtualClock clock, CapturingOutputSink sink) = CreateEngine();
+        try
+        {
+            Task<DialogResult<string>> inputTask = PostInputDialog(engine);
+            await SettleAsync(engine, clock);
+            Assert.IsType<InputDialog>(sink.LastModel?.ActiveOverlay);
+
+            // Try to open a select Dialog — must fault.
+            Dialog selectDialog = new(multiSelect: false, modal: true);
+            selectDialog.List.SetItems([PlainLine("X")]);
+
+            TaskCompletionSource<int> secondTcs = new(TaskCreationOptions.RunContinuationsAsynchronously);
+            engine.Post(new OpenDialogCommand(
+                selectDialog,
+                () => secondTcs.TrySetResult(0),
+                () => secondTcs.TrySetException(new InvalidOperationException("A dialog is already active."))));
+
+            await SettleAsync(engine, clock);
+
+            InvalidOperationException ex =
+                await Assert.ThrowsAsync<InvalidOperationException>(() => secondTcs.Task);
+            Assert.Contains("already active", ex.Message, StringComparison.OrdinalIgnoreCase);
+
+            // The InputDialog must still be active.
+            Assert.IsType<InputDialog>(sink.LastModel?.ActiveOverlay);
+
+            // Clean up.
+            engine.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Escape), Modifiers.None));
+            await SettleAsync(engine, clock);
+            DialogResult<string> result = await inputTask;
+            Assert.Equal(DialogOutcome.Cancelled, result.Outcome);
+        }
+        finally { engine.Dispose(); }
+    }
+
+    // ── InputDialog render: prompt row is prepended ───────────────────────────
+
+    [Fact]
+    public void InputDialogRenderPrependsPromptRow()
+    {
+        Line prompt = PlainLine("Enter value:");
+        InputDialog dialog = new(prompt, @default: null, isSecret: false);
+        dialog.MaxRows = 10;
+
+        IReadOnlyList<Line> rows = dialog.Render(80);
+
+        Assert.True(rows.Count >= 1);
+        Assert.Same(prompt, rows[0]);
+    }
+
+    // ── InputDialog: MaxRows limits total output ──────────────────────────────
+
+    [Fact]
+    public void InputDialogRenderNeverExceedsMaxRows()
+    {
+        Line prompt = PlainLine("Prompt:");
+        InputDialog dialog = new(prompt, @default: null, isSecret: false);
+        dialog.MaxRows = 2;
+
+        IReadOnlyList<Line> rows = dialog.Render(80);
+
+        Assert.True(rows.Count <= 2, $"Rows {rows.Count} exceeded MaxRows 2");
+    }
+
+    // ── InputDialog: CaretInOverlay null before first render ──────────────────
+
+    [Fact]
+    public void InputDialogCaretInOverlayNullBeforeFirstRender()
+    {
+        InputDialog dialog = new(prompt: null, @default: null, isSecret: false);
+        Assert.Null(dialog.CaretInOverlay);
+    }
+
+    // ── InputDialog: CaretInOverlay set after render; offset by prompt row ────
+
+    [Fact]
+    public void InputDialogCaretInOverlayOffsetByPromptRow()
+    {
+        Line prompt = PlainLine("Prompt:");
+        InputDialog dialog = new(prompt, @default: null, isSecret: false);
+        dialog.MaxRows = 10;
+
+        dialog.Render(80);
+
+        (int Row, int Col)? caret = dialog.CaretInOverlay;
+        Assert.NotNull(caret);
+        // With a prompt row, caret must be on row 1 (or higher), not row 0 (which is the prompt).
+        Assert.True(caret!.Value.Row >= 1, $"Expected caret row ≥ 1 with prompt, got {caret.Value.Row}");
+    }
+
+    // ── Ctrl+C is NOT inserted as text (modifier gate) ───────────────────────
+
+    [Fact]
+    public async Task CtrlCIsNotInsertedAsTextAndIsConsumedByModalCatchAll()
+    {
+        (LoopEngine engine, VirtualClock clock, _) = CreateEngine();
+        try
+        {
+            Task<DialogResult<string>> task = PostInputDialog(engine);
+
+            // Type 'a', then Ctrl+C (which must NOT insert 'c'), then Enter.
+            engine.InputWriter.TryWrite(new KeyEvent(KeyCode.FromRune(new Rune('a')), Modifiers.None));
+            engine.InputWriter.TryWrite(new KeyEvent(KeyCode.FromRune(new Rune('c')), Modifiers.Ctrl));
+            engine.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Enter), Modifiers.None));
+
+            await SettleAsync(engine, clock);
+            DialogResult<string> result = await task;
+
+            // Result must be "a", not "ac" — the Ctrl+C was consumed silently.
+            Assert.Equal(DialogOutcome.Submitted, result.Outcome);
+            Assert.Equal("a", result.Value);
+        }
+        finally { engine.Dispose(); }
+    }
+
+    // ── InputDialog: IsDismissed transitions ─────────────────────────────────
+
+    [Fact]
+    public void InputDialogIsDismissedAfterEnter()
+    {
+        InputDialog dialog = new(prompt: null, @default: null, isSecret: false);
+        Assert.False(dialog.IsDismissed);
+
+        dialog.HandleKey(new KeyEvent(KeyCode.Named(NamedKey.Enter), Modifiers.None));
+
+        Assert.True(dialog.IsDismissed);
+        Assert.Equal(OverlayCloseKind.Submit, dialog.CloseRequest);
+    }
+
+    [Fact]
+    public void InputDialogIsDismissedAfterEscape()
+    {
+        InputDialog dialog = new(prompt: null, @default: null, isSecret: false);
+        dialog.HandleKey(new KeyEvent(KeyCode.Named(NamedKey.Escape), Modifiers.None));
+
+        Assert.True(dialog.IsDismissed);
+        Assert.Equal(OverlayCloseKind.Cancel, dialog.CloseRequest);
+    }
+}
diff --git a/tests/Dcli.Tests/Internal/TerminalCapabilitiesTests.cs b/tests/Dcli.Tests/Internal/TerminalCapabilitiesTests.cs
new file mode 100644
index 0000000..631c916
--- /dev/null
+++ b/tests/Dcli.Tests/Internal/TerminalCapabilitiesTests.cs
@@ -0,0 +1,243 @@
+using Dcli.Internal;
+using Xunit;
+
+namespace Dcli.Tests.Internal;
+
+/// <summary>
+/// §13.4 (Chunk B) — Capability detection tests.
+/// All cases inject <see cref="CapabilityInputs"/> directly so no real environment
+/// variables are read.
+/// </summary>
+public sealed class TerminalCapabilitiesTests
+{
+    // ── Shared VT-capable base inputs ─────────────────────────────────────────
+    private static CapabilityInputs Base(
+        string? colorTerm = null,
+        string? termProgram = null,
+        string? term = "xterm-256color",
+        string? lang = null,
+        bool isWindows = false) =>
+        new CapabilityInputs
+        {
+            TermVariable = term,
+            IsStdinTty = true,
+            IsStdoutTty = true,
+            IsSupportedPlatform = true,
+            ColorTermVariable = colorTerm,
+            TermProgramVariable = termProgram,
+            LangVariable = lang,
+            IsWindows = isWindows,
+        };
+
+    // ── 13.4-cap-a  Truecolor detection ──────────────────────────────────────
+
+    [Fact]
+    public void TruecolorDetectedWhenColortermIsTruecolor()
+    {
+        TerminalCapabilities caps = TerminalCapabilityDetector.DetectCapabilities(Base(colorTerm: "truecolor"));
+        Assert.True(caps.HasTruecolor);
+    }
+
+    [Fact]
+    public void TruecolorDetectedWhenColortermIsTruecolorUpperCase()
+    {
+        TerminalCapabilities caps = TerminalCapabilityDetector.DetectCapabilities(Base(colorTerm: "TRUECOLOR"));
+        Assert.True(caps.HasTruecolor);
+    }
+
+    [Fact]
+    public void TruecolorDetectedWhenColortermIs24bit()
+    {
+        TerminalCapabilities caps = TerminalCapabilityDetector.DetectCapabilities(Base(colorTerm: "24bit"));
+        Assert.True(caps.HasTruecolor);
+    }
+
+    [Fact]
+    public void TruecolorDetectedWhenColortermIs24bitUpperCase()
+    {
+        TerminalCapabilities caps = TerminalCapabilityDetector.DetectCapabilities(Base(colorTerm: "24BIT"));
+        Assert.True(caps.HasTruecolor);
+    }
+
+    [Fact]
+    public void TruecolorNotDetectedWhenColortermIsUnset()
+    {
+        TerminalCapabilities caps = TerminalCapabilityDetector.DetectCapabilities(Base(colorTerm: null));
+        Assert.False(caps.HasTruecolor);
+    }
+
+    [Fact]
+    public void TruecolorNotDetectedWhenColortermIs256color()
+    {
+        // COLORTERM=256color is sometimes set but does NOT indicate truecolor.
+        TerminalCapabilities caps = TerminalCapabilityDetector.DetectCapabilities(Base(colorTerm: "256color"));
+        Assert.False(caps.HasTruecolor);
+    }
+
+    // ── 13.4-cap-b  Synchronized-output detection ────────────────────────────
+
+    [Fact]
+    public void SyncOutputDetectedWhenTermProgramIsWezTerm()
+    {
+        TerminalCapabilities caps = TerminalCapabilityDetector.DetectCapabilities(Base(termProgram: "WezTerm"));
+        Assert.True(caps.HasSynchronizedOutput);
+    }
+
+    [Fact]
+    public void SyncOutputDetectedWhenTermProgramIsWezTermLowerCase()
+    {
+        TerminalCapabilities caps = TerminalCapabilityDetector.DetectCapabilities(Base(termProgram: "wezterm"));
+        Assert.True(caps.HasSynchronizedOutput);
+    }
+
+    [Fact]
+    public void SyncOutputDetectedWhenTermProgramIsITerm()
+    {
+        TerminalCapabilities caps = TerminalCapabilityDetector.DetectCapabilities(Base(termProgram: "iTerm.app"));
+        Assert.True(caps.HasSynchronizedOutput);
+    }
+
+    [Fact]
+    public void SyncOutputDetectedWhenTermProgramIsVscode()
+    {
+        TerminalCapabilities caps = TerminalCapabilityDetector.DetectCapabilities(Base(termProgram: "vscode"));
+        Assert.True(caps.HasSynchronizedOutput);
+    }
+
+    [Fact]
+    public void SyncOutputDetectedWhenTermProgramIsWarpTerminal()
+    {
+        TerminalCapabilities caps = TerminalCapabilityDetector.DetectCapabilities(Base(termProgram: "WarpTerminal"));
+        Assert.True(caps.HasSynchronizedOutput);
+    }
+
+    [Fact]
+    public void SyncOutputDetectedWhenTermProgramIsGhostty()
+    {
+        TerminalCapabilities caps = TerminalCapabilityDetector.DetectCapabilities(Base(termProgram: "ghostty"));
+        Assert.True(caps.HasSynchronizedOutput);
+    }
+
+    [Fact]
+    public void SyncOutputDetectedWhenTermProgramIsKitty()
+    {
+        TerminalCapabilities caps = TerminalCapabilityDetector.DetectCapabilities(Base(termProgram: "kitty"));
+        Assert.True(caps.HasSynchronizedOutput);
+    }
+
+    [Fact]
+    public void SyncOutputDetectedWhenTermProgramIsFoot()
+    {
+        TerminalCapabilities caps = TerminalCapabilityDetector.DetectCapabilities(Base(termProgram: "foot"));
+        Assert.True(caps.HasSynchronizedOutput);
+    }
+
+    [Fact]
+    public void SyncOutputDetectedWhenTermIsXtermKitty()
+    {
+        TerminalCapabilities caps = TerminalCapabilityDetector.DetectCapabilities(Base(term: "xterm-kitty"));
+        Assert.True(caps.HasSynchronizedOutput);
+    }
+
+    [Fact]
+    public void SyncOutputDetectedWhenTermStartsWithAlacrittyDirect()
+    {
+        TerminalCapabilities caps = TerminalCapabilityDetector.DetectCapabilities(Base(term: "alacritty-direct"));
+        Assert.True(caps.HasSynchronizedOutput);
+    }
+
+    [Fact]
+    public void SyncOutputDetectedWhenTermIsAlacrittyExact()
+    {
+        TerminalCapabilities caps = TerminalCapabilityDetector.DetectCapabilities(Base(term: "alacritty"));
+        Assert.True(caps.HasSynchronizedOutput);
+    }
+
+    [Fact]
+    public void SyncOutputDetectedWhenIsWindows()
+    {
+        TerminalCapabilities caps = TerminalCapabilityDetector.DetectCapabilities(
+            Base(termProgram: null, term: "xterm-256color", isWindows: true));
+        Assert.True(caps.HasSynchronizedOutput);
+    }
+
+    [Fact]
+    public void SyncOutputNotDetectedWhenTerminalIsUnknown()
+    {
+        TerminalCapabilities caps = TerminalCapabilityDetector.DetectCapabilities(
+            Base(termProgram: null, term: "xterm-256color", isWindows: false));
+        Assert.False(caps.HasSynchronizedOutput);
+    }
+
+    [Fact]
+    public void SyncOutputNotDetectedWhenNothingIsSet()
+    {
+        TerminalCapabilities caps = TerminalCapabilityDetector.DetectCapabilities(
+            Base(termProgram: null, term: null, isWindows: false));
+        Assert.False(caps.HasSynchronizedOutput);
+    }
+
+    // ── 13.4-cap-c  East-Asian ambiguous-wide detection ──────────────────────
+
+    [Fact]
+    public void AmbiguousWideDetectedWhenLangIsJapanese()
+    {
+        TerminalCapabilities caps = TerminalCapabilityDetector.DetectCapabilities(Base(lang: "ja_JP.UTF-8"));
+        Assert.True(caps.TreatAmbiguousAsWide);
+    }
+
+    [Fact]
+    public void AmbiguousWideDetectedWhenLangIsChinese()
+    {
+        TerminalCapabilities caps = TerminalCapabilityDetector.DetectCapabilities(Base(lang: "zh_CN.UTF-8"));
+        Assert.True(caps.TreatAmbiguousAsWide);
+    }
+
+    [Fact]
+    public void AmbiguousWideDetectedWhenLangIsKorean()
+    {
+        TerminalCapabilities caps = TerminalCapabilityDetector.DetectCapabilities(Base(lang: "ko_KR.UTF-8"));
+        Assert.True(caps.TreatAmbiguousAsWide);
+    }
+
+    [Fact]
+    public void AmbiguousWideNotDetectedWhenLangIsEnglish()
+    {
+        TerminalCapabilities caps = TerminalCapabilityDetector.DetectCapabilities(Base(lang: "en_US.UTF-8"));
+        Assert.False(caps.TreatAmbiguousAsWide);
+    }
+
+    [Fact]
+    public void AmbiguousWideDetectedWhenLangIsNullAndLcAllIsJapanese()
+    {
+        // LangVariable carries the LC_ALL fallback when LANG is null.
+        CapabilityInputs inputs = new CapabilityInputs
+        {
+            TermVariable = "xterm-256color",
+            IsStdinTty = true,
+            IsStdoutTty = true,
+            IsSupportedPlatform = true,
+            LangVariable = "ja_JP.UTF-8",
+        };
+        TerminalCapabilities caps = TerminalCapabilityDetector.DetectCapabilities(inputs);
+        Assert.True(caps.TreatAmbiguousAsWide);
+    }
+
+    [Fact]
+    public void AmbiguousWideNotDetectedWhenBothLangAndLcAllAreNull()
+    {
+        TerminalCapabilities caps = TerminalCapabilityDetector.DetectCapabilities(Base(lang: null));
+        Assert.False(caps.TreatAmbiguousAsWide);
+    }
+
+    // ── Default value ─────────────────────────────────────────────────────────
+
+    [Fact]
+    public void DefaultCapabilitiesHaveAllFlagsFalse()
+    {
+        TerminalCapabilities def = TerminalCapabilities.Default;
+        Assert.False(def.HasTruecolor);
+        Assert.False(def.HasSynchronizedOutput);
+        Assert.False(def.TreatAmbiguousAsWide);
+    }
+}
diff --git a/tests/Dcli.Tests/LineWrapperTests.cs b/tests/Dcli.Tests/LineWrapperTests.cs
new file mode 100644
index 0000000..2efb103
--- /dev/null
+++ b/tests/Dcli.Tests/LineWrapperTests.cs
@@ -0,0 +1,321 @@
+using Xunit;
+
+namespace Dcli.Tests;
+
+/// <summary>
+/// Behavioural tests for Section 3.2/3.3: width-aware wrapping of a <see cref="Line"/> into
+/// visual rows.  Every test asserts a specific spec scenario from the Section-3 brief.
+/// </summary>
+public sealed class LineWrapperTests
+{
+    private static Style Bold => new(Format: Format.Bold);
+    private static Style Italic => new(Format: Format.Italic);
+
+    // -----------------------------------------------------------------------------------------
+    // Edge cases
+    // -----------------------------------------------------------------------------------------
+
+    [Fact]
+    public void EmptyLineProducesExactlyOneEmptyRow()
+    {
+        Line empty = new(Array.Empty<Segment>());
+        IReadOnlyList<Line> rows = LineWrapper.Wrap(empty, 80);
+        Assert.Single(rows);
+        Assert.Empty(rows[0].Segments);
+    }
+
+    [Fact]
+    public void LineWithOnlyEmptySegmentsProducesOneEmptyRow()
+    {
+        Line line = new(new[] { new Segment(""), new Segment("") });
+        IReadOnlyList<Line> rows = LineWrapper.Wrap(line, 80);
+        Assert.Single(rows);
+    }
+
+    [Fact]
+    public void ZeroOrNegativeWidthDoesNotLoopOrThrow()
+    {
+        // Width <= 0 is clamped to 1 internally; must terminate and return at least one row.
+        Line line = new LineBuilder().Text("abc").Build();
+        IReadOnlyList<Line> rows0 = LineWrapper.Wrap(line, 0);
+        IReadOnlyList<Line> rowsNeg = LineWrapper.Wrap(line, -5);
+        Assert.NotEmpty(rows0);
+        Assert.NotEmpty(rowsNeg);
+    }
+
+    [Fact]
+    public void SingleCharWiderThanWidthIsEmittedOnItsOwnRow()
+    {
+        // Width = 1, single wide (2-col) emoji: must not be dropped.
+        Line line = new LineBuilder().Text("\U0001F600").Build();
+        IReadOnlyList<Line> rows = LineWrapper.Wrap(line, 1);
+        int totalWidth = rows.Sum(r => r.Segments.Sum(s => DisplayWidth.Measure(s.Text)));
+        Assert.True(totalWidth > 0, "Wide char must not be dropped");
+        // Must appear in exactly one row
+        Assert.Single(rows, r => r.Segments.Any(s => s.Text.Contains("\U0001F600", StringComparison.Ordinal)));
+    }
+
+    [Fact]
+    public void OverWideEmojiAloneProducesExactlyOneRow()
+    {
+        // B1 regression: a single wide char wider than width must NOT produce a phantom empty
+        // leading row.  The only output should be the one row containing the emoji.
+        Line line = new LineBuilder().Text("\U0001F600").Build();
+        IReadOnlyList<Line> rows = LineWrapper.Wrap(line, 1);
+        Assert.Single(rows);
+    }
+
+    [Fact]
+    public void OverWideCjkAloneProducesExactlyOneRow()
+    {
+        // B1 regression: same defect for a CJK ideograph (2-col) at width 1.
+        Line line = new LineBuilder().Text("中").Build();
+        IReadOnlyList<Line> rows = LineWrapper.Wrap(line, 1);
+        Assert.Single(rows);
+    }
+
+    [Fact]
+    public void OverWideCharMidStringHasNoEmptyInterleaved()
+    {
+        // B1 regression: "A中B" at width 1 → ["A", "中", "B"] — no zero-content rows.
+        Line line = new LineBuilder().Text("A中B").Build();
+        IReadOnlyList<Line> rows = LineWrapper.Wrap(line, 1);
+        foreach (Line row in rows)
+        {
+            int rowWidth = row.Segments.Sum(s => DisplayWidth.Measure(s.Text));
+            Assert.True(rowWidth > 0, "No empty (zero-width) rows should be interleaved");
+        }
+    }
+
+    // -----------------------------------------------------------------------------------------
+    // Basic wrapping
+    // -----------------------------------------------------------------------------------------
+
+    [Fact]
+    public void ShortLineDoesNotWrap()
+    {
+        Line line = new LineBuilder().Text("hello").Build();
+        IReadOnlyList<Line> rows = LineWrapper.Wrap(line, 80);
+        Assert.Single(rows);
+        Assert.Equal("hello", rows[0].Segments[0].Text);
+    }
+
+    [Fact]
+    public void LineExactlyWidthDoesNotWrap()
+    {
+        // "abcde" = 5 columns, width = 5 → one row.
+        Line line = new LineBuilder().Text("abcde").Build();
+        IReadOnlyList<Line> rows = LineWrapper.Wrap(line, 5);
+        Assert.Single(rows);
+    }
+
+    [Fact]
+    public void LineOneCharOverWidthWrapsTwoRows()
+    {
+        // "abcdef" = 6 cols, width = 5 → two rows: "abcde" + "f".
+        Line line = new LineBuilder().Text("abcdef").Build();
+        IReadOnlyList<Line> rows = LineWrapper.Wrap(line, 5);
+        Assert.Equal(2, rows.Count);
+        Assert.Equal("abcde", rows[0].Segments[0].Text);
+        Assert.Equal("f", rows[1].Segments[0].Text);
+    }
+
+    [Fact]
+    public void LongLineWrapsCorrectNumberOfRows()
+    {
+        // 12 ASCII chars, width 4 → 3 rows.
+        Line line = new LineBuilder().Text("abcdefghijkl").Build();
+        IReadOnlyList<Line> rows = LineWrapper.Wrap(line, 4);
+        Assert.Equal(3, rows.Count);
+    }
+
+    // -----------------------------------------------------------------------------------------
+    // Wide char handling
+    // -----------------------------------------------------------------------------------------
+
+    [Fact]
+    public void WideCharDoesNotStraddle()
+    {
+        // Width = 3, text "AB中D": A(1)+B(1)=2, 中(2) would push to col 4 > 3 → row break.
+        // Row 0: "AB", Row 1: "中D"
+        Line line = new LineBuilder().Text("AB中D").Build();
+        IReadOnlyList<Line> rows = LineWrapper.Wrap(line, 3);
+
+        string row0text = string.Concat(rows[0].Segments.Select(s => s.Text));
+        string row1text = string.Concat(rows[1].Segments.Select(s => s.Text));
+
+        Assert.Equal("AB", row0text);
+        Assert.Contains("中", row1text, StringComparison.Ordinal);
+        Assert.Contains("D", row1text, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void WideCharsWrapAtExactlyWidth()
+    {
+        // Width = 4, "中文" = 2+2 = 4 → one row.
+        Line line = new LineBuilder().Text("中文").Build();
+        IReadOnlyList<Line> rows = LineWrapper.Wrap(line, 4);
+        Assert.Single(rows);
+    }
+
+    [Fact]
+    public void WideCharsWrapWhenExceedingWidth()
+    {
+        // Width = 3, "中文A": 中(2)+文(2)=4>3 → row break after 中.
+        // Row 0: "中", Row 1: "文A"
+        Line line = new LineBuilder().Text("中文A").Build();
+        IReadOnlyList<Line> rows = LineWrapper.Wrap(line, 3);
+        Assert.Equal(2, rows.Count);
+        string row0 = string.Concat(rows[0].Segments.Select(s => s.Text));
+        string row1 = string.Concat(rows[1].Segments.Select(s => s.Text));
+        Assert.Equal("中", row0);
+        Assert.Equal("文A", row1);
+    }
+
+    [Fact]
+    public void WideCharAtWidth1IsEmittedOwnRow()
+    {
+        // Width = 1, "A中B": A fits (1=1), 中(2)>1 → break; 中 alone on next row even though >width.
+        Line line = new LineBuilder().Text("A中B").Build();
+        IReadOnlyList<Line> rows = LineWrapper.Wrap(line, 1);
+
+        // All chars must appear (none dropped)
+        string all = string.Concat(rows.SelectMany(r => r.Segments).Select(s => s.Text));
+        Assert.Contains("A", all, StringComparison.Ordinal);
+        Assert.Contains("中", all, StringComparison.Ordinal);
+        Assert.Contains("B", all, StringComparison.Ordinal);
+    }
+
+    // -----------------------------------------------------------------------------------------
+    // Style preservation
+    // -----------------------------------------------------------------------------------------
+
+    [Fact]
+    public void SplitSegmentPreservesStyleOnBothHalves()
+    {
+        // Single bold segment "ABCDE" split at width 3 → "ABC" bold + "DE" bold.
+        Line line = new(new[] { new Segment("ABCDE", Bold) });
+        IReadOnlyList<Line> rows = LineWrapper.Wrap(line, 3);
+
+        Assert.Equal(2, rows.Count);
+        Assert.All(rows[0].Segments, s => Assert.Equal(Bold, s.Style));
+        Assert.All(rows[1].Segments, s => Assert.Equal(Bold, s.Style));
+    }
+
+    [Fact]
+    public void MultiStyleSegmentsPreserveTheirStyles()
+    {
+        // Row 0: "AB" (bold), Row 1: "C" (bold) + "DE" (italic) + "F" (italic)
+        // width=3, "AB"(bold) "CDEF"(italic): AB=2<3, C(1)→AB+C=3=row full, new row "DEF"
+        // Actually: "AB"(bold)=2, then C of "CDEF"(italic): col goes to 3 exactly, row ends.
+        // Row 0: "AB", "C"; Row 1: "DEF"
+        Segment boldSeg = new("AB", Bold);
+        Segment italicSeg = new("CDEF", Italic);
+        Line line = new(new[] { boldSeg, italicSeg });
+        IReadOnlyList<Line> rows = LineWrapper.Wrap(line, 3);
+
+        Assert.Equal(2, rows.Count);
+
+        // Row 0 must contain bold "AB" and italic "C"
+        bool hasAB = rows[0].Segments.Any(s => s.Text == "AB" && s.Style == Bold);
+        bool hasC = rows[0].Segments.Any(s => s.Text == "C" && s.Style == Italic);
+        Assert.True(hasAB, "Row 0 must contain bold 'AB'");
+        Assert.True(hasC, "Row 0 must contain italic 'C'");
+
+        // Row 1 must contain italic "DEF"
+        bool hasDEF = rows[1].Segments.Any(s => s.Text == "DEF" && s.Style == Italic);
+        Assert.True(hasDEF, "Row 1 must contain italic 'DEF'");
+    }
+
+    [Fact]
+    public void WrapAcrossSegmentBoundaryPreservesStyles()
+    {
+        // Width 4: seg1="AB"(bold, 2 cols), seg2="CD"(italic, 2 cols) → exactly fills row 0.
+        // seg3="EF"(bold) → row 1.
+        Segment s1 = new("AB", Bold);
+        Segment s2 = new("CD", Italic);
+        Segment s3 = new("EF", Bold);
+        Line line = new(new[] { s1, s2, s3 });
+        IReadOnlyList<Line> rows = LineWrapper.Wrap(line, 4);
+
+        Assert.Equal(2, rows.Count);
+
+        string row0 = string.Concat(rows[0].Segments.Select(s => s.Text));
+        string row1 = string.Concat(rows[1].Segments.Select(s => s.Text));
+        Assert.Equal("ABCD", row0);
+        Assert.Equal("EF", row1);
+
+        // Styles must be correct on each row
+        Assert.Contains(rows[0].Segments, s => s.Text == "AB" && s.Style == Bold);
+        Assert.Contains(rows[0].Segments, s => s.Text == "CD" && s.Style == Italic);
+        Assert.Contains(rows[1].Segments, s => s.Text == "EF" && s.Style == Bold);
+    }
+
+    // -----------------------------------------------------------------------------------------
+    // Total content preservation
+    // -----------------------------------------------------------------------------------------
+
+    [Fact]
+    public void WrapPreservesAllTextContent()
+    {
+        // All characters from the source line must appear across the rows, in order.
+        string text = "Hello, 世界! This is a longer string.";
+        Line line = new LineBuilder().Text(text).Build();
+        IReadOnlyList<Line> rows = LineWrapper.Wrap(line, 10);
+
+        string rejoined = string.Concat(rows.SelectMany(r => r.Segments).Select(s => s.Text));
+        Assert.Equal(text, rejoined);
+    }
+
+    [Fact]
+    public void EachRowWidthIsWithinBound()
+    {
+        string text = "Hello, 世界! Testing wide char wrap.";
+        Line line = new LineBuilder().Text(text).Build();
+        const int width = 10;
+        IReadOnlyList<Line> rows = LineWrapper.Wrap(line, width);
+
+        foreach (Line row in rows)
+        {
+            int rowWidth = row.Segments.Sum(s => DisplayWidth.Measure(s.Text));
+            // Allow a single wide char that forced its own row to exceed (documented edge case):
+            // a row is exempt if it contains exactly one segment whose entire text is a single
+            // scalar value with display-width > the target width.
+            bool hasSingleWideChar =
+                row.Segments.Count == 1 &&
+                DisplayWidth.Measure(row.Segments[0].Text) > width &&
+                row.Segments[0].Text.Length is 1 or 2; // BMP rune = 1 code unit; supplementary = 2
+
+            if (!hasSingleWideChar)
+            {
+                Assert.True(rowWidth <= width,
+                    $"Row width {rowWidth} exceeds bound {width}: [{string.Concat(row.Segments.Select(s => s.Text))}]");
+            }
+        }
+    }
+
+    // -----------------------------------------------------------------------------------------
+    // Tab handling inside wrap
+    // -----------------------------------------------------------------------------------------
+
+    [Fact]
+    public void TabExpandsCorrectlyDuringWrap()
+    {
+        // "\tA" at width 9: tab from col 0 → 8, then A → col 9. Fits in one row at width 9.
+        Line line = new LineBuilder().Text("\tA").Build();
+        IReadOnlyList<Line> rows = LineWrapper.Wrap(line, 9);
+        Assert.Single(rows);
+    }
+
+    [Fact]
+    public void TabCausesWrapWhenItExceedsWidth()
+    {
+        // "\tA" at width 8: tab from col 0 → 8 (fills exactly), A → col 9 > 8.
+        // Expect at least 2 rows: row0 = tab, row1 = "A".
+        Line line = new LineBuilder().Text("\tA").Build();
+        IReadOnlyList<Line> rows = LineWrapper.Wrap(line, 8);
+        // We just confirm A appears after the tab row and overall content is preserved.
+        string all = string.Concat(rows.SelectMany(r => r.Segments).Select(s => s.Text));
+        Assert.Contains("A", all, StringComparison.Ordinal);
+    }
+}
diff --git a/tests/Dcli.Tests/OverlayRoutingTests.cs b/tests/Dcli.Tests/OverlayRoutingTests.cs
new file mode 100644
index 0000000..41cdb2e
--- /dev/null
+++ b/tests/Dcli.Tests/OverlayRoutingTests.cs
@@ -0,0 +1,595 @@
+using System.Text;
+using Dcli.Internal.FixedRegion;
+using Dcli.Internal.RenderLoop;
+using Xunit;
+
+namespace Dcli.Tests;
+
+/// <summary>
+/// Tests for tasks 11.2 and 11.5: OverlayState invariant, intercept-chain key routing,
+/// and FixedRegionComposer overlay slotting / budget / cursor placement.
+/// </summary>
+public sealed class OverlayRoutingTests
+{
+    // ── Helpers ───────────────────────────────────────────────────────────────
+
+    private static RenderModel MakeModel(int cols = 80, int rows = 24)
+    {
+        ConstantSizeSource size = new(cols, rows);
+        return new RenderModel(size);
+    }
+
+    private sealed class ConstantSizeSource : ITerminalSizeSource
+    {
+        private readonly int _cols;
+        private readonly int _rows;
+        internal ConstantSizeSource(int cols, int rows) { _cols = cols; _rows = rows; }
+        public (int Columns, int Rows) GetSize() => (_cols, _rows);
+    }
+
+    private static Line PlainLine(string text) => new([new Segment(text)]);
+
+    private static Autocomplete MakeAutocomplete(TextBuffer? buffer = null)
+    {
+        buffer ??= new TextBuffer();
+        Autocomplete ac = new(buffer);
+        ac.Show([new AutocompleteCandidate("item1", PlainLine("item1")), new AutocompleteCandidate("item2", PlainLine("item2"))]);
+        return ac;
+    }
+
+    private static Dialog MakeDialog(bool modal = true) => new(modal: modal);
+
+    // ── LoopEngine test infrastructure (mirrors FixedRegionTests) ─────────────
+
+    private sealed class CapturingOutputSink : IOutputSink
+    {
+        internal RenderModel? LastModel { get; private set; }
+        public void Paint(RenderModel model) { LastModel = model; }
+        public void EmitRestoreSequence() { }
+    }
+
+    private sealed class VirtualClock : IClock
+    {
+        private readonly object _lock = new();
+        private TimeSpan _now;
+        private readonly List<(TimeSpan Deadline, TaskCompletionSource Tcs, CancellationTokenSource LinkedCts)> _waiters = [];
+
+        internal VirtualClock(TimeSpan? initial = null) => _now = initial ?? TimeSpan.Zero;
+
+        public TimeSpan Now { get { lock (_lock) { return _now; } } }
+
+        internal void Advance(TimeSpan by)
+        {
+            List<(TaskCompletionSource Tcs, CancellationTokenSource LinkedCts)> toComplete = [];
+            lock (_lock)
+            {
+                _now += by;
+                for (int i = _waiters.Count - 1; i >= 0; i--)
+                {
+                    if (_waiters[i].Deadline <= _now)
+                    {
+                        toComplete.Add((_waiters[i].Tcs, _waiters[i].LinkedCts));
+                        _waiters.RemoveAt(i);
+                    }
+                }
+            }
+            foreach ((TaskCompletionSource tcs, CancellationTokenSource linkedCts) in toComplete)
+            {
+                tcs.TrySetResult();
+                linkedCts.Dispose();
+            }
+        }
+
+        public Task WaitUntilAsync(TimeSpan deadline, CancellationToken cancellationToken)
+        {
+            lock (_lock)
+            {
+                if (deadline <= _now)
+                    return Task.CompletedTask;
+            }
+            CancellationTokenSource linkedCts = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
+            TaskCompletionSource tcs = new(TaskCreationOptions.RunContinuationsAsynchronously);
+            linkedCts.Token.Register(
+                static state => ((TaskCompletionSource)state!).TrySetCanceled(),
+                tcs,
+                useSynchronizationContext: false);
+            lock (_lock)
+            {
+                if (deadline <= _now) { linkedCts.Dispose(); return Task.CompletedTask; }
+                _waiters.Add((deadline, tcs, linkedCts));
+            }
+            return tcs.Task;
+        }
+    }
+
+    /// <summary>Posts an ILoopCommand to inject an overlay into the model.</summary>
+    private sealed class ShowAutocompleteCommand(Autocomplete autocomplete) : ILoopCommand
+    {
+        void ILoopCommand.Apply(RenderModel model)
+        {
+            model.ShowAutocomplete(autocomplete);
+            model.MarkDirty();
+        }
+    }
+
+    private sealed class ShowDialogCommand(Dialog dialog) : ILoopCommand
+    {
+        void ILoopCommand.Apply(RenderModel model)
+        {
+            model.ShowModal(dialog);
+            model.MarkDirty();
+        }
+    }
+
+    // ── 11.2 — RenderModel overlay invariant ─────────────────────────────────
+
+    [Fact]
+    public void ShowDialogWhileAutocompleteActiveReplacesIt()
+    {
+        RenderModel model = MakeModel();
+        Autocomplete ac = MakeAutocomplete();
+        Dialog dialog = MakeDialog();
+
+        model.ShowAutocomplete(ac);
+        Assert.IsType<Autocomplete>(model.ActiveOverlay);
+
+        model.ShowModal(dialog);
+        Assert.IsType<Dialog>(model.ActiveOverlay);
+        Assert.Same(dialog, model.ActiveOverlay);
+    }
+
+    [Fact]
+    public void ShowAutocompleteWhileDialogActiveIsNoOp()
+    {
+        RenderModel model = MakeModel();
+        Dialog dialog = MakeDialog();
+        Autocomplete ac = MakeAutocomplete();
+
+        model.ShowModal(dialog);
+        model.ShowAutocomplete(ac); // must be ignored
+
+        Assert.IsType<Dialog>(model.ActiveOverlay);
+        Assert.Same(dialog, model.ActiveOverlay);
+    }
+
+    [Fact]
+    public void ClearOverlayRemovesActiveOverlay()
+    {
+        RenderModel model = MakeModel();
+        model.ShowModal(MakeDialog());
+        Assert.NotNull(model.ActiveOverlay);
+
+        model.ClearOverlay();
+        Assert.Null(model.ActiveOverlay);
+    }
+
+    [Fact]
+    public void ShowAutocompleteWhenNoOverlayIsActive()
+    {
+        RenderModel model = MakeModel();
+        Autocomplete ac = MakeAutocomplete();
+        model.ShowAutocomplete(ac);
+        Assert.Same(ac, model.ActiveOverlay);
+    }
+
+    // ── 11.2 — LoopEngine intercept-chain routing ─────────────────────────────
+
+    [Fact]
+    public async Task AutocompleteArrowConsumedNoKeyPressedEvent()
+    {
+        ConstantSizeSource size = new(80, 24);
+        CapturingOutputSink sink = new();
+        VirtualClock clock = new();
+        using LoopEngine engine = new(size, clock, sink, minFrameInterval: TimeSpan.Zero);
+
+        Autocomplete ac = new(new TextBuffer());
+        ac.Show([new AutocompleteCandidate("a", PlainLine("a")), new AutocompleteCandidate("b", PlainLine("b"))]);
+        engine.Post(new ShowAutocompleteCommand(ac));
+
+        // ↓ should be consumed by autocomplete (moves selection); no KeyPressed emitted.
+        engine.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Down), Modifiers.None));
+
+        clock.Advance(TimeSpan.FromMilliseconds(100));
+        using CancellationTokenSource cts = new(TimeSpan.FromSeconds(5));
+        await engine.SettleAsync(cts.Token);
+
+        Assert.Equal(0, engine.OutboundEvents.Count);
+
+        // Verify the selection moved (index should now be 1).
+        // We can observe this by snapshotting; the second item renders as selected.
+        // (We simply assert no event emitted — selection state is internal to ScrollableList.)
+    }
+
+    [Fact]
+    public async Task AutocompleteUpConsumedNoKeyPressedEvent()
+    {
+        ConstantSizeSource size = new(80, 24);
+        CapturingOutputSink sink = new();
+        VirtualClock clock = new();
+        using LoopEngine engine = new(size, clock, sink, minFrameInterval: TimeSpan.Zero);
+
+        TextBuffer buf = new();
+        Autocomplete ac = new(buf);
+        ac.Show([new AutocompleteCandidate("x", PlainLine("x")), new AutocompleteCandidate("y", PlainLine("y"))]);
+        engine.Post(new ShowAutocompleteCommand(ac));
+
+        // First move down to index 1, then up to index 0.
+        engine.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Down), Modifiers.None));
+        engine.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Up), Modifiers.None));
+
+        clock.Advance(TimeSpan.FromMilliseconds(100));
+        using CancellationTokenSource cts = new(TimeSpan.FromSeconds(5));
+        await engine.SettleAsync(cts.Token);
+
+        // Both keys consumed by autocomplete — no events on outbound channel.
+        Assert.Equal(0, engine.OutboundEvents.Count);
+    }
+
+    [Fact]
+    public async Task AutocompletePrintableFallsThroughToEditor()
+    {
+        ConstantSizeSource size = new(80, 24);
+        CapturingOutputSink sink = new();
+        VirtualClock clock = new();
+        using LoopEngine engine = new(size, clock, sink, minFrameInterval: TimeSpan.Zero);
+
+        TextBuffer buf = new TextBuffer();
+        Autocomplete ac = new(buf);
+        ac.Show([new AutocompleteCandidate("item", PlainLine("item"))]);
+        engine.Post(new ShowAutocompleteCommand(ac));
+
+        // A printable character — autocomplete doesn't consume printables, so it falls through
+        // to the editor and inserts the character.
+        engine.InputWriter.TryWrite(new KeyEvent(KeyCode.FromRune(new Rune('z')), Modifiers.None));
+
+        clock.Advance(TimeSpan.FromMilliseconds(100));
+        using CancellationTokenSource cts = new(TimeSpan.FromSeconds(5));
+        await engine.SettleAsync(cts.Token);
+
+        // No KeyPressed on outbound (the editor consumed it). However, 'z' changes the buffer
+        // so InputChanged is emitted. Exactly one outbound event.
+        Assert.Equal(1, engine.OutboundEvents.Count);
+        Assert.True(engine.OutboundEvents.TryRead(out TerminalEvent? printableEv));
+        Assert.IsType<InputChanged>(printableEv);
+
+        // Verify the character is in the editor.
+        Assert.NotNull(sink.LastModel);
+        string fixedText = string.Concat(
+            sink.LastModel!.FixedRegionRows.SelectMany(r => r.Segments).Select(s => s.Text));
+        Assert.Contains("z", fixedText, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public async Task ModalDialogPreventsEditorMutation()
+    {
+        ConstantSizeSource size = new(80, 24);
+        CapturingOutputSink sink = new();
+        VirtualClock clock = new();
+        using LoopEngine engine = new(size, clock, sink, minFrameInterval: TimeSpan.Zero);
+
+        Dialog dialog = new(modal: true);
+        engine.Post(new ShowDialogCommand(dialog));
+
+        // Type a printable — modal dialog consumes all non-exit keys.
+        engine.InputWriter.TryWrite(new KeyEvent(KeyCode.FromRune(new Rune('A')), Modifiers.None));
+        // Arrow — also consumed.
+        engine.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Left), Modifiers.None));
+
+        clock.Advance(TimeSpan.FromMilliseconds(100));
+        using CancellationTokenSource cts = new(TimeSpan.FromSeconds(5));
+        await engine.SettleAsync(cts.Token);
+
+        // No KeyPressed emitted (modal ate all keys).
+        Assert.Equal(0, engine.OutboundEvents.Count);
+
+        // Editor buffer must be unchanged (empty).
+        Assert.NotNull(sink.LastModel);
+        string editorText = sink.LastModel!.FixedRegion.Editor.Text;
+        Assert.Equal(string.Empty, editorText);
+    }
+
+    [Fact]
+    public async Task ModalDialogEnterClosesOverlay()
+    {
+        ConstantSizeSource size = new(80, 24);
+        CapturingOutputSink sink = new();
+        VirtualClock clock = new();
+        using LoopEngine engine = new(size, clock, sink, minFrameInterval: TimeSpan.Zero);
+
+        Dialog dialog = new(modal: true);
+        engine.Post(new ShowDialogCommand(dialog));
+
+        // Enter closes the dialog (Submit).
+        engine.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Enter), Modifiers.None));
+
+        clock.Advance(TimeSpan.FromMilliseconds(100));
+        using CancellationTokenSource cts = new(TimeSpan.FromSeconds(5));
+        await engine.SettleAsync(cts.Token);
+
+        // Dialog set CloseRequest = Submit and was dismissed → cleared from the model.
+        Assert.Equal(OverlayCloseKind.Submit, dialog.CloseRequest);
+        Assert.Null(sink.LastModel?.ActiveOverlay);
+    }
+
+    [Fact]
+    public async Task AutocompleteEscapeDismissesOverlay()
+    {
+        ConstantSizeSource size = new(80, 24);
+        CapturingOutputSink sink = new();
+        VirtualClock clock = new();
+        using LoopEngine engine = new(size, clock, sink, minFrameInterval: TimeSpan.Zero);
+
+        Autocomplete ac = new(new TextBuffer());
+        ac.Show([new AutocompleteCandidate("x", PlainLine("x"))]);
+        engine.Post(new ShowAutocompleteCommand(ac));
+
+        // Escape dismisses the autocomplete.
+        engine.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Escape), Modifiers.None));
+
+        clock.Advance(TimeSpan.FromMilliseconds(100));
+        using CancellationTokenSource cts = new(TimeSpan.FromSeconds(5));
+        await engine.SettleAsync(cts.Token);
+
+        // Autocomplete is dismissed → cleared from the model.
+        Assert.True(ac.IsDismissed);
+        Assert.Null(sink.LastModel?.ActiveOverlay);
+    }
+
+    // ── 11.5 — Composer: cursor placement and row order ───────────────────────
+
+    [Fact]
+    public void AutocompleteBelowInputCursorVisibleAndRowOrder()
+    {
+        // Autocomplete (BelowInput): cursor visible, EditorCaretLocal is in input rows,
+        // and row order is [input…][autocomplete…][status…].
+        RenderModel model = MakeModel(cols: 80, rows: 24);
+        StatusLine status = new() { Rows = [PlainLine("status")] };
+        TextBuffer editor = new();
+        editor.Insert("hi");
+        FixedRegionComposer composer = new(editor, status);
+
+        Autocomplete ac = new(editor);
+        ac.Show([new AutocompleteCandidate("hello", PlainLine("hello")), new AutocompleteCandidate("hi", PlainLine("hi"))]);
+        model.ShowAutocomplete(ac);
+
+        composer.Compose(model);
+
+        // Cursor visible.
+        Assert.True(model.IsCursorVisible);
+        Assert.NotNull(model.EditorCaretLocal);
+
+        // Row order: input (not offset by overlay rows above), then autocomplete, then status.
+        // Status is the last row.
+        IReadOnlyList<Line> rows = model.FixedRegionRows;
+        Assert.True(rows.Count >= 2, "Should have at least input + status rows");
+
+        // Status must be at the very bottom.
+        Line lastRow = rows[^1];
+        Assert.Contains(lastRow.Segments, s => s.Text.Contains("status", StringComparison.Ordinal));
+
+        // Autocomplete should appear before status but after input (BelowInput).
+        // The caret row is within input, so EditorCaretLocal.Row < (rows before status).
+        (int caretRow, _) = model.EditorCaretLocal!.Value;
+        // No above-overlay rows, so caretRow == editor-internal caret row (0 for single-line input).
+        Assert.True(caretRow >= 0 && caretRow < rows.Count - 1 /* exclude status */,
+            $"Caret row {caretRow} should be within the input section");
+    }
+
+    [Fact]
+    public void ModalDialogAboveInputCursorHiddenAndRowOrder()
+    {
+        // Modal Dialog (AboveInput, HidesCursor=true): cursor hidden, EditorCaretLocal null,
+        // and row order is [dialog…][input…][status…].
+        RenderModel model = MakeModel(cols: 80, rows: 24);
+        StatusLine status = new() { Rows = [PlainLine("status")] };
+        TextBuffer editor = new();
+        editor.Insert("hi");
+        FixedRegionComposer composer = new(editor, status);
+
+        Dialog dialog = new(modal: true);
+        dialog.List.SetItems([PlainLine("option A"), PlainLine("option B")]);
+        model.ShowModal(dialog);
+
+        composer.Compose(model);
+
+        // Cursor hidden.
+        Assert.False(model.IsCursorVisible);
+        Assert.Null(model.EditorCaretLocal);
+        Assert.Null(model.CaretPosition);
+
+        // Row order: dialog (above) then input then status.
+        IReadOnlyList<Line> rows = model.FixedRegionRows;
+        Assert.True(rows.Count >= 3, "Should have dialog + input + status rows");
+
+        // Status at the bottom.
+        Line lastRow = rows[^1];
+        Assert.Contains(lastRow.Segments, s => s.Text.Contains("status", StringComparison.Ordinal));
+    }
+
+    [Fact]
+    public void BudgetSqueezeTotalNeverExceedsCap()
+    {
+        // On a tight terminal: cap = 8 (10 rows, 50%=5<8 → floor=8), status=2.
+        // budget = 6. Input=5 rows (multiline), overlay gets budget - inputRows ≥ 0.
+        // Assert total ≤ cap.
+        RenderModel model = MakeModel(cols: 80, rows: 10);
+        StatusLine status = new() { Rows = [PlainLine("s1"), PlainLine("s2")] };
+        TextBuffer editor = new();
+        editor.Insert("line1\nline2\nline3\nline4\nline5\nline6");
+        FixedRegionComposer composer = new(editor, status);
+
+        Autocomplete ac = new(editor);
+        ac.Show(Enumerable.Range(1, 20).Select(i => new AutocompleteCandidate($"item{i}", PlainLine($"item{i}"))).ToList());
+        model.ShowAutocomplete(ac);
+
+        composer.Compose(model);
+
+        int cap = FixedRegionComposer.ComputeCap(10, null); // 8
+        Assert.True(model.FixedRegionRows.Count <= cap,
+            $"Fixed region {model.FixedRegionRows.Count} exceeds cap {cap}");
+
+        // Status must be fully present (sacred) — 2 rows at the bottom.
+        IReadOnlyList<Line> rows = model.FixedRegionRows;
+        Assert.Contains(rows, r => r.Segments.Any(s => s.Text.Contains("s1", StringComparison.Ordinal)));
+        Assert.Contains(rows, r => r.Segments.Any(s => s.Text.Contains("s2", StringComparison.Ordinal)));
+    }
+
+    [Fact]
+    public void BudgetSqueezeOverlayShrinksNotStatus()
+    {
+        // cap=8, status=2 → budget=6.
+        // Large input (6 rows) → input gets 6 rows, overlay gets 0 (it absorbs the squeeze).
+        // Total = 6 + 0 + 2 = 8 = cap. Status still present.
+        RenderModel model = MakeModel(cols: 80, rows: 10);
+        StatusLine status = new() { Rows = [PlainLine("s1"), PlainLine("s2")] };
+        TextBuffer editor = new();
+        editor.Insert("line1\nline2\nline3\nline4\nline5\nline6");
+        FixedRegionComposer composer = new(editor, status);
+
+        Autocomplete ac = new(editor);
+        ac.Show([new AutocompleteCandidate("suggestion", PlainLine("suggestion"))]);
+        model.ShowAutocomplete(ac);
+
+        composer.Compose(model);
+
+        // Status rows both present.
+        IReadOnlyList<Line> rows = model.FixedRegionRows;
+        int cap = FixedRegionComposer.ComputeCap(10, null); // 8
+        Assert.True(rows.Count <= cap);
+
+        // Status at the bottom two rows.
+        Assert.Contains(rows[^1].Segments, s => s.Text.Contains("s2", StringComparison.Ordinal));
+        Assert.Contains(rows[^2].Segments, s => s.Text.Contains("s1", StringComparison.Ordinal));
+    }
+
+    [Fact]
+    public void NoOverlayNormalComposeBehaviourUnchanged()
+    {
+        // No active overlay: behaviour is identical to the §10 path.
+        RenderModel model = MakeModel(cols: 80, rows: 24);
+        StatusLine status = new() { Rows = [PlainLine("status")] };
+        TextBuffer editor = new();
+        editor.Insert("hello");
+        FixedRegionComposer composer = new(editor, status);
+
+        composer.Compose(model);
+
+        Assert.Equal(2, model.FixedRegionRows.Count); // input row + status row
+        Assert.True(model.IsCursorVisible);
+        Assert.NotNull(model.EditorCaretLocal);
+    }
+
+    /// <summary>
+    /// Wires an Autocomplete to the engine's own TextBuffer (obtained from the model at
+    /// Apply-time) and then shows it. This ensures Accept() writes into the same buffer
+    /// that the engine and sink observe.
+    /// </summary>
+    private sealed class ShowAutocompleteOnEngineBufferCommand(string insertText) : ILoopCommand
+    {
+        private Autocomplete? _created;
+        internal Autocomplete Created => _created ?? throw new InvalidOperationException("Not yet applied.");
+
+        void ILoopCommand.Apply(RenderModel model)
+        {
+            TextBuffer buf = model.FixedRegion.Editor;
+            _created = new Autocomplete(buf);
+            _created.Show([new AutocompleteCandidate(insertText, PlainLine(insertText))]);
+            model.ShowAutocomplete(_created);
+            model.MarkDirty();
+        }
+    }
+
+    // ── 11.2 — Autocomplete Accept clears overlay at the engine level ────────
+
+    [Theory]
+    [InlineData(NamedKey.Enter)]
+    [InlineData(NamedKey.Tab)]
+    public async Task AutocompleteAcceptClearsOverlay(NamedKey acceptKey)
+    {
+        ConstantSizeSource size = new(80, 24);
+        CapturingOutputSink sink = new();
+        VirtualClock clock = new();
+        using LoopEngine engine = new(size, clock, sink, minFrameInterval: TimeSpan.Zero);
+
+        // Use a command that wires the autocomplete to the engine's own editor buffer,
+        // so that Accept() writes into the same TextBuffer the model/sink observe.
+        ShowAutocompleteOnEngineBufferCommand cmd = new("accepted_text");
+        engine.Post(cmd);
+
+        // Let the command apply so the autocomplete is wired.
+        clock.Advance(TimeSpan.FromMilliseconds(50));
+        using CancellationTokenSource cts1 = new(TimeSpan.FromSeconds(5));
+        await engine.SettleAsync(cts1.Token);
+
+        // Accept the candidate — Enter and Tab are both accept keys.
+        engine.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(acceptKey), Modifiers.None));
+
+        clock.Advance(TimeSpan.FromMilliseconds(100));
+        using CancellationTokenSource cts2 = new(TimeSpan.FromSeconds(5));
+        await engine.SettleAsync(cts2.Token);
+
+        // Overlay must be cleared.
+        Assert.True(cmd.Created.IsDismissed);
+        Assert.Null(sink.LastModel?.ActiveOverlay);
+
+        // The accepted InsertText must be in the engine's editor buffer.
+        Assert.NotNull(sink.LastModel);
+        Assert.Equal("accepted_text", sink.LastModel!.FixedRegion.Editor.Text);
+    }
+
+    // ── 11.5 — Zero-cap guard: overlay contributes exactly zero rows ──────────
+
+    [Fact]
+    public void OverlayContributesNoRowsWhenBudgetFullyConsumed()
+    {
+        // cap=8 (rows=10, ComputeCap), status=2 → budget=6.
+        // Input has 6 lines → inputRows == 6 → overlayCap = budget - inputRows = 0.
+        // The autocomplete has candidates but must contribute zero rows.
+        RenderModel model = MakeModel(cols: 80, rows: 10);
+        StatusLine status = new() { Rows = [PlainLine("s1"), PlainLine("s2")] };
+        TextBuffer editor = new();
+        editor.Insert("line1\nline2\nline3\nline4\nline5\nline6");
+        FixedRegionComposer composer = new(editor, status);
+
+        Autocomplete ac = new(editor);
+        ac.Show(Enumerable.Range(1, 5).Select(i =>
+            new AutocompleteCandidate($"ZZ_CANDIDATE_ZZ_{i}", PlainLine($"ZZ_CANDIDATE_ZZ_{i}"))).ToList());
+        model.ShowAutocomplete(ac);
+
+        composer.Compose(model);
+
+        int cap = FixedRegionComposer.ComputeCap(10, null); // 8
+        IReadOnlyList<Line> rows = model.FixedRegionRows;
+
+        // Total rows must equal cap exactly (status + input fill it, overlay gets 0).
+        Assert.Equal(cap, rows.Count);
+
+        // No sentinel candidate text must appear anywhere in the fixed region.
+        string allText = string.Concat(rows.SelectMany(r => r.Segments).Select(s => s.Text));
+        Assert.DoesNotContain("ZZ_CANDIDATE_ZZ", allText, StringComparison.Ordinal);
+    }
+
+    // ── 11.5 — Autocomplete defensive copy ───────────────────────────────────
+
+    [Fact]
+    public void AutocompleteShowStoresDefensiveCopy()
+    {
+        TextBuffer buf = new();
+        Autocomplete ac = new(buf);
+
+        List<AutocompleteCandidate> candidates =
+        [
+            new AutocompleteCandidate("original", PlainLine("original")),
+        ];
+        ac.Show(candidates);
+
+        // Mutate the original list after Show.
+        candidates.Add(new AutocompleteCandidate("added", PlainLine("added")));
+
+        // The autocomplete should still render only the original candidate.
+        // (MaxRows must be set first, or the list renders 0 rows.)
+        ac.MaxRows = 10;
+        IReadOnlyList<Line> rows = ac.Render(width: 80);
+        Assert.Single(rows); // only the original, not the added one
+    }
+}
diff --git a/tests/Dcli.Tests/RenderLoop/ResizeReflowTests.cs b/tests/Dcli.Tests/RenderLoop/ResizeReflowTests.cs
new file mode 100644
index 0000000..5aef1c3
--- /dev/null
+++ b/tests/Dcli.Tests/RenderLoop/ResizeReflowTests.cs
@@ -0,0 +1,293 @@
+using Dcli.Internal;
+using Dcli.Internal.Input;
+using Dcli.Internal.RenderLoop;
+using Dcli.Internal.Scrollback;
+using Xunit;
+using DcliTerminal = Dcli.Terminal;
+
+namespace Dcli.Tests.RenderLoop;
+
+/// <summary>
+/// Behavioural tests for §13.2: terminal resize reflows the live window and fixed region.
+/// All tests use injected edges — no real tty, no real raw mode.
+/// </summary>
+public sealed class ResizeReflowTests
+{
+    // ─────────────────────────────────────────────────────────────────────────
+    // Test doubles
+    // ─────────────────────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// A controllable <see cref="IResizeWatcher"/> for tests. Stores the callback registered
+    /// via <see cref="Start"/> and exposes <see cref="Fire"/> to simulate a resize event.
+    /// <see cref="Dispose"/> is idempotent and does not null the callback, so the watcher
+    /// remains usable after the Terminal takes ownership and disposes it.
+    /// </summary>
+    private sealed class FakeResizeWatcher : IResizeWatcher
+    {
+        private Action<int, int>? _callback;
+
+        public void Start(Action<int, int> onResize) => _callback = onResize;
+
+        /// <summary>Idempotent no-op; does not clear the callback so <see cref="Fire"/> still works.</summary>
+        public void Dispose() { }
+
+        /// <summary>Simulates a terminal resize by invoking the registered callback.</summary>
+        internal void Fire(int columns, int rows) =>
+            _callback?.Invoke(columns, rows);
+    }
+
+    /// <summary>Captures the most-recent painted <see cref="RenderModel"/> for assertion.</summary>
+    private sealed class CapturingOutputSink : IOutputSink
+    {
+        private volatile RenderModel? _lastModel;
+
+        internal RenderModel? LastModel => _lastModel;
+
+        public void Paint(RenderModel model) => _lastModel = model;
+
+        public void EmitRestoreSequence() { }
+    }
+
+    /// <summary>Byte source that immediately times out — no data.</summary>
+    private sealed class ImmediateTimeoutByteSource : IInputByteSource
+    {
+        public int Read(Span<byte> buffer) => 0;
+    }
+
+    /// <summary>Fixed terminal dimensions.</summary>
+    private sealed class FixedSizeSource(int cols, int rows) : ITerminalSizeSource
+    {
+        public (int Columns, int Rows) GetSize() => (cols, rows);
+    }
+
+    /// <summary>A <see cref="ILoopCommand"/> that appends a line to the scrollback model.</summary>
+    private sealed class AppendLineCmd(Line line) : ILoopCommand
+    {
+        void ILoopCommand.Apply(RenderModel model) =>
+            model.Scrollback.Append(new TextBlock(line), model);
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // Factory helpers
+    // ─────────────────────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Creates a headless terminal with the given dimensions and the caller-supplied watcher.
+    /// Ownership of the watcher is transferred to the returned Terminal.
+    /// </summary>
+    private static (DcliTerminal Term, CapturingOutputSink Sink)
+        CreateTerminal(int cols, int rows, FakeResizeWatcher watcher)
+    {
+        using RecordingRawModeSession session = new();
+        RestoreCoordinator coordinator = RestoreCoordinator.Wire(session);
+        CapturingOutputSink sink = new();
+
+        // VirtualClock at TimeSpan.Zero + minFrameInterval=Zero means every dirty state paints
+        // immediately — no real-time dependency, fully deterministic.
+        DcliTerminal term = DcliTerminal.StartCore(
+            session,
+            coordinator,
+            watcher,
+            new ImmediateTimeoutByteSource(),
+            new VirtualClock(TimeSpan.Zero),
+            sink,
+            new FixedSizeSource(cols, rows),
+            minFrameInterval: TimeSpan.Zero);
+
+        // session and coordinator transferred to term; DisposeAsync disposes them.
+        // The `using` also calls Dispose() on scope exit, but RecordingRawModeSession.Dispose is idempotent.
+        return (term, sink);
+    }
+
+    private static Task SettleAsync(DcliTerminal term) =>
+        term.Loop.SettleAsync().WaitAsync(TimeSpan.FromSeconds(5));
+
+    /// <summary>
+    /// Minimal virtual clock: always reports TimeSpan.Zero, WaitUntilAsync returns immediately.
+    /// With minFrameInterval=Zero, the loop paints on every dirty-state settle.
+    /// </summary>
+    private sealed class VirtualClock(TimeSpan initial) : IClock
+    {
+        public TimeSpan Now => initial;
+        public Task WaitUntilAsync(TimeSpan deadline, CancellationToken cancellationToken) =>
+            Task.CompletedTask;
+    }
+
+    private static Line PlainLine(string text) => new([new Segment(text)]);
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // 13.2-a — Live window reflows on width change
+    // ─────────────────────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// A line that wraps into multiple rows at width 20 should reflow to fewer rows at width 40.
+    /// </summary>
+    [Fact]
+    public async Task LiveWindowReflowsOnWidthChange()
+    {
+        // "01234567890123456789ABCDEFGHIJ" = 30 chars.
+        // At width 20: wraps to 2 rows. At width 40: fits in 1 row.
+        const string text = "01234567890123456789ABCDEFGHIJ";
+
+        using FakeResizeWatcher watcher = new();
+        (DcliTerminal term, CapturingOutputSink sink) = CreateTerminal(cols: 20, rows: 24, watcher);
+        await using (term)
+        {
+            term.Loop.Post(new AppendLineCmd(PlainLine(text)));
+            await SettleAsync(term);
+
+            RenderModel? modelAt20 = sink.LastModel;
+            Assert.NotNull(modelAt20);
+            int rowsAt20 = modelAt20.LiveWindowRows.Count;
+            Assert.True(rowsAt20 >= 2, $"Expected ≥2 live rows at width 20; got {rowsAt20}.");
+
+            // Resize to width 40 — the same text should now fit in 1 row.
+            watcher.Fire(40, 24);
+            await SettleAsync(term);
+
+            RenderModel? modelAt40 = sink.LastModel;
+            Assert.NotNull(modelAt40);
+            Assert.Equal(40, modelAt40.Columns);
+            int rowsAt40 = modelAt40.LiveWindowRows.Count;
+            Assert.True(rowsAt40 < rowsAt20,
+                $"Expected fewer live rows at width 40 (got {rowsAt40}) than at width 20 ({rowsAt20}).");
+        }
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // 13.2-b — Fixed-region cap recomputes on row change
+    // ─────────────────────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Shrinking the terminal row count causes the fixed-region cap to recompute.
+    /// The paint frame after the resize must use the new row budget.
+    /// </summary>
+    [Fact]
+    public async Task FixedRegionCapRecomputesOnRowChange()
+    {
+        using FakeResizeWatcher watcher = new();
+        (DcliTerminal term, CapturingOutputSink sink) = CreateTerminal(cols: 80, rows: 24, watcher);
+        await using (term)
+        {
+            // Force a paint at rows=24.
+            term.Loop.Post(new AppendLineCmd(PlainLine("initial")));
+            await SettleAsync(term);
+
+            RenderModel? modelAt24 = sink.LastModel;
+            Assert.NotNull(modelAt24);
+            Assert.Equal(24, modelAt24.Rows);
+
+            // Resize to rows=10 — the frame must reflect the new row count.
+            watcher.Fire(80, 10);
+            await SettleAsync(term);
+
+            RenderModel? modelAt10 = sink.LastModel;
+            Assert.NotNull(modelAt10);
+            Assert.Equal(10, modelAt10.Rows);
+
+            // The fixed region total rows must be ≤ the row budget of 10.
+            int fixedRows = modelAt10.FixedRegionRows.Count;
+            Assert.True(fixedRows <= 10,
+                $"Fixed region rows {fixedRows} exceed the row budget of 10.");
+        }
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // 13.2-c — Committed rows are NOT re-emitted on resize
+    // ─────────────────────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Rows already committed (above the horizon) must not appear in NewlyCommittedRows
+    /// after a resize. ClearCommitted semantics guarantee this; this test pins it.
+    /// </summary>
+    [Fact]
+    public async Task CommittedRowsAreNotReEmittedOnResize()
+    {
+        // rows=5 so the live window is very small; appending many lines commits some.
+        using FakeResizeWatcher watcher = new();
+        (DcliTerminal term, CapturingOutputSink sink) = CreateTerminal(cols: 80, rows: 5, watcher);
+        await using (term)
+        {
+            // Append enough lines to overflow the live window → some commit above the horizon.
+            for (int i = 0; i < 10; i++)
+                term.Loop.Post(new AppendLineCmd(PlainLine($"line {i}")));
+
+            await SettleAsync(term);
+
+            RenderModel? postAppend = sink.LastModel;
+            Assert.NotNull(postAppend);
+
+            // After the paint, ClearCommitted ran → NewlyCommittedRows is empty.
+            Assert.Empty(postAppend.NewlyCommittedRows);
+
+            // Fire a resize at the same size — no new overflows should occur.
+            watcher.Fire(80, 5);
+            await SettleAsync(term);
+
+            RenderModel? postResize = sink.LastModel;
+            Assert.NotNull(postResize);
+
+            // The post-resize frame must not re-emit already-committed rows.
+            Assert.Empty(postResize.NewlyCommittedRows);
+        }
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // 13.2-d — Resized outbound event is emitted
+    // ─────────────────────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Firing the watcher must produce exactly one <see cref="Resized"/> event on the
+    /// outbound channel with the new dimensions.
+    /// </summary>
+    [Fact]
+    public async Task ResizedOutboundEventIsEmittedOnResize()
+    {
+        using FakeResizeWatcher watcher = new();
+        (DcliTerminal term, _) = CreateTerminal(cols: 80, rows: 24, watcher);
+        await using (term)
+        {
+            watcher.Fire(120, 40);
+            await SettleAsync(term);
+
+            // Drain the outbound channel.
+            List<TerminalEvent> events = [];
+            while (term.Events.TryRead(out TerminalEvent? ev))
+                events.Add(ev);
+
+            Resized? resized = events.OfType<Resized>().FirstOrDefault();
+            Assert.NotNull(resized);
+            Assert.Equal(120, resized.Columns);
+            Assert.Equal(40, resized.Rows);
+        }
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // 13.2-e — Volatile snapshot updates after resize
+    // ─────────────────────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// After a resize settles, <see cref="DcliTerminal.GetTerminalSize()"/> must return
+    /// the new dimensions.
+    /// </summary>
+    [Fact]
+    public async Task VolatileSnapshotUpdatesAfterResize()
+    {
+        using FakeResizeWatcher watcher = new();
+        (DcliTerminal term, _) = CreateTerminal(cols: 80, rows: 24, watcher);
+        await using (term)
+        {
+            (int cols0, int rows0) = term.GetTerminalSize();
+            Assert.Equal(80, cols0);
+            Assert.Equal(24, rows0);
+
+            watcher.Fire(160, 48);
+            await SettleAsync(term);
+
+            (int cols1, int rows1) = term.GetTerminalSize();
+            Assert.Equal(160, cols1);
+            Assert.Equal(48, rows1);
+        }
+    }
+}
diff --git a/tests/Dcli.Tests/RenderLoop/SgrTranslatorTests.cs b/tests/Dcli.Tests/RenderLoop/SgrTranslatorTests.cs
new file mode 100644
index 0000000..f0b7575
--- /dev/null
+++ b/tests/Dcli.Tests/RenderLoop/SgrTranslatorTests.cs
@@ -0,0 +1,110 @@
+using System.Text;
+using Dcli.Internal;
+using Dcli.Internal.RenderLoop;
+using Xunit;
+
+namespace Dcli.Tests.RenderLoop;
+
+/// <summary>
+/// §13.4 (Chunk B) — SgrTranslator truecolor and 256-indexed downgrade tests.
+/// </summary>
+public sealed class SgrTranslatorTests
+{
+    private static SgrTranslator MakeSgr(bool hasTruecolor) =>
+        new(new TerminalCapabilities { HasTruecolor = hasTruecolor });
+
+    /// <summary>Emits a foreground SGR and returns only the code portion (between ESC[ and m).</summary>
+    private static string FgSgr(SgrTranslator sgr, byte r, byte g, byte b)
+    {
+        StringBuilder sb = new();
+        sgr.AppendOpenSgr(new Style(Foreground: Color.FromRgb(r, g, b)), sb);
+        string s = sb.ToString();
+        // Strip ESC[ prefix and trailing m.
+        return s.Length > 3 ? s[2..^1] : s;
+    }
+
+    private static string BgSgr(SgrTranslator sgr, byte r, byte g, byte b)
+    {
+        StringBuilder sb = new();
+        sgr.AppendOpenSgr(new Style(Background: Color.FromRgb(r, g, b)), sb);
+        string s = sb.ToString();
+        return s.Length > 3 ? s[2..^1] : s;
+    }
+
+    // ── 13.4-sgr-a  HasTruecolor=true emits 24-bit sequences ─────────────────
+
+    [Fact]
+    public void WithTruecolorRgbForegroundEmits24bitSequence()
+    {
+        SgrTranslator sgr = MakeSgr(hasTruecolor: true);
+        // (255,0,0) → ESC[38;2;255;0;0m
+        Assert.Equal("38;2;255;0;0", FgSgr(sgr, 255, 0, 0));
+    }
+
+    [Fact]
+    public void WithTruecolorArbitraryRgbEmits24bitForeground()
+    {
+        SgrTranslator sgr = MakeSgr(hasTruecolor: true);
+        // (10,20,30) — preserves original VtFrameRendererTests SGR assertion
+        Assert.Equal("38;2;10;20;30", FgSgr(sgr, 10, 20, 30));
+    }
+
+    // ── 13.4-sgr-b  HasTruecolor=false downgrades to 256-indexed ─────────────
+
+    [Fact]
+    public void WithoutTruecolorRedDowngradesToCubeIndex196()
+    {
+        // (255,0,0) → cube(5,0,0) = 16 + 36*5 = 196
+        SgrTranslator sgr = MakeSgr(hasTruecolor: false);
+        Assert.Equal("38;5;196", FgSgr(sgr, 255, 0, 0));
+    }
+
+    [Fact]
+    public void WithoutTruecolorWhiteDowngradesToCubeIndex231()
+    {
+        // (255,255,255) → cube(5,5,5) = 16 + 180 + 30 + 5 = 231
+        SgrTranslator sgr = MakeSgr(hasTruecolor: false);
+        Assert.Equal("38;5;231", FgSgr(sgr, 255, 255, 255));
+    }
+
+    [Fact]
+    public void WithoutTruecolorBlackDowngradesToCubeIndex16()
+    {
+        // (0,0,0) → cube(0,0,0) = 16; grey ramp dist = 3*64 = 192 → cube wins
+        SgrTranslator sgr = MakeSgr(hasTruecolor: false);
+        Assert.Equal("38;5;16", FgSgr(sgr, 0, 0, 0));
+    }
+
+    [Fact]
+    public void WithoutTruecolorMidGreyDowngradesToGreyRamp244()
+    {
+        // (128,128,128): cube nearest = (135,135,135) → dist = 3*49 = 147
+        // grey ramp: level = 12 → grey = 8 + 12*10 = 128 → dist = 0 → grey wins
+        // index = 232 + 12 = 244
+        SgrTranslator sgr = MakeSgr(hasTruecolor: false);
+        Assert.Equal("38;5;244", FgSgr(sgr, 128, 128, 128));
+    }
+
+    [Fact]
+    public void WithoutTruecolorBlackBackgroundDowngradesToIndex16()
+    {
+        // Background variant: ESC[48;5;16m
+        SgrTranslator sgr = MakeSgr(hasTruecolor: false);
+        Assert.Equal("48;5;16", BgSgr(sgr, 0, 0, 0));
+    }
+
+    // ── NearestXterm256 pure-function matrix ──────────────────────────────────
+
+    [Theory]
+    [InlineData(255, 0, 0, 196)]       // pure red → cube (5,0,0)
+    [InlineData(255, 255, 255, 231)]   // white → cube (5,5,5)
+    [InlineData(0, 0, 0, 16)]          // black → cube (0,0,0)
+    [InlineData(128, 128, 128, 244)]   // mid-grey → grey ramp level 12
+    [InlineData(0, 255, 0, 46)]        // pure green → cube (0,5,0) = 16+30 = 46
+    [InlineData(0, 0, 255, 21)]        // pure blue → cube (0,0,5) = 16+5 = 21
+    public void NearestXterm256MapsKnownColorsToExpectedIndex(byte r, byte g, byte b, int expectedIndex)
+    {
+        int actual = SgrTranslator.NearestXterm256(r, g, b);
+        Assert.Equal(expectedIndex, actual);
+    }
+}
diff --git a/tests/Dcli.Tests/ScrollableListTests.cs b/tests/Dcli.Tests/ScrollableListTests.cs
new file mode 100644
index 0000000..8e8a986
--- /dev/null
+++ b/tests/Dcli.Tests/ScrollableListTests.cs
@@ -0,0 +1,641 @@
+using Dcli.Internal.FixedRegion;
+using Xunit;
+
+namespace Dcli.Tests;
+
+/// <summary>
+/// Unit tests for <see cref="ScrollableList"/> — task 11.1.
+/// </summary>
+public sealed class ScrollableListTests
+{
+    // ── Helpers ──────────────────────────────────────────────────────────────────────────────
+
+    /// <summary>Creates a plain-text item <see cref="Line"/>.</summary>
+    private static Line Item(string text) =>
+        new LineBuilder().Text(text).Build();
+
+    /// <summary>Creates a list of plain-text items.</summary>
+    private static List<Line> Items(params string[] texts) =>
+        texts.Select(Item).ToList();
+
+    /// <summary>
+    /// Returns true when every segment in <paramref name="line"/> has <see cref="Format.Reverse"/>
+    /// set.
+    /// </summary>
+    private static bool AllReversed(Line line) =>
+        line.Segments.Count > 0 &&
+        line.Segments.All(s => (s.Style.Format & Format.Reverse) == Format.Reverse);
+
+    /// <summary>
+    /// Returns true when NO segment in <paramref name="line"/> has <see cref="Format.Reverse"/>
+    /// set.
+    /// </summary>
+    private static bool NoneReversed(Line line) =>
+        line.Segments.All(s => (s.Style.Format & Format.Reverse) == Format.None);
+
+    /// <summary>
+    /// Measures the total display width of all segments in a row.
+    /// </summary>
+    private static int RowDisplayWidth(Line line) =>
+        line.Segments.Sum(s => DisplayWidth.Measure(s.Text));
+
+    /// <summary>Returns true if any segment text in the line contains <paramref name="substring"/>.</summary>
+    private static bool AnySegmentContains(Line line, string substring) =>
+        line.Segments.Any(s => s.Text.Contains(substring, StringComparison.Ordinal));
+
+    // ── Initial state ────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void InitiallyEmptySelectedIndexIsMinusOne()
+    {
+        ScrollableList list = new();
+        Assert.Equal(-1, list.SelectedIndex);
+        Assert.Equal(0, list.Count);
+    }
+
+    [Fact]
+    public void AfterSetItemsNonEmptySelectedIndexIsZero()
+    {
+        ScrollableList list = new();
+        list.SetItems(Items("a", "b", "c"));
+        Assert.Equal(0, list.SelectedIndex);
+        Assert.Equal(3, list.Count);
+    }
+
+    [Fact]
+    public void AfterSetItemsEmptySelectedIndexIsMinusOne()
+    {
+        ScrollableList list = new();
+        list.SetItems(Items());
+        Assert.Equal(-1, list.SelectedIndex);
+    }
+
+    [Fact]
+    public void DefaultMaxRowsIsTen()
+    {
+        ScrollableList list = new();
+        Assert.Equal(10, list.MaxRows);
+    }
+
+    [Fact]
+    public void MaxRowsBelowOneTreatedAsOne()
+    {
+        ScrollableList list = new();
+        list.MaxRows = 0;
+        Assert.Equal(1, list.MaxRows);
+        list.MaxRows = -5;
+        Assert.Equal(1, list.MaxRows);
+    }
+
+    // ── MoveDown / MoveUp ────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void MoveDownAdvancesSelection()
+    {
+        ScrollableList list = new();
+        list.SetItems(Items("a", "b", "c"));
+        list.MoveDown();
+        Assert.Equal(1, list.SelectedIndex);
+        list.MoveDown();
+        Assert.Equal(2, list.SelectedIndex);
+    }
+
+    [Fact]
+    public void MoveDownClampsAtLastItem()
+    {
+        ScrollableList list = new();
+        list.SetItems(Items("a", "b"));
+        list.MoveDown(); // → 1 (last)
+        list.MoveDown(); // no-op
+        Assert.Equal(1, list.SelectedIndex);
+    }
+
+    [Fact]
+    public void MoveUpMovesSelectionUp()
+    {
+        ScrollableList list = new();
+        list.SetItems(Items("a", "b", "c"));
+        list.MoveDown();
+        list.MoveDown();
+        list.MoveUp();
+        Assert.Equal(1, list.SelectedIndex);
+    }
+
+    [Fact]
+    public void MoveUpClampsAtZero()
+    {
+        ScrollableList list = new();
+        list.SetItems(Items("a", "b"));
+        list.MoveUp(); // already at 0, no-op
+        Assert.Equal(0, list.SelectedIndex);
+    }
+
+    [Fact]
+    public void MoveDownEmptyListIsNoOp()
+    {
+        ScrollableList list = new();
+        list.MoveDown();
+        Assert.Equal(-1, list.SelectedIndex);
+    }
+
+    [Fact]
+    public void MoveUpEmptyListIsNoOp()
+    {
+        ScrollableList list = new();
+        list.MoveUp();
+        Assert.Equal(-1, list.SelectedIndex);
+    }
+
+    // ── Auto-scroll (viewport) ───────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void RenderReturnsAtMostMaxRows()
+    {
+        ScrollableList list = new();
+        list.MaxRows = 3;
+        list.SetItems(Items("a", "b", "c", "d", "e"));
+        IReadOnlyList<Line> rows = list.Render(80);
+        Assert.Equal(3, rows.Count);
+    }
+
+    [Fact]
+    public void RenderShowsAllItemsWhenCountLessThanMaxRows()
+    {
+        ScrollableList list = new();
+        list.MaxRows = 10;
+        list.SetItems(Items("a", "b"));
+        IReadOnlyList<Line> rows = list.Render(80);
+        Assert.Equal(2, rows.Count);
+    }
+
+    [Fact]
+    public void AutoScrollMovingDownPastBottomScrollsViewport()
+    {
+        // 5 items, viewport = 3. Start: shows [0,1,2]. Move down until selected = 3 (past bottom).
+        ScrollableList list = new();
+        list.MaxRows = 3;
+        list.SetItems(Items("a", "b", "c", "d", "e"));
+
+        list.MoveDown(); // → 1
+        list.MoveDown(); // → 2
+        list.MoveDown(); // → 3, top should scroll to 1
+
+        IReadOnlyList<Line> rows = list.Render(80);
+        Assert.Equal(3, rows.Count);
+
+        // The selected row must appear somewhere in the rendered output and carry Format.Reverse.
+        Assert.Contains(rows, AllReversed);
+    }
+
+    [Fact]
+    public void AutoScrollSelectedItemAlwaysVisibleAfterMovingDown()
+    {
+        // 10 items, viewport = 3. Move to the last item and confirm it's in the viewport.
+        ScrollableList list = new();
+        list.MaxRows = 3;
+        list.SetItems(Items("0", "1", "2", "3", "4", "5", "6", "7", "8", "9"));
+
+        for (int i = 0; i < 9; i++)
+            list.MoveDown();
+
+        Assert.Equal(9, list.SelectedIndex);
+
+        IReadOnlyList<Line> rows = list.Render(80);
+        Assert.Equal(3, rows.Count);
+        // Last rendered row should be the selected (reversed) row.
+        Assert.True(AllReversed(rows[^1]));
+    }
+
+    [Fact]
+    public void AutoScrollMovingUpScrollsBackMinimally()
+    {
+        // 5 items, viewport = 3. Move down past viewport then back up.
+        ScrollableList list = new();
+        list.MaxRows = 3;
+        list.SetItems(Items("a", "b", "c", "d", "e"));
+
+        list.MoveDown();
+        list.MoveDown();
+        list.MoveDown(); // selected = 3, top = 1, viewport shows [1,2,3]
+        list.MoveUp();   // selected = 2, still within [1,2,3] — no scroll needed
+
+        IReadOnlyList<Line> rows = list.Render(80);
+        Assert.Equal(3, rows.Count);
+        // Selected (item index 2) is the middle row of the viewport [1,2,3].
+        Assert.True(AllReversed(rows[1]));
+    }
+
+    [Fact]
+    public void AutoScrollMoveUpScrollsViewportWhenSelectionLeavesTop()
+    {
+        // Move selection back to 0 from scrolled-down position.
+        ScrollableList list = new();
+        list.MaxRows = 3;
+        list.SetItems(Items("a", "b", "c", "d", "e"));
+
+        // Scroll down
+        list.MoveDown();
+        list.MoveDown();
+        list.MoveDown(); // selected = 3, top scrolled to 1
+
+        // Scroll all the way back up
+        list.MoveUp();
+        list.MoveUp();
+        list.MoveUp();
+        Assert.Equal(0, list.SelectedIndex);
+
+        IReadOnlyList<Line> rows = list.Render(80);
+        Assert.Equal(3, rows.Count);
+        Assert.True(AllReversed(rows[0]));
+    }
+
+    // ── Selected-row highlight ───────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void SelectedRowHasFormatReverse()
+    {
+        ScrollableList list = new();
+        list.SetItems(Items("alpha", "beta", "gamma"));
+        list.MoveDown(); // selected = 1
+
+        IReadOnlyList<Line> rows = list.Render(80);
+
+        Assert.True(AllReversed(rows[1]));
+    }
+
+    [Fact]
+    public void NonSelectedRowsDoNotHaveFormatReverse()
+    {
+        ScrollableList list = new();
+        list.SetItems(Items("alpha", "beta", "gamma"));
+
+        IReadOnlyList<Line> rows = list.Render(80);
+
+        // row 0 is selected; rows 1 and 2 must not be reversed.
+        Assert.True(NoneReversed(rows[1]));
+        Assert.True(NoneReversed(rows[2]));
+    }
+
+    [Fact]
+    public void SelectedRowPreservesExistingFgBgAndOtherFormats()
+    {
+        // Item has Bold + Dim + a foreground color; Reverse must be ORed in, not replacing.
+        Line item = new LineBuilder()
+            .Append("hello", new Style(Foreground: Color.FromRgb(1, 2, 3), Format: Format.Bold | Format.Dim))
+            .Build();
+
+        ScrollableList list = new();
+        list.SetItems([item]);
+
+        IReadOnlyList<Line> rows = list.Render(80);
+        Segment seg = rows[0].Segments[0];
+
+        Assert.Equal(Color.FromRgb(1, 2, 3), seg.Style.Foreground);
+        Assert.True((seg.Style.Format & Format.Bold) == Format.Bold);
+        Assert.True((seg.Style.Format & Format.Dim) == Format.Dim);
+        Assert.True((seg.Style.Format & Format.Reverse) == Format.Reverse);
+    }
+
+    [Fact]
+    public void SelectedRowPaddedToFullWidth()
+    {
+        // Short item (3 cols) with width = 20: padded row should be exactly 20 cols wide.
+        ScrollableList list = new();
+        list.SetItems([Item("abc")]);
+
+        IReadOnlyList<Line> rows = list.Render(20);
+        int totalWidth = RowDisplayWidth(rows[0]);
+
+        Assert.Equal(20, totalWidth);
+    }
+
+    [Fact]
+    public void NonSelectedRowNotPadded()
+    {
+        ScrollableList list = new();
+        list.SetItems(Items("abc", "xyz"));
+        // selected is "abc" (row 0); "xyz" (row 1) must NOT be padded.
+        IReadOnlyList<Line> rows = list.Render(20);
+        int secondRowWidth = RowDisplayWidth(rows[1]);
+        Assert.True(secondRowWidth <= 3); // "xyz" = 3 cols, no padding
+    }
+
+    // ── Width-aware truncation ────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void RenderTruncatesOverWideItemToSingleRow()
+    {
+        // Item is wider than the available width.
+        ScrollableList list = new();
+        list.SetItems([Item("abcdefghijklmnop")]); // 16 chars
+        IReadOnlyList<Line> rows = list.Render(5);
+        Assert.Single(rows);
+    }
+
+    [Fact]
+    public void RenderTruncatedRowDisplayWidthDoesNotExceedWidth()
+    {
+        // Non-selected row must not exceed the given width.
+        // Use a two-item list so the first item is selected and the second is not.
+        ScrollableList list = new();
+        list.SetItems(Items("xxxxxx_short", "abcdefghijklmnop")); // second item > 5 cols
+        list.MoveDown(); // selected = 1 (the over-wide item)
+        IReadOnlyList<Line> rows = list.Render(5);
+        // row 1 is the selected (padded) row — check its width <= 5.
+        // row 0 is not padded — also check <= 5.
+        Assert.True(RowDisplayWidth(rows[0]) <= 5);
+        Assert.True(RowDisplayWidth(rows[1]) <= 5);
+    }
+
+    [Fact]
+    public void RenderDoesNotSplitWideChar()
+    {
+        // CJK char is 2 cols. At width = 2, "A中" → first row is "A" (1 col) because "中" (2 cols)
+        // would exceed width when combined with "A" (total 3 > 2). LineWrapper moves it to the
+        // second row; we discard that second row.
+        Line item = new LineBuilder().Text("A中").Build();
+        ScrollableList list = new();
+        list.SetItems([item]);
+
+        IReadOnlyList<Line> rows = list.Render(2);
+        Assert.Single(rows);
+
+        // First row must be at most 2 display columns and must not end mid-wide-char.
+        int w = RowDisplayWidth(rows[0]);
+        Assert.InRange(w, 0, 2);
+    }
+
+    // ── Multi-select ─────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void ToggleCurrentAddsThenRemovesFromCheckedIndices()
+    {
+        ScrollableList list = new(multiSelect: true);
+        list.SetItems(Items("a", "b", "c"));
+
+        list.ToggleCurrent(); // check index 0
+        Assert.Contains(0, list.CheckedIndices);
+
+        list.ToggleCurrent(); // uncheck index 0
+        Assert.DoesNotContain(0, list.CheckedIndices);
+    }
+
+    [Fact]
+    public void ToggleCurrentMultipleItemsCheckedIndicesAscending()
+    {
+        ScrollableList list = new(multiSelect: true);
+        list.SetItems(Items("a", "b", "c", "d"));
+
+        list.MoveDown();      // selected = 1
+        list.ToggleCurrent(); // check 1
+        list.MoveDown();      // selected = 2
+        list.ToggleCurrent(); // check 2
+        list.MoveUp();        // selected = 1
+        list.MoveUp();        // selected = 0
+        list.ToggleCurrent(); // check 0
+
+        int[] expected = [0, 1, 2];
+        Assert.Equal(expected, list.CheckedIndices.ToArray());
+    }
+
+    [Fact]
+    public void ToggleCurrentWhenNotMultiSelectIsNoOp()
+    {
+        ScrollableList list = new(multiSelect: false);
+        list.SetItems(Items("a", "b"));
+        list.ToggleCurrent();
+        Assert.Empty(list.CheckedIndices);
+    }
+
+    [Fact]
+    public void ToggleCurrentEmptyListIsNoOp()
+    {
+        ScrollableList list = new(multiSelect: true);
+        list.ToggleCurrent();
+        Assert.Empty(list.CheckedIndices);
+    }
+
+    [Fact]
+    public void RenderCheckedRowShowsCheckedMarker()
+    {
+        ScrollableList list = new(multiSelect: true);
+        list.SetItems(Items("alpha"));
+        list.ToggleCurrent(); // check index 0
+
+        IReadOnlyList<Line> rows = list.Render(80);
+        Assert.True(AnySegmentContains(rows[0], "[x]"));
+    }
+
+    [Fact]
+    public void RenderUncheckedRowShowsUncheckedMarker()
+    {
+        ScrollableList list = new(multiSelect: true);
+        list.SetItems(Items("alpha", "beta"));
+        list.ToggleCurrent(); // check 0
+        list.MoveDown();      // selected = 1 (unchecked)
+
+        IReadOnlyList<Line> rows = list.Render(80);
+        // row 0 = item[0]: checked → "[x]"
+        Assert.True(AnySegmentContains(rows[0], "[x]"));
+        // row 1 = item[1]: not checked → "[ ]"
+        Assert.True(AnySegmentContains(rows[1], "[ ]"));
+    }
+
+    [Fact]
+    public void RenderNoMarkersWhenNotMultiSelect()
+    {
+        ScrollableList list = new(multiSelect: false);
+        list.SetItems(Items("hello"));
+        IReadOnlyList<Line> rows = list.Render(80);
+        string allText = string.Concat(rows[0].Segments.Select(s => s.Text));
+        Assert.DoesNotContain("[", allText, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void MultiSelectCheckedStateVisibleIndependentlyOfSelection()
+    {
+        // Item[0] is checked but NOT selected; item[1] is selected but NOT checked.
+        ScrollableList list = new(multiSelect: true);
+        list.SetItems(Items("a", "b"));
+        list.ToggleCurrent(); // check 0
+        list.MoveDown();      // selected = 1
+
+        IReadOnlyList<Line> rows = list.Render(80);
+        // row 0 = item[0]: checked, not selected.
+        Assert.True(AnySegmentContains(rows[0], "[x]"));
+        Assert.True(NoneReversed(rows[0]));
+
+        // row 1 = item[1]: not checked, selected.
+        Assert.True(AnySegmentContains(rows[1], "[ ]"));
+        Assert.True(AllReversed(rows[1]));
+    }
+
+    // ── SetItems resets state ────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void SetItemsResetsSelectionToZero()
+    {
+        ScrollableList list = new();
+        list.SetItems(Items("a", "b", "c"));
+        list.MoveDown();
+        list.MoveDown();
+        Assert.Equal(2, list.SelectedIndex);
+
+        list.SetItems(Items("x", "y"));
+        Assert.Equal(0, list.SelectedIndex);
+    }
+
+    [Fact]
+    public void SetItemsResetsScrollToTop()
+    {
+        // Scroll down, then SetItems — first rendered row should be item[0] again.
+        ScrollableList list = new();
+        list.MaxRows = 2;
+        list.SetItems(Items("a", "b", "c", "d"));
+        list.MoveDown();
+        list.MoveDown(); // selected = 2, scrolled down
+
+        list.SetItems(Items("x", "y", "z"));
+        IReadOnlyList<Line> rows = list.Render(80);
+
+        // After reset the first rendered row is item[0] ("x") and it should be the selected (reversed) row.
+        Assert.True(AllReversed(rows[0]));
+    }
+
+    [Fact]
+    public void SetItemsClearsCheckedIndices()
+    {
+        ScrollableList list = new(multiSelect: true);
+        list.SetItems(Items("a", "b", "c"));
+        list.ToggleCurrent();
+        list.MoveDown();
+        list.ToggleCurrent();
+        Assert.Equal(2, list.CheckedIndices.Count);
+
+        list.SetItems(Items("x", "y"));
+        Assert.Empty(list.CheckedIndices);
+    }
+
+    [Fact]
+    public void SetItemsToEmptySelectedIndexMinusOne()
+    {
+        ScrollableList list = new();
+        list.SetItems(Items("a"));
+        list.SetItems([]);
+        Assert.Equal(-1, list.SelectedIndex);
+        Assert.Equal(0, list.Count);
+    }
+
+    // ── Empty-list edge cases ────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void RenderEmptyListReturnsEmpty()
+    {
+        ScrollableList list = new();
+        Assert.Empty(list.Render(80));
+    }
+
+    [Fact]
+    public void EmptyListMoveDownMoveUpAreNoOps()
+    {
+        ScrollableList list = new();
+        list.MoveDown();
+        list.MoveUp();
+        Assert.Equal(-1, list.SelectedIndex);
+    }
+
+    [Fact]
+    public void EmptyListToggleCurrentIsNoOp()
+    {
+        ScrollableList list = new(multiSelect: true);
+        list.ToggleCurrent();
+        Assert.Empty(list.CheckedIndices);
+    }
+
+    // ── Width edge cases ─────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void RenderWidthBelowOneTreatedAsOne()
+    {
+        ScrollableList list = new();
+        list.SetItems(Items("abc"));
+        IReadOnlyList<Line> rows = list.Render(0);
+        Assert.Single(rows);
+
+        rows = list.Render(-5);
+        Assert.Single(rows);
+    }
+
+    // ── MaxRows change between renders (B1 / B2 regression) ─────────────────────────────────
+
+    [Fact]
+    public void RenderDoesNotThrowWhenMaxRowsGrowsAfterScrollingDown()
+    {
+        // B1 repro: 10 items, MaxRows=3, scroll to bottom (_top=7), then grow MaxRows to 10.
+        // Before the fix, end = 7 + 10 = 17 → IndexOutOfRange on _items[17..].
+        ScrollableList list = new();
+        list.MaxRows = 3;
+        list.SetItems(Items("0", "1", "2", "3", "4", "5", "6", "7", "8", "9"));
+
+        for (int i = 0; i < 9; i++)
+            list.MoveDown();
+
+        Assert.Equal(9, list.SelectedIndex);
+
+        list.MaxRows = 10; // grow before render
+
+        IReadOnlyList<Line> rows = list.Render(80); // must not throw
+        Assert.Equal(10, rows.Count);               // all 10 items visible now
+        Assert.Contains(rows, AllReversed);          // selected row present and highlighted
+    }
+
+    [Fact]
+    public void RenderKeepsSelectionVisibleWhenMaxRowsShrinks()
+    {
+        // B2 repro: 10 items, MaxRows=10, navigate to last item (selection=9, _top=0),
+        // then shrink MaxRows to 3. Before the fix, Render returned items [0,1,2] and
+        // the selected row (index 9) was absent from the output.
+        ScrollableList list = new();
+        list.MaxRows = 10;
+        list.SetItems(Items("0", "1", "2", "3", "4", "5", "6", "7", "8", "9"));
+
+        for (int i = 0; i < 9; i++)
+            list.MoveDown();
+
+        Assert.Equal(9, list.SelectedIndex);
+
+        list.MaxRows = 3; // shrink before render
+
+        IReadOnlyList<Line> rows = list.Render(80);
+        Assert.Equal(3, rows.Count);         // exactly MaxRows rows returned
+        Assert.Contains(rows, AllReversed);  // selected row must appear within the window
+        // The last rendered row must be item[9] (the selected one), highlighted.
+        Assert.True(AllReversed(rows[^1]));
+    }
+
+    // ── CheckedIndices returns a snapshot (N1) ───────────────────────────────────────────────
+
+    [Fact]
+    public void CheckedIndicesReturnsSnapshotNotLiveSet()
+    {
+        ScrollableList list = new(multiSelect: true);
+        list.SetItems(Items("a", "b", "c"));
+        list.ToggleCurrent(); // check index 0
+
+        IReadOnlyCollection<int> snapshot = list.CheckedIndices;
+        Assert.Contains(0, snapshot);
+
+        // Toggle again (uncheck 0, check 1) — snapshot must be unaffected.
+        list.ToggleCurrent();          // uncheck 0
+        list.MoveDown();
+        list.ToggleCurrent();          // check 1
+
+        // The snapshot captured before those mutations still shows only [0].
+        Assert.Contains(0, snapshot);
+        Assert.Single(snapshot);
+
+        // The current CheckedIndices reflects the new state.
+        Assert.Contains(1, list.CheckedIndices);
+        Assert.DoesNotContain(0, list.CheckedIndices);
+    }
+}
diff --git a/tests/Dcli.Tests/ScrollbackModelTests.cs b/tests/Dcli.Tests/ScrollbackModelTests.cs
new file mode 100644
index 0000000..b25a505
--- /dev/null
+++ b/tests/Dcli.Tests/ScrollbackModelTests.cs
@@ -0,0 +1,755 @@
+using Dcli.Internal.RenderLoop;
+using Dcli.Internal.Scrollback;
+using Dcli.Internal.Scrollback.Commands;
+using Xunit;
+
+namespace Dcli.Tests;
+
+/// <summary>
+/// Behavioural tests for §9 (Chunk A): <see cref="ILineObject"/> / <see cref="TextBlock"/>,
+/// the commit-horizon / live-window overflow logic in <see cref="ScrollbackModel"/>, and the
+/// <see cref="LiveBlock"/> streaming-block contract.
+/// </summary>
+/// <remarks>
+/// All tests operate purely on the model (no real terminal, no loop thread). The
+/// <see cref="RenderModel"/> is constructed with a <see cref="FixedSizeSource"/> so terminal
+/// dimensions are explicit.
+/// </remarks>
+public sealed class ScrollbackModelTests
+{
+    // ─── Helpers ─────────────────────────────────────────────────────────────
+
+    private static RenderModel MakeModel(int cols = 80, int rows = 24) =>
+        new(new FixedSizeSource(cols, rows));
+
+    private static Line PlainLine(string text) =>
+        new([new Segment(text)]);
+
+    private static Line StyledLine(string text, Style style) =>
+        new([new Segment(text, style)]);
+
+    /// <summary>Extracts all text from a sequence of visual rows.</summary>
+    private static string JoinRows(IEnumerable<Line> rows) =>
+        string.Join("\n", rows.Select(r => string.Concat(r.Segments.Select(s => s.Text))));
+
+    private sealed class FixedSizeSource(int cols, int rows) : ITerminalSizeSource
+    {
+        public (int Columns, int Rows) GetSize() => (cols, rows);
+    }
+
+    // ─── 9.1: TextBlock.Render ───────────────────────────────────────────────
+
+    [Fact]
+    public void TextBlockSingleLineRendersAsOneRow()
+    {
+        TextBlock block = new(PlainLine("hello"));
+        IReadOnlyList<Line> rows = block.Render(80);
+
+        Assert.Single(rows);
+        Assert.Equal("hello", rows[0].Segments[0].Text);
+    }
+
+    [Fact]
+    public void TextBlockLongLineWrapsToMultipleRows()
+    {
+        // A 10-char line rendered at width 4 should wrap to at least 3 rows.
+        TextBlock block = new(PlainLine("0123456789"));
+        IReadOnlyList<Line> rows = block.Render(4);
+
+        Assert.True(rows.Count >= 3, $"expected ≥3 rows at width 4, got {rows.Count}");
+        // All chars must be present across all rows.
+        string combined = JoinRows(rows).Replace("\n", string.Empty, StringComparison.Ordinal);
+        Assert.Equal("0123456789", combined);
+    }
+
+    [Fact]
+    public void TextBlockCjkCharactersCountedAs2Columns()
+    {
+        // Each CJK char is 2 columns wide. At width 4, each pair of CJK chars fills one row.
+        // "日本語" = 3 × 2 = 6 columns → at width 4: ["日本", "語"] = 2 rows.
+        TextBlock block = new(PlainLine("日本語"));
+        IReadOnlyList<Line> rows = block.Render(4);
+
+        Assert.Equal(2, rows.Count);
+        Assert.Equal("日本", rows[0].Segments[0].Text);
+        Assert.Equal("語", rows[1].Segments[0].Text);
+    }
+
+    [Fact]
+    public void TextBlockZeroWidthCharacterDoesNotInflateRowCount()
+    {
+        // U+200B (ZWSP) is zero-width. "AB" + ZWSP + "C" at width 3:
+        // "AB" (2 cols) + ZWSP (0 cols) = 2 cols → fits in row 1; "C" (1 col) in row 2 or same.
+        // The ZWSP must not force an extra wrap by being counted as a column.
+        string text = "AB​C";
+        TextBlock block = new(PlainLine(text));
+        IReadOnlyList<Line> rows = block.Render(3);
+        // Without ZWSP "ABC" at width 3 is 1 row; with ZWSP still fits in ≤2 rows.
+        Assert.True(rows.Count <= 2, $"ZWSP inflated row count to {rows.Count}");
+    }
+
+    [Fact]
+    public void TextBlockMultipleLogicalLinesEachWrapIndependently()
+    {
+        // Two 5-char lines at width 3 → each wraps into 2 rows → 4 total.
+        TextBlock block = new([PlainLine("ABCDE"), PlainLine("12345")]);
+        IReadOnlyList<Line> rows = block.Render(3);
+
+        Assert.Equal(4, rows.Count);
+    }
+
+    [Fact]
+    public void TextBlockEmptyBlockProducesOneEmptyRow()
+    {
+        TextBlock block = new(Array.Empty<Line>());
+        IReadOnlyList<Line> rows = block.Render(80);
+
+        Assert.Single(rows);
+        Assert.Empty(rows[0].Segments);
+    }
+
+    [Fact]
+    public void TextBlockStylePreservedAcrossWrappedRows()
+    {
+        Style bold = new(Format: Format.Bold);
+        TextBlock block = new(StyledLine("ABCDE", bold));
+        IReadOnlyList<Line> rows = block.Render(3);
+
+        foreach (Line row in rows)
+        {
+            foreach (Segment seg in row.Segments)
+                Assert.Equal(bold, seg.Style);
+        }
+    }
+
+    // ─── 9.2: Commit horizon + bounded live window ───────────────────────────
+
+    [Fact]
+    public void ScrollbackAppendWithinBudgetKeepsAllRowsLive()
+    {
+        // 3 single-row lines in a 10-row terminal: all stay live.
+        RenderModel model = MakeModel(cols: 80, rows: 10);
+        ScrollbackModel sm = new();
+
+        sm.Append(new TextBlock(PlainLine("line1")), model);
+        sm.Append(new TextBlock(PlainLine("line2")), model);
+        sm.Append(new TextBlock(PlainLine("line3")), model);
+        sm.PrePaint(model);
+
+        Assert.Equal(3, model.LiveWindowRows.Count);
+        Assert.Empty(model.NewlyCommittedRows);
+    }
+
+    [Fact]
+    public void ScrollbackOverflowCommitsTopLine()
+    {
+        // maxHeight = rows - fixedHeight = 3 - 0 = 3.
+        // Append 4 single-row lines → the topmost one must commit.
+        RenderModel model = MakeModel(cols: 80, rows: 3);
+        ScrollbackModel sm = new();
+
+        sm.Append(new TextBlock(PlainLine("A")), model);
+        sm.Append(new TextBlock(PlainLine("B")), model);
+        sm.Append(new TextBlock(PlainLine("C")), model);
+        sm.Append(new TextBlock(PlainLine("D")), model);
+        sm.PrePaint(model);
+
+        // Live window must be exactly maxHeight = 3.
+        Assert.Equal(3, model.LiveWindowRows.Count);
+        // One row committed — the first one appended.
+        Assert.Single(model.NewlyCommittedRows);
+        Assert.Equal("A", model.NewlyCommittedRows[0].Segments[0].Text);
+    }
+
+    [Fact]
+    public void ScrollbackLiveWindowNeverExceedsMaxHeight()
+    {
+        // maxHeight = 2. Append 10 single-row lines; after each PrePaint live ≤ 2.
+        RenderModel model = MakeModel(cols: 80, rows: 2);
+        ScrollbackModel sm = new();
+
+        for (int i = 0; i < 10; i++)
+        {
+            sm.Append(new TextBlock(PlainLine($"line{i}")), model);
+            sm.PrePaint(model);
+            Assert.True(model.LiveWindowRows.Count <= 2,
+                $"after append {i}: live={model.LiveWindowRows.Count} > maxHeight 2");
+            sm.ClearCommitted(model);
+        }
+    }
+
+    [Fact]
+    public void ScrollbackCommittedRowsResetAfterClearCommitted()
+    {
+        // Frame 1: overflow → NewlyCommittedRows non-empty.
+        // After ClearCommitted → empty.
+        // Frame 2 with no new content → still empty.
+        RenderModel model = MakeModel(cols: 80, rows: 2);
+        ScrollbackModel sm = new();
+
+        sm.Append(new TextBlock(PlainLine("A")), model);
+        sm.Append(new TextBlock(PlainLine("B")), model);
+        sm.Append(new TextBlock(PlainLine("C")), model);
+
+        sm.PrePaint(model);
+        Assert.NotEmpty(model.NewlyCommittedRows);
+
+        sm.ClearCommitted(model);
+        Assert.Empty(model.NewlyCommittedRows);
+
+        // Second frame with no additional appends.
+        model.MarkDirty();
+        sm.PrePaint(model);
+        Assert.Empty(model.NewlyCommittedRows);
+    }
+
+    [Fact]
+    public void ScrollbackCommittedRowsEmittedOnlyOnce()
+    {
+        // The core "commit-once / never-rewrite" invariant: rows that commit in frame 1
+        // must not appear in NewlyCommittedRows on frame 2.
+        RenderModel model = MakeModel(cols: 80, rows: 2);
+        ScrollbackModel sm = new();
+
+        sm.Append(new TextBlock(PlainLine("A")), model);
+        sm.Append(new TextBlock(PlainLine("B")), model);
+        sm.Append(new TextBlock(PlainLine("overflow")), model);
+
+        sm.PrePaint(model);
+        Assert.NotEmpty(model.NewlyCommittedRows);
+
+        sm.ClearCommitted(model);
+        model.ClearDirty();
+
+        // Frame 2: no new appends.
+        model.MarkDirty();
+        sm.PrePaint(model);
+        Assert.Empty(model.NewlyCommittedRows);
+    }
+
+    [Fact]
+    public void ScrollbackZeroRowTerminalAllLinesCommit()
+    {
+        // maxHeight = max(0, 0 - 0) = 0 → every appended row immediately commits.
+        RenderModel model = MakeModel(cols: 80, rows: 0);
+        ScrollbackModel sm = new();
+
+        sm.Append(new TextBlock(PlainLine("A")), model);
+        sm.Append(new TextBlock(PlainLine("B")), model);
+        sm.PrePaint(model);
+
+        Assert.Empty(model.LiveWindowRows);
+        Assert.Equal(2, model.NewlyCommittedRows.Count);
+    }
+
+    [Fact]
+    public void ScrollbackWrappedBlockRowCountUsedForOverflowMath()
+    {
+        // width=2 → "ABCD" wraps to 2 rows ("AB", "CD"). maxHeight = 3.
+        // Two 2-row blocks → 4 rows > 3 → first block (2 rows) commits.
+        RenderModel model = MakeModel(cols: 2, rows: 3);
+        ScrollbackModel sm = new();
+
+        sm.Append(new TextBlock(PlainLine("ABCD")), model);
+        sm.Append(new TextBlock(PlainLine("EFGH")), model);
+        sm.PrePaint(model);
+
+        Assert.Equal(2, model.NewlyCommittedRows.Count); // "AB" and "CD"
+        Assert.Equal(2, model.LiveWindowRows.Count);     // "EF" and "GH"
+    }
+
+    [Fact]
+    public void ScrollbackFixedRegionReducesLiveWindowBudget()
+    {
+        // rows=5, fixedRegion has 2 rows → maxHeight = 3.
+        // Append 4 single-row lines → overflow.
+        RenderModel model = MakeModel(cols: 80, rows: 5);
+        model.FixedRegionRows =
+        [
+            new Line([new Segment("status1")]),
+            new Line([new Segment("status2")])
+        ];
+        ScrollbackModel sm = new();
+
+        for (int i = 0; i < 4; i++)
+            sm.Append(new TextBlock(PlainLine($"line{i}")), model);
+        sm.PrePaint(model);
+
+        Assert.True(model.LiveWindowRows.Count <= 3,
+            $"live={model.LiveWindowRows.Count} > maxHeight 3 with fixedRegion=2");
+        Assert.NotEmpty(model.NewlyCommittedRows);
+    }
+
+    // ─── 9.3: LiveBlock streaming ─────────────────────────────────────────────
+
+    [Fact]
+    public void LiveBlockAppendTextAppearsInLiveWindow()
+    {
+        RenderModel model = MakeModel(cols: 80, rows: 10);
+        ScrollbackModel sm = new();
+
+        LiveBlock block = sm.BeginLive(model);
+        block.AppendText("hello ");
+        block.AppendText("world");
+        sm.PrePaint(model);
+
+        string liveText = JoinRows(model.LiveWindowRows);
+        Assert.Contains("hello world", liveText, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void LiveBlockSetContentReplacesAppendedText()
+    {
+        RenderModel model = MakeModel(cols: 80, rows: 10);
+        ScrollbackModel sm = new();
+
+        LiveBlock block = sm.BeginLive(model);
+        block.AppendText("initial");
+        block.SetContent([PlainLine("replaced content")]);
+        sm.PrePaint(model);
+
+        string liveText = JoinRows(model.LiveWindowRows);
+        Assert.Contains("replaced content", liveText, StringComparison.Ordinal);
+        Assert.DoesNotContain("initial", liveText, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void LiveBlockAppendTextAfterSetContentIsIgnored()
+    {
+        RenderModel model = MakeModel(cols: 80, rows: 10);
+        ScrollbackModel sm = new();
+
+        LiveBlock block = sm.BeginLive(model);
+        block.SetContent([PlainLine("replacement")]);
+        block.AppendText("ignored");
+        sm.PrePaint(model);
+
+        string liveText = JoinRows(model.LiveWindowRows);
+        Assert.Contains("replacement", liveText, StringComparison.Ordinal);
+        Assert.DoesNotContain("ignored", liveText, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void LiveBlockCommitRemovesBlockFromLiveWindow()
+    {
+        RenderModel model = MakeModel(cols: 80, rows: 10);
+        ScrollbackModel sm = new();
+
+        LiveBlock block = sm.BeginLive(model);
+        block.AppendText("streaming...");
+
+        // Frame 1: block is live.
+        sm.PrePaint(model);
+        Assert.NotEmpty(model.LiveWindowRows);
+        sm.ClearCommitted(model);
+        model.ClearDirty();
+
+        // Commit the block then paint frame 2.
+        sm.CommitLiveBlock(block, model);
+        sm.PrePaint(model);
+
+        Assert.Empty(model.LiveWindowRows);                // no longer live
+        Assert.NotEmpty(model.NewlyCommittedRows);         // drained into committed
+        Assert.Contains("streaming...",
+            model.NewlyCommittedRows[0].Segments[0].Text,
+            StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void LiveBlockCommitAfterSetContentFreezesReplacedContent()
+    {
+        // "replace-then-commit": the replaced content is what freezes.
+        RenderModel model = MakeModel(cols: 80, rows: 10);
+        ScrollbackModel sm = new();
+
+        LiveBlock block = sm.BeginLive(model);
+        block.AppendText("draft");
+        block.SetContent([PlainLine("final")]);
+        sm.CommitLiveBlock(block, model);
+        sm.PrePaint(model);
+
+        string committed = JoinRows(model.NewlyCommittedRows);
+        Assert.Contains("final", committed, StringComparison.Ordinal);
+        Assert.DoesNotContain("draft", committed, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void LiveBlockAppendTextAfterCommitIsIgnored()
+    {
+        RenderModel model = MakeModel(cols: 80, rows: 10);
+        ScrollbackModel sm = new();
+
+        LiveBlock block = sm.BeginLive(model);
+        block.AppendText("before");
+        sm.CommitLiveBlock(block, model);
+        block.AppendText("after-commit-ignored");
+
+        sm.PrePaint(model);
+
+        string committed = JoinRows(model.NewlyCommittedRows);
+        Assert.Contains("before", committed, StringComparison.Ordinal);
+        Assert.DoesNotContain("after-commit-ignored", committed, StringComparison.Ordinal);
+    }
+
+    // ─── Commands ─────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void AppendLineCommandAddsLineToLiveWindow()
+    {
+        RenderModel model = MakeModel();
+        ScrollbackModel sm = new();
+        ILoopCommand cmd = new AppendLineCommand(sm, PlainLine("via command"));
+        cmd.Apply(model);
+        sm.PrePaint(model);
+
+        Assert.Contains("via command", JoinRows(model.LiveWindowRows), StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void BeginLiveCommandCapturesBlockRef()
+    {
+        RenderModel model = MakeModel();
+        ScrollbackModel sm = new();
+
+        LiveBlock? captured = null;
+        ILoopCommand cmd = new BeginLiveCommand(sm, b => captured = b);
+        cmd.Apply(model);
+
+        Assert.NotNull(captured);
+        captured.AppendText("test");
+        sm.PrePaint(model);
+
+        Assert.Contains("test", JoinRows(model.LiveWindowRows), StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void AppendTextCommandGrowsLiveBlock()
+    {
+        RenderModel model = MakeModel();
+        ScrollbackModel sm = new();
+        LiveBlock block = sm.BeginLive(model);
+
+        ILoopCommand cmd = new AppendTextCommand(block, "token");
+        cmd.Apply(model);
+        sm.PrePaint(model);
+
+        Assert.Contains("token", JoinRows(model.LiveWindowRows), StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void SetContentCommandReplacesContent()
+    {
+        RenderModel model = MakeModel();
+        ScrollbackModel sm = new();
+        LiveBlock block = sm.BeginLive(model);
+        block.AppendText("old");
+
+        ILoopCommand cmd = new SetContentCommand(block, [PlainLine("new content")]);
+        cmd.Apply(model);
+        sm.PrePaint(model);
+
+        string liveText = JoinRows(model.LiveWindowRows);
+        Assert.Contains("new content", liveText, StringComparison.Ordinal);
+        Assert.DoesNotContain("old", liveText, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void CommitLiveBlockCommandFreezesBlock()
+    {
+        RenderModel model = MakeModel();
+        ScrollbackModel sm = new();
+        LiveBlock block = sm.BeginLive(model);
+        block.AppendText("frozen content");
+
+        ILoopCommand cmd = new CommitLiveBlockCommand(sm, block);
+        cmd.Apply(model);
+
+        Assert.True(block.IsCommitted);
+    }
+
+    // ─── 9.2 nit: FIFO sweep stops at first non-committed object ──────────────
+
+    [Fact]
+    public void CommittedLiveBlockBehindNonCommittedObjectIsNotSweptUntilObjectAheadCommits()
+    {
+        // FIFO invariant: the sweep in PrePaint walks from the front and stops at the first
+        // object that is NOT a committed LiveBlock. A committed LiveBlock sitting behind a
+        // non-committed TextBlock is therefore NOT swept in the same frame.
+        RenderModel model = MakeModel(cols: 80, rows: 20);
+        ScrollbackModel sm = new();
+
+        // layout: [TextBlock (never committed)] [LiveBlock (committed immediately)]
+        sm.Append(new TextBlock(PlainLine("text-ahead")), model);
+        LiveBlock block = sm.BeginLive(model);
+        block.AppendText("behind-text");
+        sm.CommitLiveBlock(block, model);
+
+        sm.PrePaint(model);
+
+        // The committed LiveBlock is behind a TextBlock which has not committed.
+        // The sweep stops at the TextBlock → the committed LiveBlock is NOT drained into
+        // NewlyCommittedRows; it stays in the live window.
+        string liveText = JoinRows(model.LiveWindowRows);
+        Assert.Contains("text-ahead", liveText, StringComparison.Ordinal);
+        Assert.Contains("behind-text", liveText, StringComparison.Ordinal);
+        // Nothing committed because the TextBlock at the front did not overflow.
+        Assert.Empty(model.NewlyCommittedRows);
+    }
+
+    // ─── 9.4: Collapsible one-way expand ─────────────────────────────────────
+
+    [Fact]
+    public void CollapsibleCreatedCollapsedRendersOnlySummary()
+    {
+        Collapsible c = new(PlainLine("summary"), [PlainLine("hidden line 1"), PlainLine("hidden line 2")]);
+        IReadOnlyList<Line> rows = c.Render(80);
+
+        Assert.Single(rows);
+        Assert.Equal("summary", rows[0].Segments[0].Text);
+    }
+
+    [Fact]
+    public void CollapsibleExpandRevealsSummaryPlusHiddenLines()
+    {
+        Collapsible c = new(PlainLine("summary"), [PlainLine("detail A"), PlainLine("detail B")]);
+        c.Expand();
+        IReadOnlyList<Line> rows = c.Render(80);
+
+        // summary + 2 hidden = 3 rows
+        Assert.Equal(3, rows.Count);
+        Assert.Equal("summary", rows[0].Segments[0].Text);
+        Assert.Equal("detail A", rows[1].Segments[0].Text);
+        Assert.Equal("detail B", rows[2].Segments[0].Text);
+    }
+
+    [Fact]
+    public void CollapsibleExpandIsIdempotent()
+    {
+        // Second Expand call must not change state or throw.
+        Collapsible c = new(PlainLine("summary"), [PlainLine("hidden")]);
+        c.Expand();
+        c.Expand(); // no-op
+
+        Assert.True(c.IsExpanded);
+        IReadOnlyList<Line> rows = c.Render(80);
+        Assert.Equal(2, rows.Count); // not duplicated
+    }
+
+    [Fact]
+    public void CollapsibleCannotRecollapseAfterExpand()
+    {
+        // There is no re-collapse operation; verify Expand stays expanded forever.
+        Collapsible c = new(PlainLine("summary"), [PlainLine("hidden")]);
+        c.Expand();
+        Assert.True(c.IsExpanded);
+
+        // No collapse API — IsExpanded can only go false→true.
+        // Verify Render still returns expanded content.
+        IReadOnlyList<Line> rows = c.Render(80);
+        Assert.Equal(2, rows.Count);
+    }
+
+    [Fact]
+    public void CollapsibleInLiveWindowShowsSummaryOnly()
+    {
+        RenderModel model = MakeModel(cols: 80, rows: 10);
+        ScrollbackModel sm = new();
+
+        Collapsible c = sm.BeginCollapsible(PlainLine("summary"), [PlainLine("hidden")], model);
+        sm.PrePaint(model);
+
+        string liveText = JoinRows(model.LiveWindowRows);
+        Assert.Contains("summary", liveText, StringComparison.Ordinal);
+        Assert.DoesNotContain("hidden", liveText, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void CollapsibleExpandViaModelBecomesLive()
+    {
+        RenderModel model = MakeModel(cols: 80, rows: 10);
+        ScrollbackModel sm = new();
+
+        Collapsible c = sm.BeginCollapsible(PlainLine("summary"), [PlainLine("detail")], model);
+        sm.ExpandCollapsible(c, model);
+        sm.PrePaint(model);
+
+        string liveText = JoinRows(model.LiveWindowRows);
+        Assert.Contains("summary", liveText, StringComparison.Ordinal);
+        Assert.Contains("detail", liveText, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void CollapsibleExpandTwiceViaModelIsNoOp()
+    {
+        RenderModel model = MakeModel(cols: 80, rows: 10);
+        ScrollbackModel sm = new();
+
+        Collapsible c = sm.BeginCollapsible(PlainLine("summary"), [PlainLine("detail")], model);
+        sm.ExpandCollapsible(c, model);
+        sm.ExpandCollapsible(c, model); // idempotent
+
+        sm.PrePaint(model);
+        string liveText = JoinRows(model.LiveWindowRows);
+
+        // "detail" appears exactly once (not doubled).
+        int count = 0;
+        int idx = 0;
+        while ((idx = liveText.IndexOf("detail", idx, StringComparison.Ordinal)) >= 0)
+        {
+            count++;
+            idx += "detail".Length;
+        }
+        Assert.Equal(1, count);
+    }
+
+    // ─── 9.4: Freeze-collapsed-at-horizon ────────────────────────────────────
+
+    [Fact]
+    public void CollapsibleFreezeCollapsedAtHorizonMakesExpandNoOp()
+    {
+        // maxHeight = 2 rows. Append enough content to force the collapsible (at the front)
+        // to commit, then attempt to expand it — must be a clean no-op.
+        RenderModel model = MakeModel(cols: 80, rows: 2);
+        ScrollbackModel sm = new();
+
+        // The collapsible occupies 1 row (summary). With maxHeight=2, adding 2 more single-row
+        // TextBlocks will push the collapsible past the horizon.
+        Collapsible c = sm.BeginCollapsible(PlainLine("frozen-summary"), [PlainLine("frozen-hidden")], model);
+        sm.Append(new TextBlock(PlainLine("filler1")), model);
+        sm.Append(new TextBlock(PlainLine("filler2")), model);
+
+        sm.PrePaint(model); // collapsible overflows → committed and dropped
+        sm.ClearCommitted(model);
+
+        // The collapsible is now frozen in native scrollback. Expand must be a no-op.
+        sm.ExpandCollapsible(c, model);
+        sm.PrePaint(model);
+
+        // The live window contains the two filler lines, not the expanded hidden content.
+        string liveText = JoinRows(model.LiveWindowRows);
+        Assert.DoesNotContain("frozen-hidden", liveText, StringComparison.Ordinal);
+        // Nothing new commits (the collapsible was already committed in a previous frame).
+        Assert.Empty(model.NewlyCommittedRows);
+    }
+
+    // ─── 9.5: Oversized expansion reprints into flow ─────────────────────────
+
+    [Fact]
+    public void CollapsibleOversizedExpansionReprintsHiddenLinesIntoCommittedFlow()
+    {
+        // maxHeight = 3 rows. The collapsible summary = 1 row; hidden = 5 rows (5 lines).
+        // Expanded height = 6 > 3 → oversized path: hidden lines go to NewlyCommittedRows,
+        // block stays a collapsed summary in the live window.
+        RenderModel model = MakeModel(cols: 80, rows: 3);
+        ScrollbackModel sm = new();
+
+        IReadOnlyList<Line> hiddenLines =
+        [
+            PlainLine("h1"), PlainLine("h2"), PlainLine("h3"),
+            PlainLine("h4"), PlainLine("h5")
+        ];
+        Collapsible c = sm.BeginCollapsible(PlainLine("summary"), hiddenLines, model);
+
+        sm.ExpandCollapsible(c, model);
+
+        // Block must still be collapsed (not expanded).
+        Assert.False(c.IsExpanded);
+
+        sm.PrePaint(model);
+
+        // Live window contains only the summary (collapsed marker).
+        string liveText = JoinRows(model.LiveWindowRows);
+        Assert.Contains("summary", liveText, StringComparison.Ordinal);
+        Assert.DoesNotContain("h1", liveText, StringComparison.Ordinal);
+
+        // Hidden lines were emitted into committed flow.
+        string committed = JoinRows(model.NewlyCommittedRows);
+        Assert.Contains("h1", committed, StringComparison.Ordinal);
+        Assert.Contains("h5", committed, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void CollapsibleOversizedReprintEmittedOnlyOnce()
+    {
+        // The reprinted rows must not appear in NewlyCommittedRows on the next frame.
+        RenderModel model = MakeModel(cols: 80, rows: 3);
+        ScrollbackModel sm = new();
+
+        IReadOnlyList<Line> hidden = [PlainLine("h1"), PlainLine("h2"), PlainLine("h3"), PlainLine("h4")];
+        Collapsible c = sm.BeginCollapsible(PlainLine("summary"), hidden, model);
+        sm.ExpandCollapsible(c, model);
+        sm.PrePaint(model);
+        Assert.NotEmpty(model.NewlyCommittedRows); // frame 1: emitted
+
+        sm.ClearCommitted(model);
+        model.MarkDirty();
+        sm.PrePaint(model);
+        Assert.Empty(model.NewlyCommittedRows); // frame 2: not re-emitted
+    }
+
+    [Fact]
+    public void CollapsibleFittingExpansionStaysLiveRenderable()
+    {
+        // maxHeight = 10, summary = 1 row, hidden = 2 rows → expanded = 3 ≤ 10.
+        // Normal path: block is marked expanded; live window shows all 3 rows.
+        RenderModel model = MakeModel(cols: 80, rows: 10);
+        ScrollbackModel sm = new();
+
+        Collapsible c = sm.BeginCollapsible(PlainLine("summary"), [PlainLine("d1"), PlainLine("d2")], model);
+        sm.ExpandCollapsible(c, model);
+
+        Assert.True(c.IsExpanded);
+
+        sm.PrePaint(model);
+        string liveText = JoinRows(model.LiveWindowRows);
+        Assert.Contains("summary", liveText, StringComparison.Ordinal);
+        Assert.Contains("d1", liveText, StringComparison.Ordinal);
+        Assert.Contains("d2", liveText, StringComparison.Ordinal);
+        Assert.Empty(model.NewlyCommittedRows);
+    }
+
+    // ─── BeginCollapsibleCommand ──────────────────────────────────────────────
+
+    [Fact]
+    public void BeginCollapsibleCommandCapturesCollapsibleRef()
+    {
+        RenderModel model = MakeModel();
+        ScrollbackModel sm = new();
+
+        Collapsible? captured = null;
+        ILoopCommand cmd = new BeginCollapsibleCommand(
+            sm,
+            PlainLine("summary"),
+            [PlainLine("hidden")],
+            c => captured = c);
+        cmd.Apply(model);
+
+        Assert.NotNull(captured);
+        sm.PrePaint(model);
+        string liveText = JoinRows(model.LiveWindowRows);
+        Assert.Contains("summary", liveText, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void ExpandCollapsibleCommandExpandsBlockInLiveWindow()
+    {
+        RenderModel model = MakeModel(cols: 80, rows: 10);
+        ScrollbackModel sm = new();
+
+        Collapsible? captured = null;
+        ILoopCommand beginCmd = new BeginCollapsibleCommand(
+            sm,
+            PlainLine("summary"),
+            [PlainLine("detail")],
+            c => captured = c);
+        beginCmd.Apply(model);
+
+        Assert.NotNull(captured);
+        ILoopCommand expandCmd = new ExpandCollapsibleCommand(sm, captured);
+        expandCmd.Apply(model);
+
+        sm.PrePaint(model);
+        string liveText = JoinRows(model.LiveWindowRows);
+        Assert.Contains("detail", liveText, StringComparison.Ordinal);
+    }
+}
diff --git a/tests/Dcli.Tests/SmokeTests.cs b/tests/Dcli.Tests/SmokeTests.cs
new file mode 100644
index 0000000..3d5644c
--- /dev/null
+++ b/tests/Dcli.Tests/SmokeTests.cs
@@ -0,0 +1,17 @@
+using Xunit;
+
+namespace Dcli.Tests;
+
+/// <summary>
+/// Placeholder tests confirming the assembly loads. Behavioral tests are added per section.
+/// </summary>
+public sealed class SmokeTests
+{
+    [Fact]
+    public void AssemblyLoads()
+    {
+        // Verifies that the Dcli assembly can be referenced and loaded by the test runner.
+        System.Reflection.Assembly assembly = typeof(DcliVersion).Assembly;
+        Assert.Equal("Dcli", assembly.GetName().Name);
+    }
+}
diff --git a/tests/Dcli.Tests/StyledTextTests.cs b/tests/Dcli.Tests/StyledTextTests.cs
new file mode 100644
index 0000000..0cebb7a
--- /dev/null
+++ b/tests/Dcli.Tests/StyledTextTests.cs
@@ -0,0 +1,381 @@
+using Xunit;
+
+namespace Dcli.Tests;
+
+/// <summary>
+/// Behavioural tests for Section 2: styled-text model (Format, Color, Style, Segment, Line, LineBuilder).
+/// Each test maps to a specific spec scenario under capability styled-text.
+/// </summary>
+public sealed class StyledTextTests
+{
+    // -----------------------------------------------------------------------------------------
+    // 2.1 Format enum
+    // -----------------------------------------------------------------------------------------
+
+    [Fact]
+    public void FormatNoneHasNoFlags()
+    {
+        Assert.Equal(0, (int)Format.None);
+    }
+
+    [Fact]
+    public void FormatValuesArePowersOfTwo()
+    {
+        // Every non-None value must be a distinct power of two so they can be ORed without overlap.
+        Format[] flags = [Format.Bold, Format.Italic, Format.Underline, Format.Dim, Format.Reverse, Format.Strikethrough];
+        for (int i = 0; i < flags.Length; i++)
+        {
+            int v = (int)flags[i];
+            Assert.True(v > 0 && (v & (v - 1)) == 0, $"{flags[i]} ({v}) is not a power of two");
+            for (int j = i + 1; j < flags.Length; j++)
+            {
+                Assert.True((int)flags[i] != (int)flags[j], $"{flags[i]} and {flags[j]} have the same value");
+            }
+        }
+    }
+
+    [Fact]
+    public void FormatCombineBoldAndItalicSetsBothFlags()
+    {
+        // Spec: Combining attributes — Bold | Italic yields a value with both flags set.
+        Format combined = Format.Bold | Format.Italic;
+        Assert.True(combined.HasFlag(Format.Bold));
+        Assert.True(combined.HasFlag(Format.Italic));
+        Assert.False(combined.HasFlag(Format.Underline));
+    }
+
+    [Fact]
+    public void FormatNoneHasNoAttributes()
+    {
+        // Spec: No formatting — Format.None has no flags set.
+        Format none = Format.None;
+        Assert.False(none.HasFlag(Format.Bold));
+        Assert.False(none.HasFlag(Format.Italic));
+        Assert.False(none.HasFlag(Format.Underline));
+        Assert.False(none.HasFlag(Format.Dim));
+        Assert.False(none.HasFlag(Format.Reverse));
+        Assert.False(none.HasFlag(Format.Strikethrough));
+    }
+
+    [Fact]
+    public void FormatAllFlagsCombinedContainsAll()
+    {
+        Format all = Format.Bold | Format.Italic | Format.Underline | Format.Dim | Format.Reverse | Format.Strikethrough;
+        Assert.True(all.HasFlag(Format.Bold));
+        Assert.True(all.HasFlag(Format.Italic));
+        Assert.True(all.HasFlag(Format.Underline));
+        Assert.True(all.HasFlag(Format.Dim));
+        Assert.True(all.HasFlag(Format.Reverse));
+        Assert.True(all.HasFlag(Format.Strikethrough));
+    }
+
+    // -----------------------------------------------------------------------------------------
+    // 2.2 Color
+    // -----------------------------------------------------------------------------------------
+
+    [Fact]
+    public void ColorNamedRoundTrips()
+    {
+        Color c = Color.Named(Color.AnsiColor.BrightCyan);
+        Assert.Equal(Color.ColorKind.Named, c.Kind);
+        Assert.Equal(Color.AnsiColor.BrightCyan, c.NamedValue);
+    }
+
+    [Fact]
+    public void ColorFromIndexRoundTrips()
+    {
+        Color c = Color.FromIndex(200);
+        Assert.Equal(Color.ColorKind.Indexed, c.Kind);
+        Assert.Equal(200, c.IndexValue);
+    }
+
+    [Fact]
+    public void ColorFromIndexZeroAndMaxRoundTrip()
+    {
+        Assert.Equal(0, Color.FromIndex(0).IndexValue);
+        Assert.Equal(255, Color.FromIndex(255).IndexValue);
+    }
+
+    [Fact]
+    public void ColorFromRgbRoundTrips()
+    {
+        // Spec: Truecolor foreground — a 24-bit RGB foreground is carried faithfully.
+        Color c = Color.FromRgb(0xDE, 0xAD, 0xBE);
+        Assert.Equal(Color.ColorKind.Rgb, c.Kind);
+        Assert.Equal(0xDE, c.R);
+        Assert.Equal(0xAD, c.G);
+        Assert.Equal(0xBE, c.B);
+    }
+
+    [Fact]
+    public void ColorEqualitySameKindAndValueAreEqual()
+    {
+        Assert.Equal(Color.FromRgb(1, 2, 3), Color.FromRgb(1, 2, 3));
+        Assert.Equal(Color.FromIndex(42), Color.FromIndex(42));
+        Assert.Equal(Color.Named(Color.AnsiColor.Red), Color.Named(Color.AnsiColor.Red));
+    }
+
+    [Fact]
+    public void ColorEqualityDifferentKindNotEqual()
+    {
+        // Index 1 and Named Red (ANSI index 1) have the same numeric value but different kinds.
+        Assert.NotEqual(Color.FromIndex(1), Color.Named(Color.AnsiColor.Red));
+    }
+
+    [Fact]
+    public void ColorWrongKindAccessorThrows()
+    {
+        Color rgb = Color.FromRgb(1, 2, 3);
+        Assert.Throws<InvalidOperationException>(() => _ = rgb.NamedValue);
+        Assert.Throws<InvalidOperationException>(() => _ = rgb.IndexValue);
+
+        Color named = Color.Named(Color.AnsiColor.Blue);
+        Assert.Throws<InvalidOperationException>(() => _ = named.R);
+    }
+
+    // -----------------------------------------------------------------------------------------
+    // 2.2 Style
+    // -----------------------------------------------------------------------------------------
+
+    [Fact]
+    public void StyleDefaultHasNullColorsAndNoneFormat()
+    {
+        // Spec: No color — unset fg/bg means terminal default colors.
+        Style s = default;
+        Assert.Null(s.Foreground);
+        Assert.Null(s.Background);
+        Assert.Equal(Format.None, s.Format);
+    }
+
+    [Fact]
+    public void StyleWithForegroundOnlyBackgroundRemainsNull()
+    {
+        Style s = new(Foreground: Color.Named(Color.AnsiColor.Green));
+        Assert.NotNull(s.Foreground);
+        Assert.Null(s.Background);
+        Assert.Equal(Format.None, s.Format);
+    }
+
+    [Fact]
+    public void StyleWithAllFieldsRoundTrips()
+    {
+        Color fg = Color.FromRgb(255, 0, 128);
+        Color bg = Color.FromIndex(16);
+        Style s = new(fg, bg, Format.Bold | Format.Underline);
+        Assert.Equal(fg, s.Foreground);
+        Assert.Equal(bg, s.Background);
+        Assert.True(s.Format.HasFlag(Format.Bold));
+        Assert.True(s.Format.HasFlag(Format.Underline));
+    }
+
+    // -----------------------------------------------------------------------------------------
+    // 2.3 Segment
+    // -----------------------------------------------------------------------------------------
+
+    [Fact]
+    public void SegmentStoresTextLiterally()
+    {
+        // Spec: Markup-like text is literal — "[bold]", "{x}", etc. are stored verbatim.
+        string markup = "[bold]{x}<em>foo</em>";
+        Segment seg = new(markup);
+        Assert.Equal(markup, seg.Text);
+    }
+
+    [Fact]
+    public void SegmentDefaultStyleIsDefault()
+    {
+        Segment seg = new("hello");
+        Assert.Equal(default, seg.Style);
+    }
+
+    [Fact]
+    public void SegmentRetainsDistinctStyle()
+    {
+        Style s = new(Foreground: Color.Named(Color.AnsiColor.Yellow), Format: Format.Bold);
+        Segment seg = new("hi", s);
+        Assert.Equal(s, seg.Style);
+        Assert.Equal("hi", seg.Text);
+    }
+
+    [Fact]
+    public void SegmentEqualitySameTextAndStyleAreEqual()
+    {
+        Style s = new(Format: Format.Italic);
+        Assert.Equal(new Segment("a", s), new Segment("a", s));
+    }
+
+    [Fact]
+    public void SegmentEqualityDifferentTextNotEqual()
+    {
+        Assert.NotEqual(new Segment("a"), new Segment("b"));
+    }
+
+    // -----------------------------------------------------------------------------------------
+    // 2.3 Line
+    // -----------------------------------------------------------------------------------------
+
+    [Fact]
+    public void LineSegmentsInOrderPreservedOnConstruction()
+    {
+        Segment s1 = new("first", new Style(Format: Format.Bold));
+        Segment s2 = new("second", new Style(Format: Format.Dim));
+        Line line = new(new[] { s1, s2 });
+        Assert.Equal(2, line.Segments.Count);
+        Assert.Equal(s1, line.Segments[0]);
+        Assert.Equal(s2, line.Segments[1]);
+    }
+
+    [Fact]
+    public void LineStructuralEqualitySameSegmentsAreEqual()
+    {
+        // Confirms that Line overrides record equality to use sequence equality, not reference equality.
+        Segment s1 = new("a");
+        Segment s2 = new("b");
+        Line line1 = new(new[] { s1, s2 });
+        Line line2 = new(new[] { s1, s2 });
+        Assert.Equal(line1, line2);
+    }
+
+    [Fact]
+    public void LineStructuralEqualityDifferentSegmentsNotEqual()
+    {
+        Line line1 = new(new[] { new Segment("a") });
+        Line line2 = new(new[] { new Segment("b") });
+        Assert.NotEqual(line1, line2);
+    }
+
+    [Fact]
+    public void LineStructuralEqualityDifferentOrderNotEqual()
+    {
+        Segment s1 = new("a");
+        Segment s2 = new("b");
+        Line line1 = new(new[] { s1, s2 });
+        Line line2 = new(new[] { s2, s1 });
+        Assert.NotEqual(line1, line2);
+    }
+
+    [Fact]
+    public void LineEmptyIsValid()
+    {
+        Line line = new(Array.Empty<Segment>());
+        Assert.Empty(line.Segments);
+    }
+
+    // -----------------------------------------------------------------------------------------
+    // 2.3 LineBuilder
+    // -----------------------------------------------------------------------------------------
+
+    [Fact]
+    public void LineBuilderAppendsSegmentsInOrder()
+    {
+        // Spec: Building a line — fragments are stored in the order they were added.
+        Line line = new LineBuilder()
+            .Text("first")
+            .Bold("second")
+            .Dim("third")
+            .Build();
+
+        Assert.Equal(3, line.Segments.Count);
+        Assert.Equal("first", line.Segments[0].Text);
+        Assert.Equal("second", line.Segments[1].Text);
+        Assert.Equal("third", line.Segments[2].Text);
+    }
+
+    [Fact]
+    public void LineBuilderTextAppliesDefaultStyle()
+    {
+        Line line = new LineBuilder().Text("hello").Build();
+        Assert.Equal(default, line.Segments[0].Style);
+    }
+
+    [Fact]
+    public void LineBuilderBoldSetsBoldFlag()
+    {
+        Line line = new LineBuilder().Bold("hi").Build();
+        Assert.True(line.Segments[0].Style.Format.HasFlag(Format.Bold));
+    }
+
+    [Fact]
+    public void LineBuilderItalicSetsItalicFlag()
+    {
+        Line line = new LineBuilder().Italic("hi").Build();
+        Assert.True(line.Segments[0].Style.Format.HasFlag(Format.Italic));
+    }
+
+    [Fact]
+    public void LineBuilderDimSetsDimFlag()
+    {
+        Line line = new LineBuilder().Dim("✻ ").Build();
+        Assert.True(line.Segments[0].Style.Format.HasFlag(Format.Dim));
+    }
+
+    [Fact]
+    public void LineBuilderAppendCarriesArbitraryStyle()
+    {
+        Style s = new(Color.FromRgb(100, 200, 50), null, Format.Bold | Format.Underline);
+        Line line = new LineBuilder().Append("styled", s).Build();
+        Assert.Equal(s, line.Segments[0].Style);
+        Assert.Equal("styled", line.Segments[0].Text);
+    }
+
+    [Fact]
+    public void LineBuilderEmptyBuildsEmptyLine()
+    {
+        Line line = new LineBuilder().Build();
+        Assert.Empty(line.Segments);
+    }
+
+    [Fact]
+    public void LineBuilderSpecExampleDimThenText()
+    {
+        // Mirrors the spec-sketch: new LineBuilder().Dim("✻ ").Text("Thinking…").Build()
+        Line line = new LineBuilder().Dim("✻ ").Text("Thinking…").Build();
+        Assert.Equal(2, line.Segments.Count);
+        Assert.Equal("✻ ", line.Segments[0].Text);
+        Assert.True(line.Segments[0].Style.Format.HasFlag(Format.Dim));
+        Assert.Equal("Thinking…", line.Segments[1].Text);
+        Assert.Equal(default, line.Segments[1].Style);
+    }
+
+    // -----------------------------------------------------------------------------------------
+    // Zone-agnostic reuse (Spec: Shared across both zones)
+    // -----------------------------------------------------------------------------------------
+
+    [Fact]
+    public void StyledTextSameTypesUsedInBothZones()
+    {
+        // Spec: Reuse in both zones — the same Segment/Line/Style types serve scrollback and
+        // fixed-region (status) content without any zone-specific coupling.
+
+        // Simulated scrollback line (content zone)
+        Line scrollbackLine = new LineBuilder()
+            .Text("User: ")
+            .Bold("hello")
+            .Build();
+
+        // Simulated status line (fixed region)
+        Line statusLine = new LineBuilder()
+            .Fg("claude-opus-4", Color.Named(Color.AnsiColor.BrightGreen))
+            .Text(" | ")
+            .Dim("thinking")
+            .Build();
+
+        // Both use the same types — no zone-specific types needed.
+        Assert.IsType<Line>(scrollbackLine);
+        Assert.IsType<Line>(statusLine);
+        Assert.Equal(2, scrollbackLine.Segments.Count);
+        Assert.Equal(3, statusLine.Segments.Count);
+    }
+
+    [Fact]
+    public void StyledTextMarkupLikeTextIsLiteralInBothZones()
+    {
+        // Markup-like strings stay verbatim in all contexts.
+        string[] literals = ["[bold]", "{x}", "<em>", "**strong**", "\x1b[1m"];
+        foreach (string literal in literals)
+        {
+            Line line = new LineBuilder().Text(literal).Build();
+            Assert.Equal(literal, line.Segments[0].Text);
+        }
+    }
+}
diff --git a/tests/Dcli.Tests/Terminal/Input/InputReaderTests.cs b/tests/Dcli.Tests/Terminal/Input/InputReaderTests.cs
new file mode 100644
index 0000000..1a460bf
--- /dev/null
+++ b/tests/Dcli.Tests/Terminal/Input/InputReaderTests.cs
@@ -0,0 +1,280 @@
+using System.Threading.Channels;
+using Dcli.Internal.Input;
+using Xunit;
+
+namespace Dcli.Tests.Terminal.Input;
+
+/// <summary>
+/// Headless tests for §6: InputReader thread, IInputByteSource edge, and Ctrl+C handling.
+/// All tests run without a real tty by injecting a scripted <see cref="FakeInputByteSource"/>.
+/// </summary>
+public sealed class InputReaderTests
+{
+    // ─── Fake byte source ─────────────────────────────────────────────────────
+
+    /// <summary>
+    /// A scripted implementation of <see cref="IInputByteSource"/> for use in tests.
+    /// Chunks of bytes are enqueued; a 0-byte chunk represents a timed-read timeout.
+    /// After all chunks are consumed each subsequent Read returns 0 (timeout) indefinitely.
+    /// </summary>
+    private sealed class FakeInputByteSource : IInputByteSource
+    {
+        private readonly Queue<byte[]> _chunks = new();
+
+        internal void Enqueue(params byte[] chunk) => _chunks.Enqueue(chunk);
+
+        /// <summary>Enqueues a 0-byte chunk to simulate a VMIN=0/VTIME=1 timeout.</summary>
+        internal void EnqueueTimeout() => _chunks.Enqueue([]);
+
+        public int Read(Span<byte> buffer)
+        {
+            if (_chunks.Count == 0)
+                return 0; // simulate idle timeout
+
+            byte[] chunk = _chunks.Dequeue();
+            if (chunk.Length == 0)
+                return 0; // explicit timeout
+
+            int count = Math.Min(chunk.Length, buffer.Length);
+            chunk.AsSpan(0, count).CopyTo(buffer);
+            return count;
+        }
+    }
+
+    // ─── Helpers ─────────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Creates a reader fed by <paramref name="source"/>, collects events until
+    /// <paramref name="expectedEventCount"/> are received or <paramref name="timeoutMs"/>
+    /// elapses, then stops the reader. Returns all collected events in order.
+    /// </summary>
+    private static List<InputEvent> RunReader(
+        FakeInputByteSource source,
+        int expectedEventCount,
+        int timeoutMs = 2000)
+    {
+        Channel<InputEvent> channel = Channel.CreateUnbounded<InputEvent>();
+        using InputReader reader = new(source, channel.Writer);
+
+        List<InputEvent> events = new();
+        using CancellationTokenSource cts = new(timeoutMs);
+        try
+        {
+            while (events.Count < expectedEventCount)
+            {
+                if (channel.Reader.TryRead(out InputEvent? ev))
+                    events.Add(ev);
+                else if (cts.IsCancellationRequested)
+                    break;
+                else
+                    Thread.Sleep(1);
+            }
+        }
+        catch (OperationCanceledException) { }
+
+        return events;
+    }
+
+    private static KeyEvent AssertKey(InputEvent ev) =>
+        Assert.IsType<KeyEvent>(ev);
+
+    // ─── §6.1 — basic byte-chunk → events ────────────────────────────────────
+
+    [Fact]
+    public void SingleAsciiByteProducesKeyEvent()
+    {
+        FakeInputByteSource source = new();
+        source.Enqueue((byte)'a');
+        source.EnqueueTimeout(); // allow loop to deliver
+
+        List<InputEvent> events = RunReader(source, expectedEventCount: 1);
+
+        Assert.Single(events);
+        KeyEvent ke = AssertKey(events[0]);
+        Assert.Equal(KeyCode.FromRune(new System.Text.Rune('a')), ke.Code);
+        Assert.Equal(Modifiers.None, ke.Modifiers);
+    }
+
+    [Fact]
+    public void MultipleAsciiChunkProducesEventsInOrder()
+    {
+        FakeInputByteSource source = new();
+        source.Enqueue((byte)'h', (byte)'i');
+        source.EnqueueTimeout();
+
+        List<InputEvent> events = RunReader(source, expectedEventCount: 2);
+
+        Assert.Equal(2, events.Count);
+        Assert.Equal(KeyCode.FromRune(new System.Text.Rune('h')), AssertKey(events[0]).Code);
+        Assert.Equal(KeyCode.FromRune(new System.Text.Rune('i')), AssertKey(events[1]).Code);
+    }
+
+    // ─── Multi-byte UTF-8 split across reads ─────────────────────────────────
+
+    [Fact]
+    public void MultiByteUtf8SplitAcrossReadsProducesOneEvent()
+    {
+        // U+00E9 LATIN SMALL LETTER E WITH ACUTE = 0xC3 0xA9 (2 bytes)
+        FakeInputByteSource source = new();
+        source.Enqueue(0xC3);           // lead byte arrives in first read
+        source.Enqueue(0xA9);           // continuation byte in second read
+        source.EnqueueTimeout();
+
+        List<InputEvent> events = RunReader(source, expectedEventCount: 1);
+
+        Assert.Single(events);
+        KeyEvent ke = AssertKey(events[0]);
+        Assert.Equal(KeyCode.FromRune(new System.Text.Rune('é')), ke.Code);
+    }
+
+    [Fact]
+    public void CsiSequenceSplitAcrossReadsProducesOneArrowEvent()
+    {
+        // Up-arrow: ESC [ A split across two reads
+        FakeInputByteSource source = new();
+        source.Enqueue(0x1B, (byte)'['); // ESC [ in first read
+        source.Enqueue((byte)'A');        // A in second read
+        source.EnqueueTimeout();
+
+        List<InputEvent> events = RunReader(source, expectedEventCount: 1);
+
+        Assert.Single(events);
+        KeyEvent ke = AssertKey(events[0]);
+        Assert.Equal(KeyCode.Named(NamedKey.Up), ke.Code);
+        Assert.Equal(Modifiers.None, ke.Modifiers);
+    }
+
+    // ─── ESC-disambiguation via Flush ─────────────────────────────────────────
+
+    [Fact]
+    public void BareEscFollowedByTimeoutProducesEscapeEvent()
+    {
+        // Lone ESC + timeout → Named(Escape)
+        FakeInputByteSource source = new();
+        source.Enqueue(0x1B);
+        source.EnqueueTimeout(); // timeout triggers Flush because HasPendingEscape
+
+        List<InputEvent> events = RunReader(source, expectedEventCount: 1);
+
+        Assert.Single(events);
+        KeyEvent ke = AssertKey(events[0]);
+        Assert.Equal(KeyCode.Named(NamedKey.Escape), ke.Code);
+    }
+
+    [Fact]
+    public void EscFollowedImmediatelyByBracketAProducesUpNotEscape()
+    {
+        // ESC arrives, then [ A arrives before any timeout — must be Up, not Escape.
+        FakeInputByteSource source = new();
+        source.Enqueue(0x1B);           // ESC
+        source.Enqueue((byte)'[', (byte)'A'); // [ A follows immediately (no timeout between)
+        source.EnqueueTimeout();
+
+        List<InputEvent> events = RunReader(source, expectedEventCount: 1);
+
+        Assert.Single(events);
+        KeyEvent ke = AssertKey(events[0]);
+        Assert.Equal(KeyCode.Named(NamedKey.Up), ke.Code);
+    }
+
+    [Fact]
+    public void PartialCsiFollowedByTimeoutDoesNotEmitSpuriousEscape()
+    {
+        // ESC [ is partial CSI (not bare ESC). A timeout must NOT flush it.
+        // Then A completes the sequence → one Up event.
+        FakeInputByteSource source = new();
+        source.Enqueue(0x1B, (byte)'['); // partial CSI
+        source.EnqueueTimeout();          // timeout — HasPendingEscape is false (state=CsiParam)
+        source.Enqueue((byte)'A');        // completes CSI to Up
+        source.EnqueueTimeout();
+
+        List<InputEvent> events = RunReader(source, expectedEventCount: 1);
+
+        // Exactly one Up event — no spurious Escape
+        Assert.Single(events);
+        KeyEvent ke = AssertKey(events[0]);
+        Assert.Equal(KeyCode.Named(NamedKey.Up), ke.Code);
+    }
+
+    [Fact]
+    public void PartialUtf8FollowedByTimeoutDoesNotTruncateScalar()
+    {
+        // U+4E2D CJK UNIFIED IDEOGRAPH 中 = 0xE4 0xB8 0xAD (3-byte UTF-8 sequence).
+        // The lead byte arrives in one read, then a 100 ms timeout fires, then the two
+        // continuation bytes arrive in the next read.
+        // The timeout must NOT flush the partial scalar as a spurious Escape because the
+        // parser is in the UTF-8 state, not the bare-ESC state (HasPendingEscape is false).
+        // Exactly one KeyEvent carrying Char(中) must be emitted.
+        FakeInputByteSource source = new();
+        source.Enqueue(0xE4);              // lead byte of 中
+        source.EnqueueTimeout();           // simulated VTIME=1 timeout between bytes
+        source.Enqueue(0xB8, 0xAD);        // continuation bytes completing 中
+        source.EnqueueTimeout();
+
+        List<InputEvent> events = RunReader(source, expectedEventCount: 1);
+
+        Assert.Single(events);
+        KeyEvent ke = AssertKey(events[0]);
+        Assert.Equal(KeyCode.FromRune(new System.Text.Rune('中')), ke.Code);
+        Assert.Equal(Modifiers.None, ke.Modifiers);
+    }
+
+    // ─── §6.2 — Ctrl+C must not terminate ────────────────────────────────────
+
+    [Fact]
+    public void CtrlCByteProducesKeyEventNotTermination()
+    {
+        // 0x03 = Ctrl+C. ISIG is cleared in raw mode, so it arrives as a byte.
+        // The reader must post it and the process must keep running.
+        FakeInputByteSource source = new();
+        source.Enqueue(0x03);
+        source.EnqueueTimeout();
+
+        List<InputEvent> events = RunReader(source, expectedEventCount: 1);
+
+        Assert.Single(events);
+        KeyEvent ke = AssertKey(events[0]);
+        Assert.Equal(KeyCode.FromRune(new System.Text.Rune('c')), ke.Code);
+        Assert.Equal(Modifiers.Ctrl, ke.Modifiers);
+        // If we reach here, the process was not terminated — test passes.
+    }
+
+    // ─── Clean shutdown ───────────────────────────────────────────────────────
+
+    [Fact]
+    public void StopCausesThreadToExitPromptly()
+    {
+        // Reader with an infinite-idle source (all timeouts).
+        // Stop() must return within ~500 ms.
+        FakeInputByteSource source = new(); // no enqueued data → all reads return 0
+
+        Channel<InputEvent> channel = Channel.CreateUnbounded<InputEvent>();
+        using InputReader reader = new(source, channel.Writer);
+
+        System.Diagnostics.Stopwatch sw = System.Diagnostics.Stopwatch.StartNew();
+        reader.Stop();
+        sw.Stop();
+
+        // The read loop wakes every ~100 ms (VTIME=1). Allow 600 ms to be safe.
+        Assert.True(sw.ElapsedMilliseconds < 600,
+            $"Stop() took {sw.ElapsedMilliseconds} ms — expected < 600 ms.");
+    }
+
+    [Fact]
+    public void DisposeCausesThreadToExitPromptly()
+    {
+        FakeInputByteSource source = new();
+
+        Channel<InputEvent> channel = Channel.CreateUnbounded<InputEvent>();
+
+        System.Diagnostics.Stopwatch sw = System.Diagnostics.Stopwatch.StartNew();
+        using (InputReader _ = new(source, channel.Writer))
+        {
+            // Dispose immediately.
+        }
+        sw.Stop();
+
+        Assert.True(sw.ElapsedMilliseconds < 600,
+            $"Dispose() took {sw.ElapsedMilliseconds} ms — expected < 600 ms.");
+    }
+}
diff --git a/tests/Dcli.Tests/Terminal/RenderLoop/RenderLoopTests.cs b/tests/Dcli.Tests/Terminal/RenderLoop/RenderLoopTests.cs
new file mode 100644
index 0000000..80fdf14
--- /dev/null
+++ b/tests/Dcli.Tests/Terminal/RenderLoop/RenderLoopTests.cs
@@ -0,0 +1,507 @@
+using System.Collections.Concurrent;
+using System.Threading.Channels;
+using Dcli.Internal.RenderLoop;
+using Xunit;
+
+namespace Dcli.Tests.Terminal.RenderLoop;
+
+/// <summary>
+/// Deterministic cadence tests for §7: render loop core.
+/// All tests use a virtual clock and an in-memory output sink — no wall-clock sleeps.
+/// </summary>
+public sealed class RenderLoopTests
+{
+    // ─────────────────────────────────────────────────────────────────────────
+    // Test doubles
+    // ─────────────────────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Virtual clock that reports a fixed "now" until the test advances it.
+    /// <see cref="WaitUntilAsync"/> returns a task that completes when <see cref="Advance"/>
+    /// moves the clock to or past the requested deadline — deterministic, no real time.
+    /// </summary>
+    private sealed class VirtualClock : IClock
+    {
+        private readonly object _lock = new();
+        private TimeSpan _now;
+        // Each entry: the deadline and a TCS to signal when the deadline is reached.
+        // The linked CTS mirrors the caller's cancellation token; disposing it cancels the TCS.
+        private readonly List<(TimeSpan Deadline, TaskCompletionSource Tcs, CancellationTokenSource LinkedCts)> _waiters = [];
+
+        internal VirtualClock(TimeSpan? initial = null) =>
+            _now = initial ?? TimeSpan.Zero;
+
+        public TimeSpan Now
+        {
+            get { lock (_lock) { return _now; } }
+        }
+
+        /// <summary>
+        /// Advance the virtual clock by <paramref name="by"/> and complete any registered
+        /// waiters whose deadline is now reached.
+        /// </summary>
+        internal void Advance(TimeSpan by)
+        {
+            List<(TaskCompletionSource Tcs, CancellationTokenSource LinkedCts)> toComplete = [];
+            lock (_lock)
+            {
+                _now += by;
+                for (int i = _waiters.Count - 1; i >= 0; i--)
+                {
+                    if (_waiters[i].Deadline <= _now)
+                    {
+                        toComplete.Add((_waiters[i].Tcs, _waiters[i].LinkedCts));
+                        _waiters.RemoveAt(i);
+                    }
+                }
+            }
+            foreach ((TaskCompletionSource tcs, CancellationTokenSource linkedCts) in toComplete)
+            {
+                tcs.TrySetResult();
+                linkedCts.Dispose();
+            }
+        }
+
+        public Task WaitUntilAsync(TimeSpan deadline, CancellationToken cancellationToken)
+        {
+            lock (_lock)
+            {
+                if (deadline <= _now)
+                    return Task.CompletedTask;
+            }
+
+            // A linked CTS cancels our TCS when the caller's token fires.
+            CancellationTokenSource linkedCts = CancellationTokenSource.CreateLinkedTokenSource(cancellationToken);
+            TaskCompletionSource tcs = new(TaskCreationOptions.RunContinuationsAsynchronously);
+            linkedCts.Token.Register(
+                static state => ((TaskCompletionSource)state!).TrySetCanceled(),
+                tcs,
+                useSynchronizationContext: false);
+
+            lock (_lock)
+            {
+                // Re-check under the lock in case Advance raced between the outer check and here.
+                if (deadline <= _now)
+                {
+                    tcs.TrySetResult();
+                    linkedCts.Dispose();
+                    return tcs.Task;
+                }
+
+                _waiters.Add((deadline, tcs, linkedCts));
+            }
+
+            return tcs.Task;
+        }
+    }
+
+    /// <summary>
+    /// In-memory output sink that counts and records each paint call.
+    /// </summary>
+    private sealed class CapturingOutputSink : IOutputSink
+    {
+        private readonly List<(int Columns, int Rows)> _frames = [];
+        private int _paintCount;
+
+        internal int PaintCount => Volatile.Read(ref _paintCount);
+
+        internal IReadOnlyList<(int Columns, int Rows)> Frames
+        {
+            get { lock (_frames) { return _frames.ToArray(); } }
+        }
+
+        public void Paint(RenderModel model)
+        {
+            lock (_frames)
+            {
+                _frames.Add((model.Columns, model.Rows));
+            }
+            Interlocked.Increment(ref _paintCount);
+        }
+
+        public void EmitRestoreSequence() { }
+    }
+
+    /// <summary>Fixed terminal dimensions for tests.</summary>
+    private sealed class FixedSizeSource : ITerminalSizeSource
+    {
+        private readonly int _cols;
+        private readonly int _rows;
+
+        internal FixedSizeSource(int cols = 80, int rows = 24)
+        {
+            _cols = cols;
+            _rows = rows;
+        }
+
+        public (int Columns, int Rows) GetSize() => (_cols, _rows);
+    }
+
+    /// <summary>A simple command that marks the model dirty (simulates an API call).</summary>
+    private sealed class DirtyCommand : ILoopCommand
+    {
+        /// <summary>Thread ID on which Apply was last called, for the no-interleave test.</summary>
+        internal int? AppliedOnThreadId { get; private set; }
+
+        void ILoopCommand.Apply(RenderModel model)
+        {
+            AppliedOnThreadId = Environment.CurrentManagedThreadId;
+            model.MarkDirty();
+        }
+    }
+
+    /// <summary>A command that records the thread ID of each Apply call.</summary>
+    private sealed class ThreadCapturingCommand : ILoopCommand
+    {
+        internal ConcurrentBag<int> ThreadIds { get; } = [];
+
+        void ILoopCommand.Apply(RenderModel model)
+        {
+            ThreadIds.Add(Environment.CurrentManagedThreadId);
+            model.MarkDirty();
+        }
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // Factory helpers
+    // ─────────────────────────────────────────────────────────────────────────
+
+    private static (LoopEngine Engine, VirtualClock Clock, CapturingOutputSink Sink) CreateEngine(
+        TimeSpan? minFrameInterval = null,
+        int cols = 80,
+        int rows = 24)
+    {
+        VirtualClock clock = new(TimeSpan.Zero);
+        CapturingOutputSink sink = new();
+        LoopEngine engine = new(
+            new FixedSizeSource(cols, rows),
+            clock,
+            sink,
+            minFrameInterval ?? TimeSpan.FromMilliseconds(16));
+        return (engine, clock, sink);
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // §7.6 — Burst coalescing
+    // ─────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task BurstCoalescingNCommandsProducesExactlyOneFrame()
+    {
+        (LoopEngine engine, _, CapturingOutputSink sink) = CreateEngine();
+        using (engine)
+        {
+            // Post N commands before any settle — all should be batched into one frame.
+            for (int i = 0; i < 10; i++)
+            {
+                engine.Post(new DirtyCommand());
+            }
+
+            using CancellationTokenSource cts = new(TimeSpan.FromSeconds(5));
+            await engine.SettleAsync(cts.Token);
+
+            Assert.Equal(1, sink.PaintCount);
+        }
+    }
+
+    [Fact]
+    public async Task BurstCoalescingNInputEventsProducesExactlyOneFrame()
+    {
+        (LoopEngine engine, _, CapturingOutputSink sink) = CreateEngine();
+        using (engine)
+        {
+            ChannelWriter<InputEvent> inputWriter = engine.InputWriter;
+            for (int i = 0; i < 5; i++)
+            {
+                inputWriter.TryWrite(new KeyEvent(KeyCode.FromRune(new System.Text.Rune('a')), Modifiers.None));
+            }
+
+            using CancellationTokenSource cts = new(TimeSpan.FromSeconds(5));
+            await engine.SettleAsync(cts.Token);
+
+            Assert.Equal(1, sink.PaintCount);
+        }
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // §7.6 — Idle: no messages → zero repaints
+    // ─────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task IdleWithNoMessagesProducesZeroFrames()
+    {
+        // Large minFrameInterval so the paint deadline never arrives during the observation window.
+        (LoopEngine engine, _, CapturingOutputSink sink) =
+            CreateEngine(minFrameInterval: TimeSpan.FromHours(1));
+        using (engine)
+        {
+            await Task.Delay(50); // give the loop time to spin if it were going to
+
+            Assert.Equal(0, sink.PaintCount);
+
+            // A truly-idle loop parks on the channel and enters the wait exactly once.
+            // A busy-poll (~1 000 Hz) would rack up hundreds of wait entries over 50 ms.
+            // Allow a small cushion (≤ 3) for startup / scheduling jitter.
+            int waitCount = engine.WaitCount;
+            Assert.True(waitCount <= 3, $"Expected ≤3 wait entries for idle loop; got {waitCount}. Loop may be busy-polling.");
+        }
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // §7.6 — Throttle: frame-rate cap
+    // ─────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task ThrottleSecondSettleBeforeIntervalElapseDoesNotPaint()
+    {
+        // minFrameInterval = 100 ms; virtual clock stays at 0 ms throughout.
+        // First settle: clock at 0, deadline at 0 → paints (0 >= 0); deadline advances to 100 ms.
+        // Second settle: clock still at 0, now (0) < deadline (100 ms) → no paint.
+        (LoopEngine engine, _, CapturingOutputSink sink) =
+            CreateEngine(minFrameInterval: TimeSpan.FromMilliseconds(100));
+        using (engine)
+        {
+            using CancellationTokenSource cts = new(TimeSpan.FromSeconds(5));
+
+            engine.Post(new DirtyCommand());
+            await engine.SettleAsync(cts.Token);
+            int afterFirst = sink.PaintCount;
+
+            engine.Post(new DirtyCommand());
+            await engine.SettleAsync(cts.Token);
+            int afterSecond = sink.PaintCount;
+
+            Assert.Equal(1, afterFirst);
+            Assert.Equal(1, afterSecond); // throttled — clock didn't advance
+        }
+    }
+
+    [Fact]
+    public async Task ThrottleAdvancingClockAllowsNextFrame()
+    {
+        // After the first frame, advance the virtual clock past the interval.
+        (LoopEngine engine, VirtualClock clock, CapturingOutputSink sink) =
+            CreateEngine(minFrameInterval: TimeSpan.FromMilliseconds(100));
+        using (engine)
+        {
+            using CancellationTokenSource cts = new(TimeSpan.FromSeconds(5));
+
+            engine.Post(new DirtyCommand());
+            await engine.SettleAsync(cts.Token);
+            Assert.Equal(1, sink.PaintCount);
+
+            // Advance the virtual clock past the frame interval.
+            clock.Advance(TimeSpan.FromMilliseconds(200));
+
+            engine.Post(new DirtyCommand());
+            await engine.SettleAsync(cts.Token);
+            Assert.Equal(2, sink.PaintCount); // second frame allowed
+        }
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // §7.6 — No interleaving: all mutations on the single loop thread
+    // ─────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task NoInterleavingConcurrentProducersAllMutationsOnLoopThread()
+    {
+        (LoopEngine engine, _, _) = CreateEngine();
+        using (engine)
+        {
+            ThreadCapturingCommand capture = new();
+
+            // Fire 20 commands from 4 concurrent producer threads.
+            Task[] producers = Enumerable.Range(0, 4)
+                .Select(_ => Task.Run(() =>
+                {
+                    for (int i = 0; i < 5; i++)
+                    {
+                        engine.Post(capture);
+                    }
+                }))
+                .ToArray();
+
+            await Task.WhenAll(producers);
+
+            using CancellationTokenSource cts = new(TimeSpan.FromSeconds(5));
+            await engine.SettleAsync(cts.Token);
+
+            // All 20 Apply calls must have happened on the same single loop thread.
+            Assert.NotEmpty(capture.ThreadIds);
+            Assert.Single(capture.ThreadIds.Distinct()); // exactly one unique thread ID
+        }
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // §7.6 — Slow consumer doesn't stall the loop
+    // ─────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task SlowConsumerNeverDrainsOutboundLoopKeepsPainting()
+    {
+        (LoopEngine engine, VirtualClock clock, CapturingOutputSink sink) = CreateEngine();
+        using (engine)
+        {
+            // Consumer never reads from OutboundEvents — events accumulate unboundedly.
+            // Use Enter (a fall-through key that emits KeyPressed) to produce outbound events.
+            ChannelWriter<InputEvent> inputWriter = engine.InputWriter;
+            for (int i = 0; i < 20; i++)
+            {
+                inputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Enter), Modifiers.None));
+            }
+
+            // Advance the virtual clock so the throttle deadline is met.
+            clock.Advance(TimeSpan.FromMilliseconds(100));
+            engine.Post(new DirtyCommand());
+
+            using CancellationTokenSource cts = new(TimeSpan.FromSeconds(5));
+            await engine.SettleAsync(cts.Token);
+
+            // Loop must have painted despite consumer never draining.
+            Assert.True(sink.PaintCount >= 1, $"Expected ≥1 paint; got {sink.PaintCount}");
+
+            // Outbound events must have accumulated unread.
+            Assert.True(engine.OutboundEvents.Count > 0, "Expected unread outbound events");
+        }
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // §7.6 — Snapshot read (terminal size)
+    // ─────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void SnapshotReadReturnsSizeWithoutRoundTrip()
+    {
+        (LoopEngine engine, _, _) = CreateEngine(cols: 120, rows: 40);
+        using (engine)
+        {
+            (int cols, int rows) = engine.GetTerminalSize();
+
+            Assert.Equal(120, cols);
+            Assert.Equal(40, rows);
+        }
+    }
+
+    [Fact]
+    public async Task SnapshotReadUpdatesAfterResizeEvent()
+    {
+        (LoopEngine engine, _, _) = CreateEngine(cols: 80, rows: 24);
+        using (engine)
+        {
+            engine.InputWriter.TryWrite(new ResizeEvent(160, 48));
+
+            using CancellationTokenSource cts = new(TimeSpan.FromSeconds(5));
+            await engine.SettleAsync(cts.Token);
+
+            (int cols, int rows) = engine.GetTerminalSize();
+
+            Assert.Equal(160, cols);
+            Assert.Equal(48, rows);
+        }
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // §7.6 — FIFO ordering
+    // ─────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task OrderingInputEventAndCommandAppliedFifoAndReflectedInSameFrame()
+    {
+        // Post an input event then a command; both must appear in the same settle frame.
+        (LoopEngine engine, _, CapturingOutputSink sink) = CreateEngine();
+        using (engine)
+        {
+            engine.InputWriter.TryWrite(new KeyEvent(KeyCode.Named(NamedKey.Enter), Modifiers.None));
+            engine.Post(new DirtyCommand());
+
+            using CancellationTokenSource cts = new(TimeSpan.FromSeconds(5));
+            await engine.SettleAsync(cts.Token);
+
+            // Both caused dirty state → one frame; Enter emits InputSubmitted (not KeyPressed).
+            Assert.Equal(1, sink.PaintCount);
+            Assert.True(engine.OutboundEvents.TryRead(out TerminalEvent? ev));
+            Assert.IsType<InputSubmitted>(ev);
+        }
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // §7.1 — TerminalEvent types are public and constructible
+    // ─────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void TerminalEventTypesArePublicAndConstructible()
+    {
+        KeyEvent ke = new(KeyCode.Named(NamedKey.Enter), Modifiers.None);
+
+        TerminalEvent e1 = new InputSubmitted("hello");
+        TerminalEvent e2 = new InputChanged("hel");
+        TerminalEvent e3 = new KeyPressed(ke);
+        TerminalEvent e4 = new Resized(80, 24);
+
+        Assert.IsType<InputSubmitted>(e1);
+        Assert.IsType<InputChanged>(e2);
+        Assert.IsType<KeyPressed>(e3);
+        Assert.IsType<Resized>(e4);
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // §7.3 — Fire-and-forget: Post returns before paint
+    // ─────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void PostReturnsImmediatelyWithoutBlockingOnLoopThread()
+    {
+        (LoopEngine engine, _, CapturingOutputSink sink) = CreateEngine();
+        using (engine)
+        {
+            // Post must return synchronously without blocking on the loop thread.
+            engine.Post(new DirtyCommand());
+
+            // If we reach here without deadlocking, the enqueue-and-return contract holds.
+            // Paint count is unobservable here without settle — that is intentional.
+            Assert.True(true);
+        }
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // §7.4 — Outbound channel: input events propagate correctly
+    // ─────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task OutboundInputEventProducesKeyPressedOnOutboundChannel()
+    {
+        (LoopEngine engine, _, _) = CreateEngine();
+        using (engine)
+        {
+            // Plain Enter with no modifier: now emits InputSubmitted (§12) instead of KeyPressed.
+            KeyEvent ke = new(KeyCode.Named(NamedKey.Enter), Modifiers.None);
+            engine.InputWriter.TryWrite(ke);
+
+            using CancellationTokenSource cts = new(TimeSpan.FromSeconds(5));
+            await engine.SettleAsync(cts.Token);
+
+            Assert.True(engine.OutboundEvents.TryRead(out TerminalEvent? ev));
+            InputSubmitted submitted = Assert.IsType<InputSubmitted>(ev);
+            Assert.Equal(string.Empty, submitted.Text); // buffer was empty
+        }
+    }
+
+    [Fact]
+    public async Task OutboundResizeEventProducesResizedOnOutboundChannel()
+    {
+        (LoopEngine engine, _, _) = CreateEngine();
+        using (engine)
+        {
+            engine.InputWriter.TryWrite(new ResizeEvent(100, 30));
+
+            using CancellationTokenSource cts = new(TimeSpan.FromSeconds(5));
+            await engine.SettleAsync(cts.Token);
+
+            Assert.True(engine.OutboundEvents.TryRead(out TerminalEvent? ev));
+            Resized resized = Assert.IsType<Resized>(ev);
+            Assert.Equal(100, resized.Columns);
+            Assert.Equal(30, resized.Rows);
+        }
+    }
+}
diff --git a/tests/Dcli.Tests/Terminal/TerminalLifecycleTests.cs b/tests/Dcli.Tests/Terminal/TerminalLifecycleTests.cs
new file mode 100644
index 0000000..a6e56a3
--- /dev/null
+++ b/tests/Dcli.Tests/Terminal/TerminalLifecycleTests.cs
@@ -0,0 +1,588 @@
+using System.Threading.Channels;
+using Dcli.Internal;
+using Dcli.Internal.Input;
+using Dcli.Internal.RenderLoop;
+using Xunit;
+using DcliTerminal = Dcli.Terminal;
+
+namespace Dcli.Tests.Terminal;
+
+/// <summary>
+/// Headless lifecycle tests for §7.5: Terminal.StartAsync / DisposeAsync.
+/// All tests use injected edges — no real tty, no real raw mode.
+/// </summary>
+public sealed class TerminalLifecycleTests
+{
+    // ─────────────────────────────────────────────────────────────────────────
+    // Test doubles
+    // ─────────────────────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Byte source that immediately returns 0 (simulates VTIME=1 timeout — no data).
+    /// The input reader thread sees this as a timed read that returned no data, checks
+    /// the stopping flag, and loops — exiting promptly when Stop() is called.
+    /// </summary>
+    private sealed class ImmediateTimeoutByteSource : IInputByteSource
+    {
+        public int Read(Span<byte> buffer) => 0; // always "timed out, no data"
+    }
+
+    /// <summary>
+    /// Output sink that records Paint calls.
+    /// </summary>
+    private sealed class CapturingOutputSink : IOutputSink
+    {
+        private int _paintCount;
+
+        internal int PaintCount => Volatile.Read(ref _paintCount);
+
+        public void Paint(RenderModel model)
+        {
+            Interlocked.Increment(ref _paintCount);
+        }
+
+        public void EmitRestoreSequence() { }
+    }
+
+    /// <summary>
+    /// Output sink that throws on any Paint call.
+    /// Used to simulate an unhandled exception on the loop thread.
+    /// </summary>
+    private sealed class ThrowingOutputSink : IOutputSink
+    {
+        public void Paint(RenderModel model) =>
+            throw new InvalidOperationException("Simulated loop-thread exception.");
+
+        public void EmitRestoreSequence() { }
+    }
+
+    /// <summary>Fixed terminal dimensions.</summary>
+    private sealed class FixedSizeSource : ITerminalSizeSource
+    {
+        public (int Columns, int Rows) GetSize() => (80, 24);
+    }
+
+    /// <summary>
+    /// Real-time clock for lifecycle tests. Uses Stopwatch for a wall-clock source
+    /// (SystemClock is internal-only in the main assembly, so we replicate the shape here).
+    /// </summary>
+    private sealed class TestSystemClock : IClock
+    {
+        private static readonly System.Diagnostics.Stopwatch _sw = System.Diagnostics.Stopwatch.StartNew();
+
+        public TimeSpan Now => _sw.Elapsed;
+
+        public Task WaitUntilAsync(TimeSpan deadline, CancellationToken cancellationToken) =>
+            Task.Delay(
+                TimeSpan.FromMilliseconds(Math.Max(0, (deadline - Now).TotalMilliseconds)),
+                cancellationToken);
+    }
+
+    /// <summary>Simple command that marks the model dirty.</summary>
+    private sealed class DirtyCommandStub : ILoopCommand
+    {
+        void ILoopCommand.Apply(RenderModel model) => model.MarkDirty();
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // Factory helpers
+    // ─────────────────────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// Wires all injected edges through Terminal.StartCore, returning a live Terminal
+    /// and the recording session for observation.
+    /// </summary>
+    private static (DcliTerminal Term, RecordingRawModeSession Session) CreateTerminal(
+        IOutputSink? sink = null,
+        TimeSpan? minFrameInterval = null)
+    {
+        RecordingRawModeSession session = new();
+        RestoreCoordinator coordinator = RestoreCoordinator.Wire(session);
+
+        // NoopResizeWatcher is a no-op disposable; ownership transfers to term on success.
+        using NoopResizeWatcher resizeWatcher = new();
+        DcliTerminal term = DcliTerminal.StartCore(
+            session,
+            coordinator,
+            resizeWatcher,
+            new ImmediateTimeoutByteSource(),
+            new TestSystemClock(),
+            sink ?? new CapturingOutputSink(),
+            new FixedSizeSource(),
+            minFrameInterval ?? TimeSpan.FromMilliseconds(16));
+
+        // coordinator and resizeWatcher transferred to term; DisposeAsync disposes them.
+        // The `using` above also calls Dispose() on scope exit, but NoopResizeWatcher.Dispose() is idempotent.
+        return (term, session);
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // §7.5 — Capability gate: non-VT inputs throw TerminalNotSupportedException
+    // ─────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task NonVtCapabilityInputsThrowTerminalNotSupportedException()
+    {
+        // StartAsync(options, capabilityInputs) throws before entering raw mode.
+        CapabilityInputs notVt = new()
+        {
+            IsSupportedPlatform = true,
+            IsStdinTty = true,
+            IsStdoutTty = true,
+            TermVariable = "dumb",   // triggers the VT check
+        };
+
+        await Assert.ThrowsAsync<TerminalNotSupportedException>(
+            () => DcliTerminal.StartAsync(new TerminalOptions(), notVt));
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // §7.5 — Start: raw mode entered (RestoreCallCount is 0 before Dispose)
+    // ─────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task StartEntersRawModeRestoreCountIsZeroBeforeDispose()
+    {
+        (DcliTerminal term, RecordingRawModeSession session) = CreateTerminal();
+
+        // Session should not have Restore called before we ask for it.
+        Assert.Equal(0, session.RestoreCallCount);
+
+        await term.DisposeAsync();
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // §7.5 — DisposeAsync restores the terminal (at least one Restore)
+    // ─────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task DisposeAsyncRestoresTerminal()
+    {
+        (DcliTerminal term, RecordingRawModeSession session) = CreateTerminal();
+        await term.DisposeAsync();
+
+        // Restore is idempotent; the count should be ≥1 (coordinator.Dispose + session.Dispose).
+        Assert.True(session.RestoreCallCount >= 1,
+            $"Expected Restore called at least once; got {session.RestoreCallCount}.");
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // §7.5 — Single DisposeAsync produces exactly one effective restore
+    // ─────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task SingleDisposeProducesExactlyOneEffectiveRestore()
+    {
+        // Even though multiple code paths call Restore (loop finally, coordinator.Dispose,
+        // session.Dispose), the CAS-idempotent guard collapses them to one effective restore.
+        (DcliTerminal term, RecordingRawModeSession session) = CreateTerminal();
+
+        await term.DisposeAsync();
+
+        Assert.Equal(1, session.EffectiveRestoreCount);
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // §7.5 — DisposeAsync is idempotent: second call does not add effective restores
+    // ─────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task DisposeAsyncIdempotentSecondCallAddsNoEffectiveRestore()
+    {
+        (DcliTerminal term, RecordingRawModeSession session) = CreateTerminal();
+
+        await term.DisposeAsync();
+        int effectiveAfterFirst = session.EffectiveRestoreCount;
+        int rawAfterFirst = session.RestoreCallCount;
+
+        await term.DisposeAsync(); // second call — Terminal guard makes this a no-op
+
+        // Terminal's guard: the second DisposeAsync does nothing at all (no new calls).
+        Assert.Equal(effectiveAfterFirst, session.EffectiveRestoreCount);
+        Assert.Equal(rawAfterFirst, session.RestoreCallCount);
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // §7.5 — Restore on loop-thread crash: loop's finally calls restoreOnExit
+    // ─────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task LoopFinallyRestoresWhenPaintThrows()
+    {
+        // The ThrowingOutputSink throws on the first Paint. To trigger a Paint, the model
+        // must be dirty AND the deadline must pass. We use a short minFrameInterval and
+        // post a DirtyCommandStub directly via LoopEngine (bypassing Terminal's public API
+        // since §9+ commands are not implemented yet).
+        ThrowingOutputSink throwingSink = new();
+        int restoreCallCount = 0;
+
+        // Create LoopEngine directly so we can post a command and observe the restore hook.
+        LoopEngine engine = new(
+            sizeSource: new FixedSizeSource(),
+            clock: new TestSystemClock(),
+            sink: throwingSink,
+            minFrameInterval: TimeSpan.FromMilliseconds(1),
+            restoreOnExit: () => Interlocked.Increment(ref restoreCallCount));
+
+        // Post a command to mark the model dirty, triggering a paint attempt.
+        engine.Post(new DirtyCommandStub());
+
+        // Wait for the loop thread to die from the exception in ThrowingOutputSink.
+        await Task.Delay(200);
+
+        // The loop's finally block must have called restoreOnExit.
+        Assert.True(Volatile.Read(ref restoreCallCount) >= 1,
+            $"Expected restoreOnExit called ≥1 time from loop finally; got {Volatile.Read(ref restoreCallCount)}");
+
+        // Dispose the engine (already stopped; Dispose is idempotent).
+        engine.Dispose();
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // §7.5 — Events channel is reachable from a started Terminal
+    // ─────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task EventsChannelReachableAndNotNull()
+    {
+        (DcliTerminal term, _) = CreateTerminal();
+        await using (term)
+        {
+            ChannelReader<TerminalEvent> events = term.Events;
+            Assert.NotNull(events);
+        }
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // §7.5 — GetTerminalSize returns the injected size
+    // ─────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task GetTerminalSizeReturnsInjectedSize()
+    {
+        (DcliTerminal term, _) = CreateTerminal();
+        await using (term)
+        {
+            (int cols, int rows) = term.GetTerminalSize();
+            Assert.Equal(80, cols);
+            Assert.Equal(24, rows);
+        }
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // §7.5 — Clean shutdown: threads join promptly
+    // ─────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task DisposeAsyncJoinsThreadsPromptly()
+    {
+        // ImmediateTimeoutByteSource returns 0 immediately, so the reader thread's
+        // timed-read loop exits on the next _stopping check after Stop() is called.
+        (DcliTerminal term, _) = CreateTerminal();
+
+        System.Diagnostics.Stopwatch sw = System.Diagnostics.Stopwatch.StartNew();
+        await term.DisposeAsync();
+        sw.Stop();
+
+        // Budget 2s = LoopEngine's 2s thread-join timeout. A real hang trips that timeout
+        // and dispose takes ≥2s; CI scheduling jitter on shared macOS runners can push the
+        // happy path to ~1.5s without anything actually wedging. Matches the 2s budget on
+        // DirtyParkShutdownViaLoopEngineJoinsPromptly below.
+        Assert.True(sw.Elapsed < TimeSpan.FromSeconds(2),
+            $"DisposeAsync took {sw.Elapsed.TotalMilliseconds:F0} ms — threads did not join promptly.");
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // Nit B — dirty-park shutdown: loop parked in dirty WaitAny joins promptly
+    // ─────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task DirtyParkShutdownViaLoopEngineJoinsPromptly()
+    {
+        // Create LoopEngine directly with a very large minFrameInterval.
+        // Post a DirtyCommand so the loop parks in the dirty WaitAny (waiting for
+        // a deadline that would not arrive for hours in real time).
+        // Then Dispose: this must cancel the CTS, unblocking both WaitAny sub-tasks
+        // and allowing the loop thread to join within the timeout.
+        CapturingOutputSink sink = new();
+        LoopEngine engine = new(
+            sizeSource: new FixedSizeSource(),
+            clock: new TestSystemClock(),
+            sink: sink,
+            minFrameInterval: TimeSpan.FromHours(1)); // very long — guarantees dirty-park
+
+        engine.Post(new DirtyCommandStub()); // model becomes dirty → loop enters dirty WaitAny
+
+        // Give the loop a moment to enter the dirty WaitAny.
+        await Task.Delay(50);
+
+        System.Diagnostics.Stopwatch sw = System.Diagnostics.Stopwatch.StartNew();
+        engine.Dispose(); // must unblock the dirty WaitAny via CTS cancellation
+        sw.Stop();
+
+        Assert.True(sw.Elapsed < TimeSpan.FromSeconds(2),
+            $"Dispose from dirty-park state took {sw.Elapsed.TotalMilliseconds:F0} ms — " +
+            "loop thread did not join promptly from dirty WaitAny.");
+    }
+
+    [Fact]
+    public async Task DirtyParkTerminalDisposeJoinsPromptly()
+    {
+        // Same dirty-park test but via the full Terminal lifecycle.
+        // Since Terminal doesn't expose Post() yet, we rely on the clean-idle state
+        // and verify Dispose completes promptly from any park state.
+        (DcliTerminal term, _) = CreateTerminal(
+            minFrameInterval: TimeSpan.FromHours(1)); // ensures dirty WaitAny if model is dirty
+
+        // Model is initially clean (no commands posted), but verify clean-idle dispose is fast too.
+        System.Diagnostics.Stopwatch sw = System.Diagnostics.Stopwatch.StartNew();
+        await term.DisposeAsync();
+        sw.Stop();
+
+        Assert.True(sw.Elapsed < TimeSpan.FromSeconds(2),
+            $"DisposeAsync took {sw.Elapsed.TotalMilliseconds:F0} ms.");
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // Nit 1 — Loop fault: LoopTerminated is faulted when the loop crashes
+    // ─────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task LoopTerminatedIsFaultedWhenLoopCrashes()
+    {
+        LoopEngine engine = new(
+            sizeSource: new FixedSizeSource(),
+            clock: new TestSystemClock(),
+            sink: new ThrowingOutputSink(),
+            minFrameInterval: TimeSpan.FromMilliseconds(1));
+
+        engine.Post(new DirtyCommandStub());
+
+        // Wait for the loop to die.
+        await Task.Delay(200);
+
+        Assert.True(engine.LoopTerminated.IsFaulted,
+            "LoopTerminated task must be faulted after a loop-thread exception.");
+        Assert.IsType<InvalidOperationException>(engine.LoopTerminated.Exception!.GetBaseException());
+
+        engine.Dispose();
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // Nit 1 — Loop fault: DisposeAsync rethrows fault and terminal is restored
+    // ─────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task DisposeAsyncRethrowsLoopFaultAndTerminalIsRestored()
+    {
+        // Use the full Terminal lifecycle with a throwing sink.
+        // Post a dirty command via the internal Loop property so paint fires.
+        (DcliTerminal term, RecordingRawModeSession session) = CreateTerminal(
+            sink: new ThrowingOutputSink(),
+            minFrameInterval: TimeSpan.FromMilliseconds(1));
+
+        term.Loop.Post(new DirtyCommandStub());
+
+        // Wait for the loop to crash.
+        await Task.Delay(200);
+
+        // (a) Terminal was restored via the loop's finally block.
+        Assert.Equal(1, session.EffectiveRestoreCount);
+
+        // (b) DisposeAsync rethrows the fault.
+        Exception? caught = null;
+        try
+        {
+            await term.DisposeAsync();
+        }
+        catch (InvalidOperationException ex)
+        {
+            caught = ex;
+        }
+
+        Assert.NotNull(caught);
+        Assert.Contains("Simulated loop-thread exception", caught.Message, StringComparison.Ordinal);
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // Nit 1 — Loop fault: SettleAsync outstanding at crash time does not hang
+    // ─────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public async Task SettleAsyncFaultsWhenLoopCrashesDuringPaint()
+    {
+        // Use a sink that blocks until signalled, then throws, so we can enqueue a
+        // SettleAsync TCS before the exception fires.
+        using SemaphoreSlim paintGate = new(0, 1);
+        using SemaphoreSlim paintStarted = new(0, 1);
+
+        BlockThenThrowSink blockSink = new(paintStarted, paintGate);
+
+        LoopEngine engine = new(
+            sizeSource: new FixedSizeSource(),
+            clock: new TestSystemClock(),
+            sink: blockSink,
+            minFrameInterval: TimeSpan.FromMilliseconds(1));
+
+        // Dirty the model so paint fires.
+        engine.Post(new DirtyCommandStub());
+
+        // Wait until paint has started (sink is blocking).
+        bool started = await paintStarted.WaitAsync(TimeSpan.FromSeconds(2));
+        Assert.True(started, "Timed out waiting for paint to start.");
+
+        // Enqueue a settle WHILE the loop is blocking inside paint.
+        Task settleTask = engine.SettleAsync();
+
+        // Now let the sink throw.
+        paintGate.Release();
+
+        // The settle task must not hang — it must fault (or cancel) within a short timeout.
+        Task faultOrCancel = await Task.WhenAny(settleTask, Task.Delay(TimeSpan.FromSeconds(2)));
+        Assert.Equal(settleTask, faultOrCancel); // completed, not timeout
+        Assert.True(settleTask.IsFaulted || settleTask.IsCanceled,
+            $"Expected settle to fault or cancel; status={settleTask.Status}");
+
+        engine.Dispose();
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // ANSI restore sequence emitted on disposal (cursor-hidden bug fix)
+    // ─────────────────────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// DisposeAsync must cause the loop's finally block to emit the ANSI restore sequence
+    /// (ESC[?2026l ESC[?25h ESC[0m) before releasing the terminal session.
+    /// This ensures cursor visibility is restored even when a dialog had hidden the cursor.
+    /// </summary>
+    [Fact]
+    public async Task DisposeAsyncEmitsAnsiRestoreSequenceBeforeTermiosRestore()
+    {
+        // Use a VtFrameRenderer backed by a StringWriter so we can inspect raw output.
+        StringWriter writer = new();
+        VtFrameRenderer renderer = new(writer);
+
+        // Build the session manually so we can attach the ordering probe before wiring.
+        // `using` satisfies CA2000; DisposeAsync also calls Restore() which is idempotent.
+        using RecordingRawModeSession session = new();
+        // Snapshot the writer's char count at the moment Restore() is first called so we
+        // can assert the ANSI bytes were already in the buffer (i.e., emitted before termios restore).
+        session.GetOutputLength = () => writer.GetStringBuilder().Length;
+
+        using RestoreCoordinator coordinator = RestoreCoordinator.Wire(session);
+        using NoopResizeWatcher resizeWatcher = new();
+        DcliTerminal term = DcliTerminal.StartCore(
+            session, coordinator, resizeWatcher,
+            new ImmediateTimeoutByteSource(), new TestSystemClock(),
+            renderer, new FixedSizeSource(),
+            minFrameInterval: TimeSpan.FromMilliseconds(16));
+
+        await term.DisposeAsync();
+
+        string output = writer.ToString();
+        const string restoreSuffix = "\x1b[?2026l\x1b[?25h\x1b[0m";
+
+        // The restore sequence must be present in the output.
+        Assert.Contains("\x1b[?2026l", output, StringComparison.Ordinal);
+        Assert.Contains("\x1b[?25h", output, StringComparison.Ordinal);
+        Assert.Contains("\x1b[0m", output, StringComparison.Ordinal);
+
+        // Termios restore must also have been called.
+        Assert.True(session.RestoreCallCount >= 1,
+            "Expected Restore called at least once alongside the ANSI restore.");
+
+        // Ordering: the ANSI bytes must have been in the buffer when Restore() was called.
+        // OutputLengthAtRestore captures the writer length at the moment of the first Restore()
+        // call. The restore suffix must fall within that prefix — not appended after.
+        long lengthAtRestore = session.OutputLengthAtRestore;
+        Assert.True(lengthAtRestore >= restoreSuffix.Length,
+            $"OutputLengthAtRestore ({lengthAtRestore}) is smaller than the restore suffix — ANSI bytes were not emitted before termios restore.");
+        string prefixAtRestore = output[..(int)lengthAtRestore];
+        Assert.EndsWith(restoreSuffix, prefixAtRestore, StringComparison.Ordinal);
+    }
+
+    /// <summary>
+    /// The restore sequence must appear at the end of the output (after any frame content),
+    /// confirming it is the last thing emitted before the terminal session is released.
+    /// </summary>
+    [Fact]
+    public async Task AnsiRestoreSequenceAppearsAtEndOfOutput()
+    {
+        StringWriter writer = new();
+        VtFrameRenderer renderer = new(writer);
+
+        (DcliTerminal term, _) = CreateTerminal(sink: renderer);
+
+        await term.DisposeAsync();
+
+        string output = writer.ToString();
+        const string restoreSuffix = "\x1b[?2026l\x1b[?25h\x1b[0m";
+
+        Assert.EndsWith(restoreSuffix, output, StringComparison.Ordinal);
+    }
+
+    /// <summary>
+    /// SimulateTerminateSignal must invoke the halt action wired by Terminal.StartCore (which
+    /// calls loop.Dispose, causing the loop's finally to emit the ANSI restore sequence on the
+    /// loop thread) and then call session.Restore via the coordinator.
+    /// </summary>
+    [Fact]
+    public async Task SimulateTerminateSignalHaltsLoopAndEmitsAnsiRestoreSequence()
+    {
+        StringWriter writer = new();
+        VtFrameRenderer renderer = new(writer);
+
+        (DcliTerminal term, RecordingRawModeSession session) = CreateTerminal(sink: renderer);
+
+        // Exercise the actual signal path: SimulateTerminateSignal → coordinator invokes
+        // the halt action (loop.Dispose) → loop.Dispose joins the thread → loop's finally
+        // emits the ANSI restore → then coordinator calls session.Restore.
+        // This is distinct from DisposeAsync(), which skips the halt action and goes directly
+        // to coordinator.Dispose(). Only this path verifies the SetHaltAction wiring.
+        term.Coordinator.SimulateTerminateSignal();
+
+        // Give the loop thread up to 2 s to join (Dispose already joined it synchronously,
+        // but LoopTerminated is the async observable handle).
+        await term.Loop.LoopTerminated.WaitAsync(TimeSpan.FromSeconds(2));
+
+        string output = writer.ToString();
+        const string restoreSuffix = "\x1b[?2026l\x1b[?25h\x1b[0m";
+
+        // The ANSI restore sequence must appear in the output (emitted by the loop finally).
+        Assert.EndsWith(restoreSuffix, output, StringComparison.Ordinal);
+
+        // Session restore must also have been called (by the coordinator after halt).
+        Assert.True(session.RestoreCallCount >= 1,
+            $"Expected RestoreCallCount ≥ 1 after SimulateTerminateSignal; got {session.RestoreCallCount}.");
+
+        // Suppress the DisposeAsync fault that may surface if the loop was already stopped.
+#pragma warning disable CA1031 // intentional: suppress any residual cleanup exception
+        try { await term.DisposeAsync(); } catch (Exception) { }
+#pragma warning restore CA1031
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // Nit 1 helper sink: blocks paint until signalled, then throws
+    // ─────────────────────────────────────────────────────────────────────────
+
+    private sealed class BlockThenThrowSink : IOutputSink
+    {
+        private readonly SemaphoreSlim _started;
+        private readonly SemaphoreSlim _gate;
+
+        internal BlockThenThrowSink(SemaphoreSlim started, SemaphoreSlim gate)
+        {
+            _started = started;
+            _gate = gate;
+        }
+
+        public void Paint(RenderModel model)
+        {
+            _started.Release(); // signal "I have started"
+            _gate.Wait();       // block until the test releases us
+            throw new InvalidOperationException("Simulated loop-thread exception.");
+        }
+
+        public void EmitRestoreSequence() { }
+    }
+}
diff --git a/tests/Dcli.Tests/TerminalDriverTests.cs b/tests/Dcli.Tests/TerminalDriverTests.cs
new file mode 100644
index 0000000..f9e3735
--- /dev/null
+++ b/tests/Dcli.Tests/TerminalDriverTests.cs
@@ -0,0 +1,343 @@
+using System.Runtime.InteropServices;
+using System.Runtime.Versioning;
+using Dcli.Internal;
+using Dcli.Internal.Posix;
+using Xunit;
+
+namespace Dcli.Tests;
+
+/// <summary>
+/// Automated tests for Section 4: terminal driver — capability detection (4.3) and
+/// guaranteed-restore wiring (4.4).
+/// <para>
+/// Raw-mode entry itself cannot be unit-tested without a real tty. Those behaviours are
+/// covered by the manual harness (task 4.5 / samples/Dcli.RawModeHarness).
+/// </para>
+/// </summary>
+public sealed class TerminalDriverTests
+{
+    // ─────────────────────────────────────────────────────────────────────────
+    // Termios struct layout guards (platform-independent Marshal.SizeOf checks)
+    //
+    // These tests run on any OS — Marshal.SizeOf measures the managed struct layout,
+    // which must match the native struct layout on the corresponding platform.
+    //
+    // Expected sizes (verified against SDK headers):
+    //   TermiosMac  (macOS): 4×ulong(32) + 20×byte(20) + 4 pad + 2×ulong(16) = 72 bytes
+    //   TermiosLinux (Linux): 4×uint(16) + byte(1) + 32×byte(32) + 3 pad + 2×uint(8) = 60 bytes
+    // ─────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    [SupportedOSPlatform("macos")]
+    public void TermiosMacSizeIs72Bytes()
+    {
+        // macOS struct termios: 4 × tcflag_t(ulong,8) + c_cc[20] + implicit pad(4) + 2 × speed_t(ulong,8)
+        // Guards against a spurious c_line byte (Linux-only) being added, which would corrupt VMIN/VTIME offsets.
+        Assert.Equal(72, Marshal.SizeOf<TermiosMac>());
+    }
+
+    [Fact]
+    [SupportedOSPlatform("linux")]
+    public void TermiosLinuxSizeIs60Bytes()
+    {
+        // Linux struct termios: 4 × tcflag_t(uint,4) + c_line(1) + c_cc[32] + implicit pad(3) + 2 × speed_t(uint,4)
+        Assert.Equal(60, Marshal.SizeOf<TermiosLinux>());
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // 4.3 Capability detection
+    // ─────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void CapabilityDetectorVtCapableDoesNotThrow()
+    {
+        CapabilityInputs inputs = new()
+        {
+            TermVariable = "xterm-256color",
+            IsStdinTty = true,
+            IsStdoutTty = true,
+            IsSupportedPlatform = true,
+        };
+
+        // Should not throw.
+        TerminalCapabilityDetector.EnsureVtCapable(inputs);
+    }
+
+    [Fact]
+    public void CapabilityDetectorDumbTermThrows()
+    {
+        CapabilityInputs inputs = new()
+        {
+            TermVariable = "dumb",
+            IsStdinTty = true,
+            IsStdoutTty = true,
+            IsSupportedPlatform = true,
+        };
+
+        TerminalNotSupportedException ex =
+            Assert.Throws<TerminalNotSupportedException>(() => TerminalCapabilityDetector.EnsureVtCapable(inputs));
+
+        Assert.Contains("TERM=dumb", ex.Message, StringComparison.OrdinalIgnoreCase);
+    }
+
+    [Fact]
+    public void CapabilityDetectorEmptyTermThrows()
+    {
+        CapabilityInputs inputs = new()
+        {
+            TermVariable = "",
+            IsStdinTty = true,
+            IsStdoutTty = true,
+            IsSupportedPlatform = true,
+        };
+
+        TerminalNotSupportedException ex =
+            Assert.Throws<TerminalNotSupportedException>(() => TerminalCapabilityDetector.EnsureVtCapable(inputs));
+
+        Assert.Contains("TERM is not set", ex.Message, StringComparison.OrdinalIgnoreCase);
+    }
+
+    [Fact]
+    public void CapabilityDetectorNullTermThrows()
+    {
+        CapabilityInputs inputs = new()
+        {
+            TermVariable = null,
+            IsStdinTty = true,
+            IsStdoutTty = true,
+            IsSupportedPlatform = true,
+        };
+
+        TerminalNotSupportedException ex =
+            Assert.Throws<TerminalNotSupportedException>(() => TerminalCapabilityDetector.EnsureVtCapable(inputs));
+
+        Assert.Contains("TERM is not set", ex.Message, StringComparison.OrdinalIgnoreCase);
+    }
+
+    [Fact]
+    public void CapabilityDetectorStdinNotTtyThrows()
+    {
+        CapabilityInputs inputs = new()
+        {
+            TermVariable = "xterm-256color",
+            IsStdinTty = false,
+            IsStdoutTty = true,
+            IsSupportedPlatform = true,
+        };
+
+        TerminalNotSupportedException ex =
+            Assert.Throws<TerminalNotSupportedException>(() => TerminalCapabilityDetector.EnsureVtCapable(inputs));
+
+        Assert.Contains("stdin", ex.Message, StringComparison.OrdinalIgnoreCase);
+    }
+
+    [Fact]
+    public void CapabilityDetectorStdoutNotTtyThrows()
+    {
+        CapabilityInputs inputs = new()
+        {
+            TermVariable = "xterm-256color",
+            IsStdinTty = true,
+            IsStdoutTty = false,
+            IsSupportedPlatform = true,
+        };
+
+        TerminalNotSupportedException ex =
+            Assert.Throws<TerminalNotSupportedException>(() => TerminalCapabilityDetector.EnsureVtCapable(inputs));
+
+        Assert.Contains("stdout", ex.Message, StringComparison.OrdinalIgnoreCase);
+    }
+
+    [Fact]
+    public void CapabilityDetectorUnsupportedPlatformThrows()
+    {
+        CapabilityInputs inputs = new()
+        {
+            TermVariable = "xterm-256color",
+            IsStdinTty = true,
+            IsStdoutTty = true,
+            IsSupportedPlatform = false,
+        };
+
+        Assert.Throws<TerminalNotSupportedException>(() => TerminalCapabilityDetector.EnsureVtCapable(inputs));
+    }
+
+    [Theory]
+    [InlineData("xterm")]
+    [InlineData("xterm-256color")]
+    [InlineData("xterm-kitty")]
+    [InlineData("screen")]
+    [InlineData("tmux-256color")]
+    [InlineData("rxvt-unicode")]
+    public void CapabilityDetectorKnownVtTermsDoNotThrow(string term)
+    {
+        CapabilityInputs inputs = new()
+        {
+            TermVariable = term,
+            IsStdinTty = true,
+            IsStdoutTty = true,
+            IsSupportedPlatform = true,
+        };
+
+        TerminalCapabilityDetector.EnsureVtCapable(inputs); // must not throw
+    }
+
+    [Fact]
+    public void TerminalNotSupportedExceptionIsPublic()
+    {
+        // The exception is documented as public so consumers can catch it.
+        Assert.True(typeof(TerminalNotSupportedException).IsPublic);
+    }
+
+    [Fact]
+    public void TerminalNotSupportedExceptionMessageConstructorSetsMessage()
+    {
+        TerminalNotSupportedException ex = new("test message");
+        Assert.Equal("test message", ex.Message);
+    }
+
+    [Fact]
+    public void TerminalNotSupportedExceptionInnerExceptionConstructorSetsInner()
+    {
+        InvalidOperationException inner = new("inner");
+        TerminalNotSupportedException ex = new("outer", inner);
+        Assert.Same(inner, ex.InnerException);
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // 4.4 Guaranteed restore — wiring logic (via RecordingRawModeSession)
+    // ─────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void DisposeRestoresSession()
+    {
+        using RecordingRawModeSession fake = new();
+        using RestoreCoordinator coord = RestoreCoordinator.Wire(fake);
+
+        coord.Dispose();
+
+        Assert.Equal(1, fake.RestoreCallCount);
+    }
+
+    [Fact]
+    public void DisposeIsIdempotent()
+    {
+        using RecordingRawModeSession fake = new();
+        using RestoreCoordinator coord = RestoreCoordinator.Wire(fake);
+
+        coord.Dispose();
+        coord.Dispose();
+        coord.Dispose();
+
+        // RestoreCoordinator.Dispose is idempotent via CAS; only one Restore reaches the session.
+        Assert.Equal(1, fake.RestoreCallCount);
+    }
+
+    [Fact]
+    public void SessionRestoreRecordsEachCall()
+    {
+        // RecordingRawModeSession.Restore records every call (it is not itself idempotent).
+        using RecordingRawModeSession fake = new();
+        fake.Restore();
+        fake.Restore();
+        fake.Restore();
+        Assert.Equal(3, fake.RestoreCallCount);
+    }
+
+    [Fact]
+    public void SimulateTerminateSignalRestoresSession()
+    {
+        using RecordingRawModeSession fake = new();
+        using RestoreCoordinator coord = RestoreCoordinator.Wire(fake);
+
+        coord.SimulateTerminateSignal();
+
+        Assert.Equal(1, fake.RestoreCallCount);
+    }
+
+    [Fact]
+    public void SimulateTerminateSignalAfterDisposeCallsRestoreAgain()
+    {
+        using RecordingRawModeSession fake = new();
+        using RestoreCoordinator coord = RestoreCoordinator.Wire(fake);
+
+        coord.Dispose(); // Restore called once here.
+        coord.SimulateTerminateSignal(); // Session's Restore called a second time (handler always delegates).
+
+        Assert.Equal(2, fake.RestoreCallCount);
+    }
+
+    [Fact]
+    public void SimulateContinueSignalReappliesRawMode()
+    {
+        using RecordingRawModeSession fake = new();
+        using RestoreCoordinator coord = RestoreCoordinator.Wire(fake);
+
+        coord.SimulateContinueSignal();
+
+        Assert.Equal(1, fake.ReapplyCallCount);
+        Assert.Equal(0, fake.RestoreCallCount);
+    }
+
+    [Fact]
+    public void OnProcessExitRestoresSession()
+    {
+        using RecordingRawModeSession fake = new();
+        using RestoreCoordinator coord = RestoreCoordinator.Wire(fake);
+
+        coord.OnProcessExit(sender: null, EventArgs.Empty);
+
+        Assert.Equal(1, fake.RestoreCallCount);
+    }
+
+    [Fact]
+    public void SessionDisposeCallsRestore()
+    {
+        using RecordingRawModeSession fake = new();
+        ((IDisposable)fake).Dispose();
+        Assert.Equal(1, fake.RestoreCallCount);
+    }
+
+    // ─────────────────────────────────────────────────────────────────────────
+    // SetHaltAction wiring: SimulateTerminateSignal must invoke the halt action
+    // ─────────────────────────────────────────────────────────────────────────
+
+    /// <summary>
+    /// When a halt action is registered via SetHaltAction, SimulateTerminateSignal must invoke
+    /// it before calling session.Restore. This is the unit-level contract for the wiring that
+    /// Terminal.StartCore establishes: coordinator.SetHaltAction(loop.Dispose).
+    /// </summary>
+    [Fact]
+    public void SimulateTerminateSignalInvokesSetHaltActionThenRestores()
+    {
+        using RecordingRawModeSession fake = new();
+        using RestoreCoordinator coord = RestoreCoordinator.Wire(fake);
+
+        int haltCallCount = 0;
+        coord.SetHaltAction(() => Interlocked.Increment(ref haltCallCount));
+
+        coord.SimulateTerminateSignal();
+
+        // The halt action must have been invoked.
+        Assert.Equal(1, Volatile.Read(ref haltCallCount));
+        // Session restore must also have been called.
+        Assert.Equal(1, fake.RestoreCallCount);
+    }
+
+    /// <summary>
+    /// Removing SetHaltAction wiring (by never calling SetHaltAction) means the halt action is
+    /// not invoked — but session.Restore still is. This is the regression baseline: if Terminal
+    /// never calls coordinator.SetHaltAction(loop.Dispose), the loop does not stop on signal.
+    /// </summary>
+    [Fact]
+    public void SimulateTerminateSignalWithNoHaltActionStillRestoresSession()
+    {
+        using RecordingRawModeSession fake = new();
+        using RestoreCoordinator coord = RestoreCoordinator.Wire(fake);
+
+        // No SetHaltAction call — mimics the broken wiring scenario.
+        coord.SimulateTerminateSignal();
+
+        Assert.Equal(1, fake.RestoreCallCount);
+    }
+}
diff --git a/tests/Dcli.Tests/Testing/FrameSnapshotTests.cs b/tests/Dcli.Tests/Testing/FrameSnapshotTests.cs
new file mode 100644
index 0000000..ac6be35
--- /dev/null
+++ b/tests/Dcli.Tests/Testing/FrameSnapshotTests.cs
@@ -0,0 +1,147 @@
+using Dcli.Testing;
+using Xunit;
+
+namespace Dcli.Tests.Testing;
+
+/// <summary>
+/// Tests for §15.4: <see cref="FrameSnapshot"/> and <see cref="FrameSnapshotPrinter"/>.
+/// </summary>
+public sealed class FrameSnapshotTests
+{
+    // ── Pre-paint empty snapshot ──────────────────────────────────────────────
+
+    [Fact]
+    public async Task SnapshotBeforeFirstPaintReturnsEmptyAndDoesNotThrow()
+    {
+        // Access Snapshot before any SettleAsync — must return empty, not throw.
+        await using HeadlessTerminal harness = await HeadlessTerminal.StartAsync();
+
+        FrameSnapshot snap = harness.Snapshot;
+
+        Assert.NotNull(snap);
+        Assert.Equal((0, 0), snap.Size);
+        Assert.Empty(snap.LiveWindowRows);
+        Assert.Empty(snap.FixedRegionRows);
+        Assert.Empty(snap.NewlyCommittedRows);
+        Assert.Null(snap.Caret);
+        Assert.Equal(OverlayKind.None, snap.Overlay.Kind);
+    }
+
+    // ── Snapshot reflects scrollback content ─────────────────────────────────
+
+    [Fact]
+    public async Task SnapshotReflectsScrollbackLine()
+    {
+        await using HeadlessTerminal harness = await HeadlessTerminal.StartAsync(
+            new HeadlessTerminalOptions { InitialColumns = 40, InitialRows = 10 });
+
+        harness.Terminal.Scrollback.Append(new Line([new Segment("hello world")]));
+        await harness.SettleAsync().WaitAsync(TimeSpan.FromSeconds(5));
+
+        FrameSnapshot snap = harness.Snapshot;
+
+        // The appended line should appear in LiveWindowRows or NewlyCommittedRows.
+        // (When the scrollback fills the live window it is committed; either location is correct.)
+        bool inLive = snap.LiveWindowRows.Any(r => r.Segments.Any(
+            s => s.Text.Contains("hello world", StringComparison.Ordinal)));
+        bool inCommitted = snap.NewlyCommittedRows.Any(r => r.Segments.Any(
+            s => s.Text.Contains("hello world", StringComparison.Ordinal)));
+
+        Assert.True(inLive || inCommitted,
+            "Appended line must appear in LiveWindowRows or NewlyCommittedRows.");
+    }
+
+    // ── Snapshot reflects active overlay ─────────────────────────────────────
+
+    [Fact]
+    public async Task SnapshotReflectsAutocompleteOverlay()
+    {
+        await using HeadlessTerminal harness = await HeadlessTerminal.StartAsync(
+            new HeadlessTerminalOptions { InitialColumns = 40, InitialRows = 10 });
+
+        // Show an autocomplete with two candidates.
+        AutocompleteCandidate[] candidates =
+        [
+            new AutocompleteCandidate("/help", new Line([new Segment("/help")])),
+            new AutocompleteCandidate("/quit", new Line([new Segment("/quit")])),
+        ];
+        harness.Terminal.Autocomplete.Show(candidates);
+        await harness.SettleAsync().WaitAsync(TimeSpan.FromSeconds(5));
+
+        FrameSnapshot snap = harness.Snapshot;
+
+        Assert.Equal(OverlayKind.Autocomplete, snap.Overlay.Kind);
+        Assert.Equal(0, snap.Overlay.SelectedIndex); // first item selected by default
+        Assert.True(snap.Overlay.VisibleRowCount > 0);
+    }
+
+    // ── Snapshot is immutable ─────────────────────────────────────────────────
+
+    [Fact]
+    public async Task SnapshotIsImmutableAcrossSubsequentPaints()
+    {
+        await using HeadlessTerminal harness = await HeadlessTerminal.StartAsync(
+            new HeadlessTerminalOptions { InitialColumns = 40, InitialRows = 10 });
+
+        // First paint: one line.
+        harness.Terminal.Scrollback.Append(new Line([new Segment("line A")]));
+        await harness.SettleAsync().WaitAsync(TimeSpan.FromSeconds(5));
+        FrameSnapshot snap1 = harness.Snapshot;
+        int rowCountAfterFirst = snap1.LiveWindowRows.Count + snap1.NewlyCommittedRows.Count;
+
+        // Second paint: additional lines.
+        harness.Terminal.Scrollback.Append(new Line([new Segment("line B")]));
+        harness.Terminal.Scrollback.Append(new Line([new Segment("line C")]));
+        await harness.SettleAsync().WaitAsync(TimeSpan.FromSeconds(5));
+
+        // The first snapshot must be unchanged.
+        int rowCountAfterFirstNow = snap1.LiveWindowRows.Count + snap1.NewlyCommittedRows.Count;
+        Assert.Equal(rowCountAfterFirst, rowCountAfterFirstNow);
+    }
+
+    // ── Pretty-printer golden frame ───────────────────────────────────────────
+
+    [Fact]
+    public async Task PrettyPrintProducesStableGoldenString()
+    {
+        await using HeadlessTerminal harness = await HeadlessTerminal.StartAsync(
+            new HeadlessTerminalOptions { InitialColumns = 40, InitialRows = 6 });
+
+        // Produce a deterministic frame: one scrollback line + a status row.
+        harness.Terminal.Scrollback.Append(new Line([new Segment("hello")]));
+        harness.Terminal.Status.SetRows([new Line([new Segment("status: ok")])]);
+        await harness.SettleAsync().WaitAsync(TimeSpan.FromSeconds(5));
+
+        FrameSnapshot snap = harness.Snapshot;
+        string pretty = FrameSnapshotPrinter.PrettyPrint(snap);
+
+        // Structural assertions that are stable regardless of exact padding.
+        Assert.Contains("hello", pretty, StringComparison.Ordinal);
+        Assert.Contains("status: ok", pretty, StringComparison.Ordinal);
+        Assert.Contains("--- horizon ---", pretty, StringComparison.Ordinal);
+        Assert.Contains("Overlay: None", pretty, StringComparison.Ordinal);
+        Assert.StartsWith("+", pretty.TrimStart('\n'), StringComparison.Ordinal);
+        Assert.EndsWith("+", pretty.TrimEnd('\n'), StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void PrettyPrintEmptySnapshotDoesNotThrow()
+    {
+        string pretty = FrameSnapshotPrinter.PrettyPrint(FrameSnapshot.Empty);
+
+        Assert.NotNull(pretty);
+        Assert.Contains("Overlay: None", pretty, StringComparison.Ordinal);
+    }
+
+    // ── §15.7-4 — Overlay state via snapshot ────────────────────────────────
+
+    [Fact]
+    public async Task SnapshotOverlayNoneWhenNoOverlayActive()
+    {
+        await using HeadlessTerminal harness = await HeadlessTerminal.StartAsync();
+        await harness.SettleAsync().WaitAsync(TimeSpan.FromSeconds(5));
+
+        FrameSnapshot snap = harness.Snapshot;
+        Assert.Equal(OverlayKind.None, snap.Overlay.Kind);
+    }
+}
diff --git a/tests/Dcli.Tests/Testing/GoldenFrameTests.cs b/tests/Dcli.Tests/Testing/GoldenFrameTests.cs
new file mode 100644
index 0000000..0b0b377
--- /dev/null
+++ b/tests/Dcli.Tests/Testing/GoldenFrameTests.cs
@@ -0,0 +1,71 @@
+using Dcli.Testing;
+using Xunit;
+
+namespace Dcli.Tests.Testing;
+
+/// <summary>
+/// End-to-end golden-frame tests (§15.6 retarget): drive commands through the harness,
+/// settle, and assert on <see cref="FrameSnapshotPrinter.PrettyPrint"/> output.
+/// These tests demonstrate the pattern that consumers should use for visual regression testing.
+/// </summary>
+/// <remarks>
+/// The assertions use structural predicates (contains, starts-with) rather than full-string
+/// equality so the golden strings survive minor rendering changes (e.g. padding adjustments).
+/// When you need strict equality, lock the full <see cref="FrameSnapshotPrinter.PrettyPrint"/>
+/// output in a constant and compare with <c>Assert.Equal</c>.
+/// </remarks>
+public sealed class GoldenFrameTests
+{
+    // ── §15.6 — Scrollback + status golden frame ─────────────────────────────
+
+    [Fact]
+    public async Task ScrollbackAndStatusRowProduceExpectedGoldenFrame()
+    {
+        // This test retargets the §8.5 representative scenario:
+        // "scrollback append + fixed-region status row" — end-to-end through the harness.
+        await using HeadlessTerminal harness = await HeadlessTerminal.StartAsync(
+            new HeadlessTerminalOptions { InitialColumns = 40, InitialRows = 6 });
+
+        harness.Terminal.Scrollback.Append(new Line([new Segment("output line")]));
+        harness.Terminal.Status.SetRows([new Line([new Segment("ready")])]);
+        await harness.SettleAsync().WaitAsync(TimeSpan.FromSeconds(5));
+
+        FrameSnapshot snap = harness.Snapshot;
+        string frame = FrameSnapshotPrinter.PrettyPrint(snap);
+
+        // Box structure
+        Assert.Contains("+", frame, StringComparison.Ordinal);
+        Assert.Contains("|", frame, StringComparison.Ordinal);
+        Assert.Contains("--- horizon ---", frame, StringComparison.Ordinal);
+
+        // Content
+        Assert.Contains("output line", frame, StringComparison.Ordinal);
+        Assert.Contains("ready", frame, StringComparison.Ordinal);
+
+        // No active overlay
+        Assert.Contains("Overlay: None", frame, StringComparison.Ordinal);
+
+        // Size was recorded
+        Assert.Equal((40, 6), snap.Size);
+    }
+
+    // ── §15.6 — Input typed into editor appears in fixed region ──────────────
+
+    [Fact]
+    public async Task TypedInputAppearsInFixedRegionRows()
+    {
+        await using HeadlessTerminal harness = await HeadlessTerminal.StartAsync(
+            new HeadlessTerminalOptions { InitialColumns = 40, InitialRows = 6 });
+
+        harness.Type("hello");
+        await harness.SettleAsync().WaitAsync(TimeSpan.FromSeconds(5));
+
+        FrameSnapshot snap = harness.Snapshot;
+        string frame = FrameSnapshotPrinter.PrettyPrint(snap);
+
+        // Typed text appears somewhere in the fixed region
+        bool inFixed = snap.FixedRegionRows.Any(r => r.Segments.Any(s => s.Text.Contains("hello", StringComparison.Ordinal)));
+        Assert.True(inFixed, "Typed text must appear in FixedRegionRows.");
+        Assert.Contains("hello", frame, StringComparison.Ordinal);
+    }
+}
diff --git a/tests/Dcli.Tests/Testing/HeadlessTerminalTests.cs b/tests/Dcli.Tests/Testing/HeadlessTerminalTests.cs
new file mode 100644
index 0000000..e69b6f0
--- /dev/null
+++ b/tests/Dcli.Tests/Testing/HeadlessTerminalTests.cs
@@ -0,0 +1,219 @@
+using Dcli.Testing;
+using Xunit;
+
+namespace Dcli.Tests.Testing;
+
+/// <summary>
+/// Tests for §15 Chunk A: <see cref="HeadlessTerminal"/>, <see cref="VirtualClock"/>,
+/// and the internal edge fakes.
+/// </summary>
+public sealed class HeadlessTerminalTests
+{
+    // ── §15.2 / §15.7-1 — StartAsync lifecycle ───────────────────────────────
+
+    [Fact]
+    public async Task StartAsyncReturnsUsableHarness()
+    {
+        await using HeadlessTerminal harness = await HeadlessTerminal.StartAsync();
+
+        Assert.NotNull(harness);
+        Assert.NotNull(harness.Terminal);
+        Assert.IsAssignableFrom<ITerminal>(harness.Terminal);
+        Assert.NotNull(harness.Clock);
+    }
+
+    [Fact]
+    public async Task DisposeAsyncIsIdempotent()
+    {
+        HeadlessTerminal harness = await HeadlessTerminal.StartAsync();
+
+        // First dispose — must not throw.
+        await harness.DisposeAsync();
+        // Second dispose — must also not throw.
+        await harness.DisposeAsync();
+    }
+
+    // ── §15.7-2 — Byte-feed → parsed event (Up arrow via ESC[A) ─────────────
+
+    [Fact]
+    public async Task FeedEscapeSequenceProducesExpectedKeyEvent()
+    {
+        await using HeadlessTerminal harness = await HeadlessTerminal.StartAsync();
+
+        // ESC O P = F1 (SS3 encoding used by xterm/macOS).
+        // F1 is not consumed by the editor so it reaches the outbound channel as KeyPressed.
+        harness.Feed([0x1B, (byte)'O', (byte)'P']);
+
+        // First settle lets the InputReader thread drain the byte queue.
+        await harness.SettleAsync().WaitAsync(TimeSpan.FromSeconds(5));
+        // Second settle ensures any KeyEvent posted by the InputReader is processed.
+        await harness.SettleAsync().WaitAsync(TimeSpan.FromSeconds(5));
+
+        List<TerminalEvent> events = await DrainEventsAsync(harness.Terminal);
+        KeyPressed? pressed = events.OfType<KeyPressed>().FirstOrDefault();
+
+        Assert.NotNull(pressed);
+        Assert.Equal(KeyCode.Named(NamedKey.F1), pressed.Key.Code);
+    }
+
+    // ── §15.3 / §15.7-3 — SettleAsync → exactly one coalesced frame ─────────
+
+    [Fact]
+    public async Task SettleAsyncMultipleAppendsProducesExactlyOneFrame()
+    {
+        await using HeadlessTerminal harness = await HeadlessTerminal.StartAsync();
+
+        // Post ≥3 scrollback lines before settling.
+        harness.Terminal.Scrollback.Append(new Line([new Segment("line 1")]));
+        harness.Terminal.Scrollback.Append(new Line([new Segment("line 2")]));
+        harness.Terminal.Scrollback.Append(new Line([new Segment("line 3")]));
+
+        int paintsBefore = harness.Sink.PaintCount;
+        await harness.SettleAsync().WaitAsync(TimeSpan.FromSeconds(5));
+        int paintsAfter = harness.Sink.PaintCount;
+
+        // Exactly one coalesced frame was produced for the batch.
+        Assert.Equal(1, paintsAfter - paintsBefore);
+    }
+
+    // ── §15.3 / §15.7-4 — Virtual clock advance triggers a throttled paint ───
+
+    [Fact]
+    public async Task VirtualClockAdvanceTriggersPaintAfterThrottle()
+    {
+        // Use a 100 ms minimum frame interval so paints are throttled.
+        HeadlessTerminalOptions options = new()
+        {
+            MinFrameInterval = TimeSpan.FromMilliseconds(100),
+        };
+        await using HeadlessTerminal harness = await HeadlessTerminal.StartAsync(options);
+
+        // Frame 1: advance clock past initial deadline (starts at zero) and settle.
+        harness.Terminal.Scrollback.Append(new Line([new Segment("first")]));
+        harness.Clock.Advance(TimeSpan.FromMilliseconds(10));
+        await harness.SettleAsync().WaitAsync(TimeSpan.FromSeconds(5));
+        int paintAfterFirst = harness.Sink.PaintCount;
+        Assert.True(paintAfterFirst >= 1, "At least one paint expected after first settle.");
+
+        // Frame 2: post dirty content, then advance past the next deadline (now+100ms).
+        // The settle will only complete once the clock fires and the loop paints.
+        harness.Terminal.Scrollback.Append(new Line([new Segment("second")]));
+        harness.Clock.Advance(TimeSpan.FromMilliseconds(200));
+        await harness.SettleAsync().WaitAsync(TimeSpan.FromSeconds(5));
+        int paintAfterSecond = harness.Sink.PaintCount;
+
+        Assert.True(paintAfterSecond > paintAfterFirst,
+            "A second paint must fire after the clock advances past the frame interval.");
+    }
+
+    // ── §15.2 / §15.7-5 — Resize delivery ───────────────────────────────────
+
+    [Fact]
+    public async Task ResizeDeliversResizedEventAndUpdatesModel()
+    {
+        await using HeadlessTerminal harness = await HeadlessTerminal.StartAsync(
+            new HeadlessTerminalOptions { InitialColumns = 80, InitialRows = 24 });
+
+        harness.Resize(60, 18);
+        await harness.SettleAsync().WaitAsync(TimeSpan.FromSeconds(5));
+
+        List<TerminalEvent> events = await DrainEventsAsync(harness.Terminal);
+        Resized? resized = events.OfType<Resized>().FirstOrDefault();
+
+        Assert.NotNull(resized);
+        Assert.Equal(60, resized.Columns);
+        Assert.Equal(18, resized.Rows);
+
+        (int cols, int rows) = harness.Terminal.GetTerminalSize();
+        Assert.Equal(60, cols);
+        Assert.Equal(18, rows);
+    }
+
+    // ── §15.2 / §15.7-6 — SendKey / Type / Paste basics ─────────────────────
+
+    [Fact]
+    public async Task SendKeyProducesKeyPressedEvent()
+    {
+        await using HeadlessTerminal harness = await HeadlessTerminal.StartAsync();
+
+        harness.SendKey(new KeyEvent(KeyCode.Named(NamedKey.Escape), Modifiers.None));
+        await harness.SettleAsync().WaitAsync(TimeSpan.FromSeconds(5));
+
+        List<TerminalEvent> events = await DrainEventsAsync(harness.Terminal);
+        Assert.Contains(events, e => e is KeyPressed { Key.Code.NamedValue: NamedKey.Escape });
+    }
+
+    [Fact]
+    public async Task TypeSingleCharacterProducesInputChangedEvent()
+    {
+        await using HeadlessTerminal harness = await HeadlessTerminal.StartAsync();
+
+        harness.Type("x");
+        await harness.SettleAsync().WaitAsync(TimeSpan.FromSeconds(5));
+
+        List<TerminalEvent> events = await DrainEventsAsync(harness.Terminal);
+        InputChanged? changed = events.OfType<InputChanged>().FirstOrDefault();
+        Assert.NotNull(changed);
+        Assert.Equal("x", changed.Text);
+    }
+
+    [Fact]
+    public async Task PastePostsToLoopWithoutCrashing()
+    {
+        await using HeadlessTerminal harness = await HeadlessTerminal.StartAsync();
+
+        // PasteEvent routing to the editor is not yet implemented (deferred to a future chunk).
+        // Verify the harness can post a paste and settle without crashing or deadlocking, and
+        // that the loop emits a paint (the model is marked dirty on every input event).
+        int paintsBefore = harness.Sink.PaintCount;
+        harness.Paste("hello");
+        await harness.SettleAsync().WaitAsync(TimeSpan.FromSeconds(5));
+
+        Assert.True(harness.Sink.PaintCount > paintsBefore,
+            "Loop must have painted at least once after processing the PasteEvent.");
+    }
+
+    // ── §15.7-7 — HeadlessRawModeSession is a no-op / independent harnesses ──
+
+    [Fact]
+    public async Task TwoHarnessesInSameProcessDoNotConflict()
+    {
+        // Two harnesses must be independent: each has its own model, session, and loop.
+        await using HeadlessTerminal first = await HeadlessTerminal.StartAsync();
+        await using HeadlessTerminal second = await HeadlessTerminal.StartAsync();
+
+        first.Type("a");
+        second.Type("b");
+
+        await first.SettleAsync().WaitAsync(TimeSpan.FromSeconds(5));
+        await second.SettleAsync().WaitAsync(TimeSpan.FromSeconds(5));
+
+        Assert.NotNull(first.Sink.LastModel);
+        Assert.NotNull(second.Sink.LastModel);
+        Assert.Equal("a", first.Sink.LastModel.FixedRegion.Editor.Text);
+        Assert.Equal("b", second.Sink.LastModel.FixedRegion.Editor.Text);
+    }
+
+    // ── Helpers ──────────────────────────────────────────────────────────────
+
+    private static async Task<List<TerminalEvent>> DrainEventsAsync(
+        ITerminal terminal,
+        int minCount = 1)
+    {
+        using CancellationTokenSource cts = new(TimeSpan.FromSeconds(5));
+        List<TerminalEvent> events = [];
+
+        while (events.Count < minCount)
+        {
+            if (terminal.Events.TryRead(out TerminalEvent? ev))
+                events.Add(ev);
+            else
+                await terminal.Events.WaitToReadAsync(cts.Token);
+        }
+
+        while (terminal.Events.TryRead(out TerminalEvent? extra))
+            events.Add(extra);
+
+        return events;
+    }
+}
diff --git a/tests/Dcli.Tests/Testing/LoopCoalescingHarnessTests.cs b/tests/Dcli.Tests/Testing/LoopCoalescingHarnessTests.cs
new file mode 100644
index 0000000..346df6f
--- /dev/null
+++ b/tests/Dcli.Tests/Testing/LoopCoalescingHarnessTests.cs
@@ -0,0 +1,110 @@
+using Dcli.Testing;
+using Xunit;
+
+namespace Dcli.Tests.Testing;
+
+/// <summary>
+/// §15.6 retarget: loop-coalescing tests (§7.6) run on the headless harness.
+/// These complement <c>RenderLoopTests</c> which drives <c>LoopEngine</c> directly.
+/// The harness-based versions verify the same behaviour through the public API
+/// so consumer code can rely on identical semantics.
+/// </summary>
+/// <remarks>
+/// The existing direct-<c>LoopEngine</c> tests in <c>RenderLoopTests</c> are kept as-is:
+/// some assert on internal state (<c>WaitCount</c>, <c>OutboundEvents.Count</c>,
+/// <c>DirtyCommand</c>) that the harness does not expose. Those remain the
+/// unit-level substrate; these tests are the integration-level substrate.
+/// </remarks>
+public sealed class LoopCoalescingHarnessTests
+{
+    // ── §7.6 — Burst coalescing ───────────────────────────────────────────────
+
+    [Fact]
+    public async Task BurstNScrollbackAppendsProducesExactlyOneFrame()
+    {
+        // Mirrors RenderLoopTests.BurstCoalescingNCommandsProducesExactlyOneFrame,
+        // but driven via the public Scrollback.Append API through the harness.
+        await using HeadlessTerminal harness = await HeadlessTerminal.StartAsync();
+
+        int paintsBefore = harness.Sink.PaintCount;
+
+        // Post 10 appends before settling.
+        for (int i = 0; i < 10; i++)
+        {
+            harness.Terminal.Scrollback.Append(new Line([new Segment($"line {i}")]));
+        }
+
+        await harness.SettleAsync().WaitAsync(TimeSpan.FromSeconds(5));
+
+        Assert.Equal(1, harness.Sink.PaintCount - paintsBefore);
+    }
+
+    [Fact]
+    public async Task BurstNTypeEventsProducesExactlyOneFrame()
+    {
+        // Mirrors RenderLoopTests.BurstCoalescingNInputEventsProducesExactlyOneFrame,
+        // but driven via harness.Type() which posts KeyEvents directly to the loop.
+        await using HeadlessTerminal harness = await HeadlessTerminal.StartAsync();
+
+        int paintsBefore = harness.Sink.PaintCount;
+
+        // Type 5 characters before settling.
+        for (int i = 0; i < 5; i++)
+        {
+            harness.Type("x");
+        }
+
+        await harness.SettleAsync().WaitAsync(TimeSpan.FromSeconds(5));
+
+        Assert.Equal(1, harness.Sink.PaintCount - paintsBefore);
+    }
+
+    // ── §7.6 — Throttle with virtual clock ───────────────────────────────────
+
+    [Fact]
+    public async Task ThrottleSecondSettleBeforeIntervalElapseDoesNotPaint()
+    {
+        // Mirrors RenderLoopTests.ThrottleSecondSettleBeforeIntervalElapseDoesNotPaint.
+        HeadlessTerminalOptions options = new()
+        {
+            MinFrameInterval = TimeSpan.FromMilliseconds(100),
+        };
+        await using HeadlessTerminal harness = await HeadlessTerminal.StartAsync(options);
+
+        // Frame 1: model is dirty from startup; first settle paints.
+        harness.Terminal.Scrollback.Append(new Line([new Segment("first")]));
+        await harness.SettleAsync().WaitAsync(TimeSpan.FromSeconds(5));
+        int afterFirst = harness.Sink.PaintCount;
+
+        // Frame 2 attempt: dirty again, but clock hasn't advanced — throttled.
+        harness.Terminal.Scrollback.Append(new Line([new Segment("second")]));
+        await harness.SettleAsync().WaitAsync(TimeSpan.FromSeconds(5));
+        int afterSecond = harness.Sink.PaintCount;
+
+        Assert.Equal(afterFirst, afterSecond); // no extra paint — throttled
+    }
+
+    [Fact]
+    public async Task ThrottleAdvancingVirtualClockAllowsNextFrame()
+    {
+        // Mirrors RenderLoopTests.ThrottleAdvancingClockAllowsNextFrame.
+        HeadlessTerminalOptions options = new()
+        {
+            MinFrameInterval = TimeSpan.FromMilliseconds(100),
+        };
+        await using HeadlessTerminal harness = await HeadlessTerminal.StartAsync(options);
+
+        // Frame 1.
+        harness.Terminal.Scrollback.Append(new Line([new Segment("first")]));
+        await harness.SettleAsync().WaitAsync(TimeSpan.FromSeconds(5));
+        int afterFirst = harness.Sink.PaintCount;
+
+        // Advance clock past the interval, then post dirty and settle.
+        harness.Clock.Advance(TimeSpan.FromMilliseconds(200));
+        harness.Terminal.Scrollback.Append(new Line([new Segment("second")]));
+        await harness.SettleAsync().WaitAsync(TimeSpan.FromSeconds(5));
+        int afterSecond = harness.Sink.PaintCount;
+
+        Assert.True(afterSecond > afterFirst, "Second frame must fire after clock advances.");
+    }
+}
diff --git a/tests/Dcli.Tests/TextBufferTests.cs b/tests/Dcli.Tests/TextBufferTests.cs
new file mode 100644
index 0000000..3f1f098
--- /dev/null
+++ b/tests/Dcli.Tests/TextBufferTests.cs
@@ -0,0 +1,774 @@
+using System.Text;
+using Dcli.Internal.FixedRegion;
+using Xunit;
+
+namespace Dcli.Tests;
+
+/// <summary>
+/// Unit tests for <see cref="TextBuffer"/> — tasks 10.1 and 10.2.
+/// </summary>
+public sealed class TextBufferTests
+{
+    // ── Helpers ─────────────────────────────────────────────────────────────────────────────
+
+    private static TextBuffer Buffer(string text = "")
+    {
+        TextBuffer buf = new();
+        if (text.Length > 0)
+            buf.Insert(text);
+        return buf;
+    }
+
+    /// <summary>
+    /// Sets the caret index directly. Only valid if the index is a valid char boundary.
+    /// </summary>
+    private static void SetCaret(TextBuffer buf, int index) =>
+        buf.SetCaretIndexForTest(index);
+
+    // ── Basic insert / caret ─────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void InsertRuneAdvancesCaretByUtf16Length()
+    {
+        TextBuffer buf = new();
+        buf.Insert(new Rune('A'));
+        Assert.Equal("A", buf.Text);
+        Assert.Equal(1, buf.CaretIndex);
+    }
+
+    [Fact]
+    public void InsertSupplementaryRuneAdvancesCaretBy2()
+    {
+        // U+1F600 😀 is a surrogate pair: Utf16SequenceLength = 2.
+        TextBuffer buf = new();
+        buf.Insert(new Rune(0x1F600));
+        Assert.Equal("\U0001F600", buf.Text);
+        Assert.Equal(2, buf.CaretIndex);
+    }
+
+    [Fact]
+    public void InsertStringCaretAtEnd()
+    {
+        TextBuffer buf = new();
+        buf.Insert("hello");
+        Assert.Equal("hello", buf.Text);
+        Assert.Equal(5, buf.CaretIndex);
+    }
+
+    [Fact]
+    public void InsertMidTextInsertsAtCaret()
+    {
+        TextBuffer buf = Buffer("ac");
+        SetCaret(buf, 1); // between a and c
+        buf.Insert(new Rune('b'));
+        Assert.Equal("abc", buf.Text);
+        Assert.Equal(2, buf.CaretIndex);
+    }
+
+    [Fact]
+    public void InsertNewlineSplitsLine()
+    {
+        TextBuffer buf = Buffer("ab");
+        SetCaret(buf, 1);
+        buf.InsertNewline();
+        Assert.Equal("a\nb", buf.Text);
+        Assert.Equal(2, buf.CaretIndex);
+    }
+
+    [Fact]
+    public void SetTextReplacesAndMovesCaretToEnd()
+    {
+        TextBuffer buf = Buffer("old");
+        buf.SetText("new content");
+        Assert.Equal("new content", buf.Text);
+        Assert.Equal(11, buf.CaretIndex);
+    }
+
+    [Fact]
+    public void ClearEmptiesBuffer()
+    {
+        TextBuffer buf = Buffer("hello");
+        buf.Clear();
+        Assert.Equal(string.Empty, buf.Text);
+        Assert.Equal(0, buf.CaretIndex);
+    }
+
+    // ── Backspace ────────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void BackspaceDeletesAsciiChar()
+    {
+        TextBuffer buf = Buffer("hello");
+        buf.Backspace();
+        Assert.Equal("hell", buf.Text);
+        Assert.Equal(4, buf.CaretIndex);
+    }
+
+    [Fact]
+    public void BackspaceAtStartIsNoOp()
+    {
+        TextBuffer buf = Buffer("hi");
+        SetCaret(buf, 0);
+        buf.Backspace();
+        Assert.Equal("hi", buf.Text);
+        Assert.Equal(0, buf.CaretIndex);
+    }
+
+    [Fact]
+    public void BackspaceDeletesWholeGraphemeClusterCombiningMark()
+    {
+        // "é" as base + combining acute = 2 UTF-16 chars, 1 grapheme cluster.
+        string text = "é"; // e + COMBINING ACUTE ACCENT
+        TextBuffer buf = Buffer(text);
+        // Caret is after the combining mark (index 2).
+        Assert.Equal(2, buf.CaretIndex);
+        buf.Backspace();
+        Assert.Equal(string.Empty, buf.Text);
+        Assert.Equal(0, buf.CaretIndex);
+    }
+
+    [Fact]
+    public void BackspaceDeletesWholeSurrogatePair()
+    {
+        // U+1F600 is a surrogate pair: 2 UTF-16 code units.
+        TextBuffer buf = Buffer("\U0001F600");
+        Assert.Equal(2, buf.CaretIndex);
+        buf.Backspace();
+        Assert.Equal(string.Empty, buf.Text);
+        Assert.Equal(0, buf.CaretIndex);
+    }
+
+    [Fact]
+    public void BackspaceDeletesEntireZwjSequence()
+    {
+        // Woman+ZWJ+Laptop: U+1F469 + U+200D + U+1F4BB  (3 code points, 5 UTF-16 units)
+        string zwjSeq = "\U0001F469‍\U0001F4BB";
+        TextBuffer buf = Buffer(zwjSeq);
+        buf.Backspace();
+        // The whole ZWJ sequence is one grapheme cluster: deleted entirely.
+        Assert.Equal(string.Empty, buf.Text);
+        Assert.Equal(0, buf.CaretIndex);
+    }
+
+    [Fact]
+    public void BackspaceDeletesEntireFlagSequence()
+    {
+        // Flag: Regional Indicator GB = U+1F1EC + U+1F1E7 (4 UTF-16 units, 1 grapheme cluster)
+        string flag = "\U0001F1EC\U0001F1E7"; // 🇬🇧
+        TextBuffer buf = Buffer(flag);
+        buf.Backspace();
+        Assert.Equal(string.Empty, buf.Text);
+        Assert.Equal(0, buf.CaretIndex);
+    }
+
+    // ── Delete (forward) ──────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void DeleteRemovesAsciiChar()
+    {
+        TextBuffer buf = Buffer("hello");
+        SetCaret(buf, 0);
+        buf.Delete();
+        Assert.Equal("ello", buf.Text);
+        Assert.Equal(0, buf.CaretIndex);
+    }
+
+    [Fact]
+    public void DeleteAtEndIsNoOp()
+    {
+        TextBuffer buf = Buffer("hi");
+        buf.Delete();
+        Assert.Equal("hi", buf.Text);
+        Assert.Equal(2, buf.CaretIndex);
+    }
+
+    [Fact]
+    public void DeleteRemovesWholeGraphemeClusterCombiningMark()
+    {
+        string text = "é"; // e + COMBINING ACUTE ACCENT (1 grapheme cluster)
+        TextBuffer buf = Buffer(text);
+        SetCaret(buf, 0);
+        buf.Delete();
+        Assert.Equal(string.Empty, buf.Text);
+    }
+
+    // ── MoveLeft / MoveRight ─────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void MoveLeftMovesOneCharBack()
+    {
+        TextBuffer buf = Buffer("abc");
+        buf.MoveLeft();
+        Assert.Equal(2, buf.CaretIndex);
+    }
+
+    [Fact]
+    public void MoveLeftAtStartIsNoOp()
+    {
+        TextBuffer buf = Buffer("a");
+        SetCaret(buf, 0);
+        buf.MoveLeft();
+        Assert.Equal(0, buf.CaretIndex);
+    }
+
+    [Fact]
+    public void MoveLeftOverCombiningMarkMovesOnePastCluster()
+    {
+        // "é" = 2 chars, 1 grapheme. Caret starts at 2, MoveLeft goes to 0.
+        string text = "é";
+        TextBuffer buf = Buffer(text);
+        Assert.Equal(2, buf.CaretIndex);
+        buf.MoveLeft();
+        Assert.Equal(0, buf.CaretIndex);
+    }
+
+    [Fact]
+    public void MoveLeftOverSupplementaryMovesByTwoChars()
+    {
+        TextBuffer buf = Buffer("\U0001F600");
+        Assert.Equal(2, buf.CaretIndex);
+        buf.MoveLeft();
+        Assert.Equal(0, buf.CaretIndex);
+    }
+
+    [Fact]
+    public void MoveLeftOverZwjSequenceMovesEntireCluster()
+    {
+        string zwjSeq = "\U0001F469‍\U0001F4BB";
+        TextBuffer buf = Buffer(zwjSeq);
+        buf.MoveLeft();
+        Assert.Equal(0, buf.CaretIndex);
+    }
+
+    [Fact]
+    public void MoveRightMovesOneCharForward()
+    {
+        TextBuffer buf = Buffer("abc");
+        SetCaret(buf, 0);
+        buf.MoveRight();
+        Assert.Equal(1, buf.CaretIndex);
+    }
+
+    [Fact]
+    public void MoveRightAtEndIsNoOp()
+    {
+        TextBuffer buf = Buffer("a");
+        buf.MoveRight();
+        Assert.Equal(1, buf.CaretIndex);
+    }
+
+    [Fact]
+    public void MoveRightOverCombiningMarkMovesOnePastCluster()
+    {
+        string text = "é"; // 2 chars, 1 grapheme
+        TextBuffer buf = Buffer(text);
+        SetCaret(buf, 0);
+        buf.MoveRight();
+        Assert.Equal(2, buf.CaretIndex);
+    }
+
+    [Fact]
+    public void MoveRightOverSupplementaryMovesByTwoChars()
+    {
+        TextBuffer buf = Buffer("\U0001F600");
+        SetCaret(buf, 0);
+        buf.MoveRight();
+        Assert.Equal(2, buf.CaretIndex);
+    }
+
+    [Fact]
+    public void MoveRightOverFlagSequenceMovesEntireCluster()
+    {
+        string flag = "\U0001F1EC\U0001F1E7"; // 🇬🇧, 4 UTF-16 units, 1 cluster
+        TextBuffer buf = Buffer(flag);
+        SetCaret(buf, 0);
+        buf.MoveRight();
+        Assert.Equal(flag.Length, buf.CaretIndex);
+    }
+
+    [Fact]
+    public void MoveLeftRightCrossesLogicalLineBoundary()
+    {
+        TextBuffer buf = Buffer("a\nb");
+        // Caret at 'b' (index 2, after the '\n').
+        SetCaret(buf, 2);
+        buf.MoveLeft(); // crosses newline: moves to 1 (the '\n')
+        Assert.Equal(1, buf.CaretIndex);
+        buf.MoveLeft(); // moves to 0 ('a')
+        Assert.Equal(0, buf.CaretIndex);
+        buf.MoveRight(); // back to 1
+        Assert.Equal(1, buf.CaretIndex);
+    }
+
+    // ── MoveHome / MoveEnd ───────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void MoveHomeSingleRowGoesToStart()
+    {
+        TextBuffer buf = Buffer("hello");
+        buf.MoveHome(80);
+        Assert.Equal(0, buf.CaretIndex);
+    }
+
+    [Fact]
+    public void MoveEndSingleRowGoesToEnd()
+    {
+        TextBuffer buf = Buffer("hello");
+        SetCaret(buf, 0);
+        buf.MoveEnd(80);
+        Assert.Equal(5, buf.CaretIndex);
+    }
+
+    [Fact]
+    public void MoveHomeWrappedRowGoesToStartOfVisualRow()
+    {
+        // "hello world" at width 5: row0="hello"(0-5), row1=" worl"(5-10), row2="d"(10-11)
+        // Caret at end (index 11, on row2 which starts at 10). MoveHome → 10.
+        TextBuffer buf = Buffer("hello world");
+        buf.MoveHome(5);
+        Assert.Equal(10, buf.CaretIndex);
+    }
+
+    [Fact]
+    public void MoveEndWrappedRowGoesToEndOfVisualRow()
+    {
+        // Caret at start (0), MoveEnd at width 5: end of first visual row "hello" → 5.
+        TextBuffer buf = Buffer("hello world");
+        SetCaret(buf, 0);
+        buf.MoveEnd(5);
+        Assert.Equal(5, buf.CaretIndex);
+    }
+
+    // ── MoveUp / MoveDown ────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void MoveUpOnFirstRowGoesToStart()
+    {
+        TextBuffer buf = Buffer("hello");
+        SetCaret(buf, 3);
+        buf.MoveUp(80);
+        Assert.Equal(0, buf.CaretIndex);
+    }
+
+    [Fact]
+    public void MoveDownOnLastRowGoesToEnd()
+    {
+        TextBuffer buf = Buffer("hello");
+        SetCaret(buf, 2);
+        buf.MoveDown(80);
+        Assert.Equal(5, buf.CaretIndex);
+    }
+
+    [Fact]
+    public void MoveUpWrappedContentMovesPreviousVisualRow()
+    {
+        // "hello world" at width 5: row0="hello"(0-5), row1=" worl"(5-10), row2="d"(10-11)
+        // Caret at 11 (end, row2 col 1). MoveUp → row1 at col 1 → index 6.
+        TextBuffer buf = Buffer("hello world");
+        SetCaret(buf, 11);
+        buf.MoveUp(5);
+        // row1 = " worl" starts at 5; col 1 = after ' ' → index 6
+        Assert.Equal(6, buf.CaretIndex);
+    }
+
+    [Fact]
+    public void MoveDownWrappedContentMovesNextVisualRow()
+    {
+        // "hello world" at width 5: row0="hello"(0-5), row1=" worl"(5-10)
+        // Caret at 3 (col 3 on row0). MoveDown → row1 at col 3 → index 5+3=8.
+        TextBuffer buf = Buffer("hello world");
+        SetCaret(buf, 3);
+        buf.MoveDown(5);
+        Assert.Equal(8, buf.CaretIndex);
+    }
+
+    [Fact]
+    public void MoveUpDownMultilogicalLines()
+    {
+        // "abc\ndef" at width 80: row0="abc", row1="def"
+        TextBuffer buf = Buffer("abc\ndef");
+        SetCaret(buf, 5); // 'e' in "def" (a=0,b=1,c=2,\n=3,d=4,e=5)
+        buf.MoveUp(80);   // row0 at col 1 → index 1
+        Assert.Equal(1, buf.CaretIndex);
+        buf.MoveDown(80); // row1 at col 1 → index 5
+        Assert.Equal(5, buf.CaretIndex);
+    }
+
+    // ── Render: wrap tracks caret ─────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void RenderShortTextProducesOneRowWithCaretAtEnd()
+    {
+        TextBuffer buf = Buffer("hello");
+        RenderResult r = buf.Render(80);
+        Assert.Single(r.VisibleRows);
+        Assert.Equal((0, 5), r.CaretPosition);
+    }
+
+    [Fact]
+    public void RenderTextExceedingWidthWrapsToMultipleRows()
+    {
+        // "hello world" at width 5 → 3 rows
+        TextBuffer buf = Buffer("hello world");
+        RenderResult r = buf.Render(5);
+        Assert.Equal(3, r.VisibleRows.Count);
+    }
+
+    [Fact]
+    public void RenderCaretAtEndOfWrappedRowIsInRange()
+    {
+        // "hello world" at width 5: row0="hello", row1=" worl", row2="d"
+        // Caret at 5 (wrap boundary). Either (0,5) or (1,0) are valid.
+        TextBuffer buf = Buffer("hello world");
+        SetCaret(buf, 5);
+        RenderResult r = buf.Render(5);
+        int row = r.CaretPosition.Row;
+        int col = r.CaretPosition.Col;
+        Assert.True((row == 0 && col == 5) || (row == 1 && col == 0),
+            $"Expected caret at end of row0 or start of row1, got ({row},{col})");
+    }
+
+    [Fact]
+    public void RenderCaretAfterWideCjkCharIsAtCol2()
+    {
+        // "中" is 2 columns. Caret after it should be at col 2.
+        TextBuffer buf = Buffer("中");
+        RenderResult r = buf.Render(80);
+        Assert.Equal((0, 2), r.CaretPosition);
+    }
+
+    [Fact]
+    public void RenderCaretAfterCombiningMarkColCountsBaseOnly()
+    {
+        // "é" as e+combining: visual width = 1.
+        TextBuffer buf = Buffer("é");
+        RenderResult r = buf.Render(80);
+        Assert.Equal((0, 1), r.CaretPosition);
+    }
+
+    [Fact]
+    public void RenderEmptyBufferProducesOneRowCaretAt00()
+    {
+        TextBuffer buf = new();
+        RenderResult r = buf.Render(80);
+        Assert.Single(r.VisibleRows);
+        Assert.Equal((0, 0), r.CaretPosition);
+    }
+
+    [Fact]
+    public void RenderMultilineBufferCaretOnCorrectRow()
+    {
+        // "abc\ndef" — caret at start of "def" = index 4
+        TextBuffer buf = Buffer("abc\ndef");
+        SetCaret(buf, 4);
+        RenderResult r = buf.Render(80);
+        Assert.Equal(2, r.VisibleRows.Count);
+        Assert.Equal(1, r.CaretPosition.Row); // second visual row
+        Assert.Equal(0, r.CaretPosition.Col); // start of row
+    }
+
+    [Fact]
+    public void RenderTrailingNewlineProducesExtraEmptyRow()
+    {
+        TextBuffer buf = Buffer("abc\n");
+        RenderResult r = buf.Render(80);
+        // "abc\n" → row0="abc", row1="" (empty trailing row)
+        Assert.Equal(2, r.VisibleRows.Count);
+        // Caret at end (index 4) → on row1, col 0
+        Assert.Equal(1, r.CaretPosition.Row);
+        Assert.Equal(0, r.CaretPosition.Col);
+    }
+
+    // ── Internal scroll ────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void RenderAllottedHeightLimitsVisibleRows()
+    {
+        // 5 lines, allotted height = 3: see exactly 3 rows.
+        TextBuffer buf = Buffer("a\nb\nc\nd\ne");
+        RenderResult r = buf.Render(80, allottedHeight: 3);
+        Assert.Equal(3, r.VisibleRows.Count);
+    }
+
+    [Fact]
+    public void RenderAllottedHeightKeepsCaretVisible()
+    {
+        // 5 lines, allotted height = 2, caret at end (last row).
+        TextBuffer buf = Buffer("a\nb\nc\nd\ne");
+        RenderResult r = buf.Render(80, allottedHeight: 2);
+        Assert.Equal(2, r.VisibleRows.Count);
+        // Caret must be within the visible window.
+        Assert.InRange(r.CaretPosition.Row, 0, 1);
+    }
+
+    [Fact]
+    public void RenderAllottedHeightScrollsToKeepCaretAtTop()
+    {
+        // Caret at 'a' (start), allotted height = 2. Window starts at row 0.
+        TextBuffer buf = Buffer("a\nb\nc\nd\ne");
+        SetCaret(buf, 0);
+        RenderResult r = buf.Render(80, allottedHeight: 2);
+        Assert.Equal(2, r.VisibleRows.Count);
+        Assert.Equal(0, r.CaretPosition.Row); // caret on first visible row
+    }
+
+    [Fact]
+    public void RenderAllottedHeightNullShowsAllRows()
+    {
+        TextBuffer buf = Buffer("a\nb\nc");
+        RenderResult r = buf.Render(80, allottedHeight: null);
+        Assert.Equal(3, r.VisibleRows.Count);
+    }
+
+    [Fact]
+    public void RenderAllottedHeightLargerThanContentShowsAll()
+    {
+        TextBuffer buf = Buffer("a\nb");
+        RenderResult r = buf.Render(80, allottedHeight: 10);
+        Assert.Equal(2, r.VisibleRows.Count);
+    }
+
+    [Fact]
+    public void RenderAllottedHeightOneKeepsCaretVisible()
+    {
+        // 3 lines, height 1: single visible row must contain the caret.
+        TextBuffer buf = Buffer("a\nb\nc");
+        SetCaret(buf, 2); // 'b' (a=0,\n=1,b=2)
+        RenderResult r = buf.Render(80, allottedHeight: 1);
+        Assert.Single(r.VisibleRows);
+        Assert.Equal(0, r.CaretPosition.Row); // caret is on the one visible row
+    }
+
+    // ── History recall ────────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void AddToHistoryThenRecallPreviousReplacesBuffer()
+    {
+        TextBuffer buf = new();
+        buf.AddToHistory("first");
+        buf.Insert("current");
+        buf.RecallPrevious();
+        Assert.Equal("first", buf.Text);
+    }
+
+    [Fact]
+    public void RecallPreviousStashesCurrentBuffer()
+    {
+        TextBuffer buf = new();
+        buf.AddToHistory("entry1");
+        buf.Insert("live");
+        buf.RecallPrevious(); // goes to "entry1", stashes "live"
+        Assert.Equal("entry1", buf.Text);
+        buf.RecallNext(); // past newest, restores stash
+        Assert.Equal("live", buf.Text);
+    }
+
+    [Fact]
+    public void RecallPreviousMultipleEntriesNavigatesOldestFirst()
+    {
+        TextBuffer buf = new();
+        buf.AddToHistory("alpha");
+        buf.AddToHistory("beta");
+        buf.AddToHistory("gamma");
+
+        buf.RecallPrevious(); // "gamma" (newest)
+        Assert.Equal("gamma", buf.Text);
+        buf.RecallPrevious(); // "beta"
+        Assert.Equal("beta", buf.Text);
+        buf.RecallPrevious(); // "alpha"
+        Assert.Equal("alpha", buf.Text);
+        buf.RecallPrevious(); // no-op at oldest
+        Assert.Equal("alpha", buf.Text);
+    }
+
+    [Fact]
+    public void RecallNextPastNewestRestoresStash()
+    {
+        TextBuffer buf = new();
+        buf.AddToHistory("a");
+        buf.AddToHistory("b");
+        buf.Insert("live");
+        buf.RecallPrevious(); // "b"
+        buf.RecallPrevious(); // "a"
+        buf.RecallNext();     // "b"
+        Assert.Equal("b", buf.Text);
+        buf.RecallNext();     // past newest → restore "live"
+        Assert.Equal("live", buf.Text);
+    }
+
+    [Fact]
+    public void RecallNextWhenNotInHistoryIsNoOp()
+    {
+        TextBuffer buf = Buffer("hello");
+        buf.RecallNext(); // no-op
+        Assert.Equal("hello", buf.Text);
+    }
+
+    [Fact]
+    public void RecallPreviousWithEmptyHistoryIsNoOp()
+    {
+        TextBuffer buf = Buffer("hello");
+        buf.RecallPrevious(); // no-op
+        Assert.Equal("hello", buf.Text);
+    }
+
+    [Fact]
+    public void AddToHistorySkipsDuplicateConsecutiveEntry()
+    {
+        TextBuffer buf = new();
+        buf.AddToHistory("dup");
+        buf.AddToHistory("dup");
+        // Only one entry; RecallPrevious goes to it; another RecallPrevious is no-op.
+        buf.RecallPrevious();
+        Assert.Equal("dup", buf.Text);
+        buf.RecallPrevious(); // no-op
+        Assert.Equal("dup", buf.Text);
+    }
+
+    [Fact]
+    public void AddToHistoryResetsHistoryNavigationAndStash()
+    {
+        TextBuffer buf = new();
+        buf.AddToHistory("entry");
+        buf.Insert("live");
+        buf.RecallPrevious(); // stash "live", go to "entry"
+        // AddToHistory resets navigation and stash.
+        buf.AddToHistory("new");
+        // History navigation now starts fresh. Next RecallPrevious → newest = "new".
+        buf.Clear();
+        buf.Insert("fresh");
+        buf.RecallPrevious(); // should go to "new"
+        Assert.Equal("new", buf.Text);
+    }
+
+    // ── Multiline editing ──────────────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void MultilineInsertCaretMovesCorrectly()
+    {
+        TextBuffer buf = new();
+        buf.Insert("line1");
+        buf.InsertNewline();
+        buf.Insert("line2");
+        Assert.Equal("line1\nline2", buf.Text);
+        Assert.Equal(11, buf.CaretIndex);
+    }
+
+    [Fact]
+    public void MultilineDeletionAtBoundaryMergesLines()
+    {
+        TextBuffer buf = Buffer("a\nb");
+        // Caret at index 2 (after '\n', before 'b'). Backspace removes '\n'.
+        SetCaret(buf, 2);
+        buf.Backspace();
+        Assert.Equal("ab", buf.Text);
+        Assert.Equal(1, buf.CaretIndex);
+    }
+
+    [Fact]
+    public void MultilineDeleteAtNewlineMergesLines()
+    {
+        TextBuffer buf = Buffer("a\nb");
+        SetCaret(buf, 1); // caret on '\n'
+        buf.Delete();     // removes '\n'
+        Assert.Equal("ab", buf.Text);
+        Assert.Equal(1, buf.CaretIndex);
+    }
+
+    // ── Caret at wide-char boundary ───────────────────────────────────────────────────────────
+
+    [Fact]
+    public void RenderCaretAtWideCharBoundaryColIsCorrect()
+    {
+        // "中A" → caret after A: col = 2 (中) + 1 (A) = 3.
+        TextBuffer buf = Buffer("中A");
+        RenderResult r = buf.Render(80);
+        Assert.Equal((0, 3), r.CaretPosition);
+    }
+
+    [Fact]
+    public void RenderCaretBetweenWideCharsColIsCorrect()
+    {
+        // "中文" — caret after 中 (U+4E2D, BMP → 1 UTF-16 char, index 1)
+        TextBuffer buf = Buffer("中文");
+        SetCaret(buf, 1); // between the two CJK chars
+        RenderResult r = buf.Render(80);
+        Assert.Equal((0, 2), r.CaretPosition); // 中 = 2 cols
+    }
+
+    // ── Grapheme cluster correctness (10.2) ───────────────────────────────────────────────────
+
+    [Fact]
+    public void GraphemeBaseAndCombiningOneVisualMove()
+    {
+        // "é" is one grapheme cluster (2 chars). Right arrow moves by 2 char positions.
+        string text = "é";
+        TextBuffer buf = Buffer(text);
+        SetCaret(buf, 0);
+        buf.MoveRight();
+        Assert.Equal(2, buf.CaretIndex);
+    }
+
+    [Fact]
+    public void GraphemeZwjSequenceOneVisualMove()
+    {
+        // Woman+ZWJ+Laptop: 5 UTF-16 units, 1 grapheme cluster.
+        string zwjSeq = "\U0001F469‍\U0001F4BB";
+        TextBuffer buf = Buffer(zwjSeq);
+        SetCaret(buf, 0);
+        buf.MoveRight(); // should jump the entire ZWJ sequence
+        Assert.Equal(zwjSeq.Length, buf.CaretIndex);
+    }
+
+    [Fact]
+    public void GraphemeFlagSequenceOneVisualMove()
+    {
+        string flag = "\U0001F1EC\U0001F1E7"; // 🇬🇧, 4 UTF-16 units
+        TextBuffer buf = Buffer(flag);
+        SetCaret(buf, 0);
+        buf.MoveRight();
+        Assert.Equal(flag.Length, buf.CaretIndex);
+    }
+
+    [Fact]
+    public void GraphemeBackspaceOverZwjSequenceDeletesAll()
+    {
+        string zwjSeq = "\U0001F469‍\U0001F4BB";
+        TextBuffer buf = Buffer(zwjSeq);
+        buf.Backspace();
+        Assert.Equal(string.Empty, buf.Text);
+    }
+
+    [Fact]
+    public void GraphemeBackspaceOverFlagSequenceDeletesAll()
+    {
+        string flag = "\U0001F1EC\U0001F1E7";
+        TextBuffer buf = Buffer(flag);
+        buf.Backspace();
+        Assert.Equal(string.Empty, buf.Text);
+    }
+
+    // ── Wrapped row caret tracking ────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void RenderWrapAtCjkCharRowsAreCorrect()
+    {
+        // "AB中" at width 3: "AB" fits (2 cols), 中 is 2 cols → wraps to next row.
+        // row0 = "AB", row1 = "中"
+        TextBuffer buf = Buffer("AB中");
+        RenderResult r = buf.Render(3);
+        Assert.Equal(2, r.VisibleRows.Count);
+        // Caret at end: on row1, col 2.
+        Assert.Equal(1, r.CaretPosition.Row);
+        Assert.Equal(2, r.CaretPosition.Col);
+    }
+
+    [Fact]
+    public void RenderOnlyWideCharsEachOnOwnRow()
+    {
+        // "中文" at width 2: each is exactly 2 cols → one per row.
+        TextBuffer buf = Buffer("中文");
+        RenderResult r = buf.Render(2);
+        Assert.Equal(2, r.VisibleRows.Count);
+    }
+}
diff --git a/tests/Dcli.Tests/VtFrameRendererTests.cs b/tests/Dcli.Tests/VtFrameRendererTests.cs
new file mode 100644
index 0000000..143b635
--- /dev/null
+++ b/tests/Dcli.Tests/VtFrameRendererTests.cs
@@ -0,0 +1,881 @@
+using Dcli.Internal;
+using Dcli.Internal.RenderLoop;
+using Xunit;
+
+namespace Dcli.Tests;
+
+/// <summary>
+/// Behavioural tests for §8 (Chunk A + Chunk B): SGR translation, single-frame VT emission,
+/// and cross-frame cursor accounting (move-to-anchor, commit choreography, frozen-never-rewritten).
+/// The injected <see cref="StringWriter"/> captures the byte stream; helper methods
+/// decode structural facts (sync fence, SGR codes, erase-to-EOL, cursor positioning)
+/// so tests assert behaviour rather than raw escape soup.
+/// </summary>
+public sealed class VtFrameRendererTests
+{
+    // ── VT escape-sequence constants used in assertions ───────────────────────
+    private const string _syncStart = "\x1b[?2026h";
+    private const string _syncEnd = "\x1b[?2026l";
+    private const string _eraseEol = "\x1b[K";
+    private const string _cursorHide = "\x1b[?25l";
+    private const string _cursorShow = "\x1b[?25h";
+    private const string _sgrReset = "\x1b[0m";
+
+    // ── Helpers ───────────────────────────────────────────────────────────────
+
+    private static (VtFrameRenderer renderer, StringWriter writer, RenderModel model) MakeRenderer(
+        int cols = 80, int rows = 24, TerminalCapabilities capabilities = default)
+    {
+        StringWriter w = new();
+        VtFrameRenderer r = new(w, capabilities);
+        RenderModel m = new(new FixedSizeSource(cols, rows));
+        return (r, w, m);
+    }
+
+    private static TerminalCapabilities WithTruecolor =>
+        new TerminalCapabilities { HasTruecolor = true };
+
+    private static Line L(params Segment[] segments) => new(segments);
+    private static Line PlainLine(string text) => L(new Segment(text));
+
+    // ── §8.2  Synchronized-output fence ──────────────────────────────────────
+
+    [Fact]
+    public void EmptyFrameWrappedInSyncFence()
+    {
+        var (renderer, writer, model) = MakeRenderer();
+        renderer.Paint(model);
+
+        string output = writer.ToString();
+        Assert.StartsWith(_syncStart, output, StringComparison.Ordinal);
+        Assert.EndsWith(_syncEnd, output, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void SyncFenceBracketsAllContent()
+    {
+        var (renderer, writer, model) = MakeRenderer();
+        model.LiveWindowRows = [PlainLine("hello")];
+        renderer.Paint(model);
+
+        string output = writer.ToString();
+        int startIdx = output.IndexOf(_syncStart, StringComparison.Ordinal);
+        int endIdx = output.LastIndexOf(_syncEnd, StringComparison.Ordinal);
+
+        Assert.True(startIdx >= 0, "_syncStart not found");
+        Assert.True(endIdx > startIdx + _syncStart.Length, "_syncEnd must appear after _syncStart");
+
+        // "hello" must be between the fence brackets.
+        string interior = output.Substring(startIdx + _syncStart.Length,
+                                           endIdx - startIdx - _syncStart.Length);
+        Assert.Contains("hello", interior, StringComparison.Ordinal);
+    }
+
+    // ── Plain row (no SGR) ────────────────────────────────────────────────────
+
+    [Fact]
+    public void PlainRowEmitsTextAndEraseEolWithNoSgr()
+    {
+        var (renderer, writer, model) = MakeRenderer();
+        model.LiveWindowRows = [PlainLine("plain text")];
+        renderer.Paint(model);
+
+        string output = writer.ToString();
+        Assert.Contains("plain text", output, StringComparison.Ordinal);
+        Assert.Contains(_eraseEol, output, StringComparison.Ordinal);
+        // No SGR attributes or reset should appear for an unstyled row.
+        Assert.DoesNotContain(_sgrReset, output, StringComparison.Ordinal);
+        Assert.DoesNotContain("\x1b[1m", output, StringComparison.Ordinal); // no bold
+    }
+
+    // ── Multi-segment styled row ──────────────────────────────────────────────
+
+    [Fact]
+    public void BoldSegmentEmitsSgr1ThenTextThenReset()
+    {
+        var (renderer, writer, model) = MakeRenderer();
+        Segment bold = new("BOLD", new Style(Format: Format.Bold));
+        model.LiveWindowRows = [L(bold)];
+        renderer.Paint(model);
+
+        string output = writer.ToString();
+        // Bold SGR code 1
+        int boldIdx = output.IndexOf("\x1b[1m", StringComparison.Ordinal);
+        int textIdx = output.IndexOf("BOLD", StringComparison.Ordinal);
+        int resetIdx = output.IndexOf(_sgrReset, StringComparison.Ordinal);
+
+        Assert.True(boldIdx >= 0, "bold SGR not found");
+        Assert.True(textIdx > boldIdx, "text must follow SGR open");
+        Assert.True(resetIdx > textIdx, "reset must follow text");
+    }
+
+    [Fact]
+    public void MultipleFormatFlagsAllCodesPresent()
+    {
+        var (renderer, writer, model) = MakeRenderer();
+        Segment seg = new("BOTH", new Style(Format: Format.Bold | Format.Underline));
+        model.LiveWindowRows = [L(seg)];
+        renderer.Paint(model);
+
+        string output = writer.ToString();
+        // Bold=1 and Underline=4 must both appear inside one SGR sequence before the text.
+        int textIdx = output.IndexOf("BOTH", StringComparison.Ordinal);
+        string beforeText = output.Substring(0, textIdx);
+        Assert.Contains("1", beforeText, StringComparison.Ordinal);   // bold code
+        Assert.Contains("4", beforeText, StringComparison.Ordinal);   // underline code
+        Assert.Contains(_sgrReset, output, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void NamedForegroundEmitsCorrectFgCode()
+    {
+        // Red = AnsiColor 1 → fg code 31
+        var (renderer, writer, model) = MakeRenderer();
+        Segment seg = new("RED", new Style(Foreground: Color.Named(Color.AnsiColor.Red)));
+        model.LiveWindowRows = [L(seg)];
+        renderer.Paint(model);
+
+        string output = writer.ToString();
+        Assert.Contains("\x1b[31m", output, StringComparison.Ordinal); // fg red
+        Assert.Contains("RED", output, StringComparison.Ordinal);
+        Assert.Contains(_sgrReset, output, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void NamedBackgroundEmitsCorrectBgCode()
+    {
+        // Green = AnsiColor 2 → bg code 42
+        var (renderer, writer, model) = MakeRenderer();
+        Segment seg = new("GRN", new Style(Background: Color.Named(Color.AnsiColor.Green)));
+        model.LiveWindowRows = [L(seg)];
+        renderer.Paint(model);
+
+        string output = writer.ToString();
+        Assert.Contains("\x1b[42m", output, StringComparison.Ordinal); // bg green
+    }
+
+    [Fact]
+    public void BrightNamedForegroundEmitsHighIntensityCode()
+    {
+        // BrightRed = AnsiColor 9 → fg code 91
+        var (renderer, writer, model) = MakeRenderer();
+        Segment seg = new("BR", new Style(Foreground: Color.Named(Color.AnsiColor.BrightRed)));
+        model.LiveWindowRows = [L(seg)];
+        renderer.Paint(model);
+
+        string output = writer.ToString();
+        Assert.Contains("91", output, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void IndexedForegroundEmitsExtendedColorSequence()
+    {
+        // 256-color fg: ESC[38;5;200m
+        var (renderer, writer, model) = MakeRenderer();
+        Segment seg = new("IDX", new Style(Foreground: Color.FromIndex(200)));
+        model.LiveWindowRows = [L(seg)];
+        renderer.Paint(model);
+
+        string output = writer.ToString();
+        Assert.Contains("38;5;200", output, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void IndexedBackgroundEmitsExtendedColorSequence()
+    {
+        // 256-color bg: ESC[48;5;100m
+        var (renderer, writer, model) = MakeRenderer();
+        Segment seg = new("IDX", new Style(Background: Color.FromIndex(100)));
+        model.LiveWindowRows = [L(seg)];
+        renderer.Paint(model);
+
+        string output = writer.ToString();
+        Assert.Contains("48;5;100", output, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void TruecolorForegroundEmitsRgbSequence()
+    {
+        // Truecolor fg: ESC[38;2;255;128;0m — requires HasTruecolor=true
+        var (renderer, writer, model) = MakeRenderer(capabilities: WithTruecolor);
+        Segment seg = new("TC", new Style(Foreground: Color.FromRgb(255, 128, 0)));
+        model.LiveWindowRows = [L(seg)];
+        renderer.Paint(model);
+
+        string output = writer.ToString();
+        Assert.Contains("38;2;255;128;0", output, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void TruecolorBackgroundEmitsRgbSequence()
+    {
+        // Truecolor bg: ESC[48;2;10;20;30m — requires HasTruecolor=true
+        var (renderer, writer, model) = MakeRenderer(capabilities: WithTruecolor);
+        Segment seg = new("TC", new Style(Background: Color.FromRgb(10, 20, 30)));
+        model.LiveWindowRows = [L(seg)];
+        renderer.Paint(model);
+
+        string output = writer.ToString();
+        Assert.Contains("48;2;10;20;30", output, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void FormatComboAndTruecolorAllCodesPresent()
+    {
+        // Bold + Italic foreground truecolor = codes 1;3 + 38;2;r;g;b — requires HasTruecolor=true
+        var (renderer, writer, model) = MakeRenderer(capabilities: WithTruecolor);
+        Segment seg = new("COMBO",
+            new Style(Foreground: Color.FromRgb(1, 2, 3), Format: Format.Bold | Format.Italic));
+        model.LiveWindowRows = [L(seg)];
+        renderer.Paint(model);
+
+        string output = writer.ToString();
+        Assert.Contains("38;2;1;2;3", output, StringComparison.Ordinal);
+        // Bold and italic codes must appear before the text.
+        int textIdx = output.IndexOf("COMBO", StringComparison.Ordinal);
+        string beforeText = output.Substring(0, textIdx);
+        Assert.Contains("1", beforeText, StringComparison.Ordinal);
+        Assert.Contains("3", beforeText, StringComparison.Ordinal);
+        Assert.Contains(_sgrReset, output, StringComparison.Ordinal);
+    }
+
+    // ── Each row ends with ESC[K ──────────────────────────────────────────────
+
+    [Fact]
+    public void EachRowEndsWithEraseEol()
+    {
+        var (renderer, writer, model) = MakeRenderer();
+        model.LiveWindowRows = [PlainLine("row1"), PlainLine("row2")];
+        renderer.Paint(model);
+
+        string output = writer.ToString();
+        // Two rows → two ESC[K occurrences.
+        int count = CountOccurrences(output, _eraseEol);
+        Assert.Equal(2, count);
+    }
+
+    [Fact]
+    public void FixedRegionRowsAlsoGetEraseEol()
+    {
+        var (renderer, writer, model) = MakeRenderer();
+        model.LiveWindowRows = [PlainLine("live")];
+        model.FixedRegionRows = [PlainLine("status")];
+        renderer.Paint(model);
+
+        string output = writer.ToString();
+        int count = CountOccurrences(output, _eraseEol);
+        Assert.Equal(2, count); // one per row across both lists
+    }
+
+    // ── Caret placement ───────────────────────────────────────────────────────
+
+    [Fact]
+    public void NoCaretCursorVisibleShowsAndNoPositioning()
+    {
+        var (renderer, writer, model) = MakeRenderer();
+        model.IsCursorVisible = true;
+        model.CaretPosition = null;
+        renderer.Paint(model);
+
+        string output = writer.ToString();
+        Assert.Contains(_cursorShow, output, StringComparison.Ordinal);
+        Assert.DoesNotContain(_cursorHide, output, StringComparison.Ordinal);
+        // No cursor-up (ESC[nA) or CHA (ESC[nG) when CaretPosition is null.
+        Assert.DoesNotContain("\x1b[1A", output, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void CaretAtLastRowNoCursorUpColumnPlaced()
+    {
+        var (renderer, writer, model) = MakeRenderer();
+        model.LiveWindowRows = [PlainLine("only row")];
+        // Caret at row 0 col 3 (the only row, which is the last row → 0 steps up).
+        model.CaretPosition = (0, 3);
+        model.IsCursorVisible = true;
+        renderer.Paint(model);
+
+        string output = writer.ToString();
+        Assert.Contains(_cursorShow, output, StringComparison.Ordinal);
+        // CHA to column 4 (1-based): ESC[4G
+        Assert.Contains("\x1b[4G", output, StringComparison.Ordinal);
+        // No cursor-up needed.
+        Assert.DoesNotContain("\x1b[1A", output, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void CaretAboveLastRowCursorUpEmitted()
+    {
+        var (renderer, writer, model) = MakeRenderer();
+        model.LiveWindowRows = [PlainLine("line0"), PlainLine("line1"), PlainLine("line2")];
+        // Caret at row 0, col 0 → 2 steps up from the last row.
+        model.CaretPosition = (0, 0);
+        model.IsCursorVisible = true;
+        renderer.Paint(model);
+
+        string output = writer.ToString();
+        Assert.Contains(_cursorShow, output, StringComparison.Ordinal);
+        // Two rows up: ESC[2A
+        Assert.Contains("\x1b[2A", output, StringComparison.Ordinal);
+        // Move to column 1 (1-based): ESC[1G
+        Assert.Contains("\x1b[1G", output, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void CaretPositioningLandsOnCorrectRow()
+    {
+        // 3 rows. After emitting the last row WITHOUT a trailing newline, the cursor rests
+        // on row index 2 (the last content row). Caret at row 1, col 5:
+        //   stepsUp = totalRows - 1 - caretRow = 3 - 1 - 1 = 1
+        //   CHA     = col + 1 = 6
+        // The emitted sequence after cursor-show must be exactly ESC[1A then ESC[6G.
+        var (renderer, writer, model) = MakeRenderer();
+        model.LiveWindowRows = [PlainLine("line0"), PlainLine("line1"), PlainLine("line2")];
+        model.CaretPosition = (1, 5);
+        model.IsCursorVisible = true;
+        renderer.Paint(model);
+
+        string output = writer.ToString();
+
+        // Verify resting-position convention: final row must NOT be followed by '\n'
+        // immediately before the sync-end fence (no spurious blank line).
+        int lastEraseIdx = output.LastIndexOf(_eraseEol, StringComparison.Ordinal);
+        Assert.True(lastEraseIdx >= 0, "ESC[K not found");
+        // The character after the last ESC[K must not be '\n' — cursor rests on that row.
+        int afterLastErase = lastEraseIdx + _eraseEol.Length;
+        Assert.True(afterLastErase < output.Length, "output ends immediately after last ESC[K");
+        Assert.NotEqual('\n', output[afterLastErase]);
+
+        // Exactly 1 step up: ESC[1A
+        Assert.Contains("\x1b[1A", output, StringComparison.Ordinal);
+        Assert.DoesNotContain("\x1b[2A", output, StringComparison.Ordinal);
+
+        // CHA to column 6: ESC[6G
+        Assert.Contains("\x1b[6G", output, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void FinalRowHasNoTrailingNewlineBeforeSyncEnd()
+    {
+        // Verifies the no-trailing-newline convention for single-row and multi-row frames.
+        // The last ESC[K in the output must not be immediately followed by '\n'.
+        var (renderer, writer, model) = MakeRenderer();
+        model.LiveWindowRows = [PlainLine("row0"), PlainLine("row1")];
+        model.FixedRegionRows = [PlainLine("status")];
+        renderer.Paint(model);
+
+        string output = writer.ToString();
+        int lastEraseIdx = output.LastIndexOf(_eraseEol, StringComparison.Ordinal);
+        Assert.True(lastEraseIdx >= 0, "ESC[K not found");
+        int afterLastErase = lastEraseIdx + _eraseEol.Length;
+        Assert.True(afterLastErase < output.Length, "output ends immediately after last ESC[K");
+        Assert.NotEqual('\n', output[afterLastErase]);
+    }
+
+    // ── §8.3  Cursor hide (modal-dialog case) ────────────────────────────────
+
+    [Fact]
+    public void CursorHiddenEmitsHideAndNoShow()
+    {
+        var (renderer, writer, model) = MakeRenderer();
+        model.IsCursorVisible = false;
+        renderer.Paint(model);
+
+        string output = writer.ToString();
+        Assert.Contains(_cursorHide, output, StringComparison.Ordinal);
+        Assert.DoesNotContain(_cursorShow, output, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void CursorHiddenNoCursorPositioning()
+    {
+        var (renderer, writer, model) = MakeRenderer();
+        model.LiveWindowRows = [PlainLine("dialog")];
+        model.IsCursorVisible = false;
+        model.CaretPosition = (0, 5); // set but should be ignored
+        renderer.Paint(model);
+
+        string output = writer.ToString();
+        Assert.Contains(_cursorHide, output, StringComparison.Ordinal);
+        // CHA sequences must not appear — caret is ignored when cursor hidden.
+        Assert.DoesNotContain("\x1b[6G", output, StringComparison.Ordinal);
+        Assert.DoesNotContain("\x1b[1G", output, StringComparison.Ordinal);
+    }
+
+    // ── LastPaintedHeight seam (Chunk B) ─────────────────────────────────────
+
+    [Fact]
+    public void LastPaintedHeightReflectsTotalRowCount()
+    {
+        var (renderer, _, model) = MakeRenderer();
+        model.LiveWindowRows = [PlainLine("a"), PlainLine("b")];
+        model.FixedRegionRows = [PlainLine("status")];
+        renderer.Paint(model);
+
+        Assert.Equal(3, renderer.LastPaintedHeight);
+    }
+
+    [Fact]
+    public void LastPaintedHeightZeroBeforeFirstPaint()
+    {
+        StringWriter w = new();
+        VtFrameRenderer r = new(w);
+        Assert.Equal(0, r.LastPaintedHeight);
+    }
+
+    // ── SGR unit tests (pure SgrTranslator, no I/O) ──────────────────────────
+
+    // Helpers: default (no-truecolor) translator and truecolor translator.
+    private static SgrTranslator DefaultSgr => new(TerminalCapabilities.Default);
+    private static SgrTranslator TruecolorSgr => new(new TerminalCapabilities { HasTruecolor = true });
+
+    [Fact]
+    public void SgrDefaultStyleReturnsEmpty()
+    {
+        string sgr = DefaultSgr.ToOpenSgr(default);
+        Assert.Equal(string.Empty, sgr);
+    }
+
+    [Fact]
+    public void SgrBoldContainsCode1()
+    {
+        string sgr = DefaultSgr.ToOpenSgr(new Style(Format: Format.Bold));
+        Assert.Contains("1", sgr, StringComparison.Ordinal);
+        Assert.StartsWith("\x1b[", sgr, StringComparison.Ordinal);
+        Assert.EndsWith("m", sgr, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void SgrDimContainsCode2()
+    {
+        string sgr = DefaultSgr.ToOpenSgr(new Style(Format: Format.Dim));
+        Assert.Contains("2", sgr, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void SgrItalicContainsCode3()
+    {
+        string sgr = DefaultSgr.ToOpenSgr(new Style(Format: Format.Italic));
+        Assert.Contains("3", sgr, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void SgrUnderlineContainsCode4()
+    {
+        string sgr = DefaultSgr.ToOpenSgr(new Style(Format: Format.Underline));
+        Assert.Contains("4", sgr, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void SgrReverseContainsCode7()
+    {
+        string sgr = DefaultSgr.ToOpenSgr(new Style(Format: Format.Reverse));
+        Assert.Contains("7", sgr, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void SgrStrikethroughContainsCode9()
+    {
+        string sgr = DefaultSgr.ToOpenSgr(new Style(Format: Format.Strikethrough));
+        Assert.Contains("9", sgr, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void SgrNamedBlackFg30()
+    {
+        string sgr = DefaultSgr.ToOpenSgr(new Style(Foreground: Color.Named(Color.AnsiColor.Black)));
+        Assert.Contains("30", sgr, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void SgrNamedWhiteFg37()
+    {
+        string sgr = DefaultSgr.ToOpenSgr(new Style(Foreground: Color.Named(Color.AnsiColor.White)));
+        Assert.Contains("37", sgr, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void SgrBrightBlackFg90()
+    {
+        string sgr = DefaultSgr.ToOpenSgr(new Style(Foreground: Color.Named(Color.AnsiColor.BrightBlack)));
+        Assert.Contains("90", sgr, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void SgrBrightWhiteFg97()
+    {
+        string sgr = DefaultSgr.ToOpenSgr(new Style(Foreground: Color.Named(Color.AnsiColor.BrightWhite)));
+        Assert.Contains("97", sgr, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void SgrNamedBgBlueCode44()
+    {
+        string sgr = DefaultSgr.ToOpenSgr(new Style(Background: Color.Named(Color.AnsiColor.Blue)));
+        Assert.Contains("44", sgr, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void SgrIndexedFgContains38With5AndN()
+    {
+        string sgr = DefaultSgr.ToOpenSgr(new Style(Foreground: Color.FromIndex(77)));
+        Assert.Contains("38;5;77", sgr, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void SgrIndexedBgContains48With5AndN()
+    {
+        string sgr = DefaultSgr.ToOpenSgr(new Style(Background: Color.FromIndex(200)));
+        Assert.Contains("48;5;200", sgr, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void SgrTruecolorFgContains38With2AndRgb()
+    {
+        // HasTruecolor=true: emit 24-bit sequence
+        string sgr = TruecolorSgr.ToOpenSgr(new Style(Foreground: Color.FromRgb(10, 20, 30)));
+        Assert.Contains("38;2;10;20;30", sgr, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void SgrTruecolorBgContains48With2AndRgb()
+    {
+        // HasTruecolor=true: emit 24-bit sequence
+        string sgr = TruecolorSgr.ToOpenSgr(new Style(Background: Color.FromRgb(100, 200, 50)));
+        Assert.Contains("48;2;100;200;50", sgr, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void SgrBoldAndTruecolorFgBothPresentCorrectOrder()
+    {
+        // Bold first (format flags before colors), then truecolor.
+        string sgr = TruecolorSgr.ToOpenSgr(
+            new Style(Foreground: Color.FromRgb(1, 2, 3), Format: Format.Bold));
+        int boldPos = sgr.IndexOf('1', StringComparison.Ordinal);
+        int colorPos = sgr.IndexOf("38;2;1;2;3", StringComparison.Ordinal);
+        Assert.True(boldPos >= 0 && colorPos > boldPos,
+            $"Expected bold code before truecolor in: {sgr}");
+    }
+
+    // ── §8.1 / §8.5  Multi-frame cross-frame cursor accounting ──────────────
+
+    // ESC[nA (cursor up n rows)
+    private static string CursorUp(int n) => $"\x1b[{n}A";
+
+    // ESC[0J (erase to end-of-screen — the region clear)
+    private const string _eraseEos = "\x1b[0J";
+
+    [Fact]
+    public void FirstFramePaintsAtCurrentPositionNoMoveUp()
+    {
+        // The very first frame must NOT emit any cursor-up sequence — it paints
+        // at whatever position the cursor is currently at.
+        var (renderer, writer, model) = MakeRenderer();
+        model.LiveWindowRows = [PlainLine("hello")];
+
+        renderer.Paint(model);
+        string output = writer.ToString();
+
+        // No ESC[nA before the content (no move-up on first frame).
+        // The only cursor-up sequences allowed are those from caret parking, which
+        // requires CaretPosition set; here it is null.
+        Assert.DoesNotContain("\x1b[1A", output, StringComparison.Ordinal);
+        Assert.Contains("hello", output, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void SecondFrameWithNoCaretMovesUpByHeightMinusOne()
+    {
+        // No caret set → cursor rests on the last row of the region (row 2 of a 3-row region).
+        // Frame 2 must emit ESC[2A (lastRestingRow = 2) to reach the anchor.
+        var (renderer, writer, model) = MakeRenderer();
+        model.LiveWindowRows = [PlainLine("r0"), PlainLine("r1"), PlainLine("r2")];
+        // CaretPosition is null and IsCursorVisible defaults to true but no position
+        // is set, so ParkCursor leaves the cursor on the last row (row 2).
+
+        renderer.Paint(model); // frame 1 — no move-up, _lastRestingRow = 2
+        writer.GetStringBuilder().Clear();
+
+        model.LiveWindowRows = [PlainLine("A"), PlainLine("B"), PlainLine("C")];
+        renderer.Paint(model); // frame 2
+
+        string output = writer.ToString();
+        // Move up 2 rows (lastRestingRow = 2 → 2 steps up to reach top).
+        Assert.Contains(CursorUp(2), output, StringComparison.Ordinal);
+        // Erase to EOS after move-up.
+        Assert.Contains(_eraseEos, output, StringComparison.Ordinal);
+        // New content present.
+        Assert.Contains("A", output, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void SecondFrameWithCaretAtRow0NoMoveUp()
+    {
+        // Visible caret on row 0 of a 3-row region: after frame 1, the cursor rests at
+        // row 0 (_lastRestingRow = 0). Frame 2 move-up must be 0 — no ESC[nA emitted
+        // before ESC[0J. Emitting ESC[1A or ESC[2A would overshoot into frozen scrollback.
+        var (renderer, writer, model) = MakeRenderer();
+        model.LiveWindowRows = [PlainLine("r0"), PlainLine("r1"), PlainLine("r2")];
+        model.CaretPosition = (0, 0);
+        model.IsCursorVisible = true;
+
+        renderer.Paint(model); // frame 1: cursor parked at row 0
+        writer.GetStringBuilder().Clear();
+
+        model.LiveWindowRows = [PlainLine("A"), PlainLine("B"), PlainLine("C")];
+        model.NewlyCommittedRows = [];
+        renderer.Paint(model); // frame 2: _lastRestingRow = 0 → stepsUp = 0
+
+        string output = writer.ToString();
+        // No cursor-up of any magnitude before ESC[0J.
+        int eraseIdx = output.IndexOf(_eraseEos, StringComparison.Ordinal);
+        Assert.True(eraseIdx >= 0, "ESC[0J must be present");
+        string beforeErase = output[..eraseIdx];
+        Assert.DoesNotContain("\x1b[1A", beforeErase, StringComparison.Ordinal);
+        Assert.DoesNotContain("\x1b[2A", beforeErase, StringComparison.Ordinal);
+        // Content still repainted.
+        Assert.Contains("A", output, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void SecondFrameWithCaretAtMidRowMovesUpByCaretRow()
+    {
+        // Visible caret on row 1 of a 3-row region: _lastRestingRow = 1.
+        // Frame 2 must emit ESC[1A (not ESC[2A) before ESC[0J.
+        var (renderer, writer, model) = MakeRenderer();
+        model.LiveWindowRows = [PlainLine("r0"), PlainLine("r1"), PlainLine("r2")];
+        model.CaretPosition = (1, 0);
+        model.IsCursorVisible = true;
+
+        renderer.Paint(model); // frame 1: cursor parked at row 1
+        writer.GetStringBuilder().Clear();
+
+        model.LiveWindowRows = [PlainLine("A"), PlainLine("B"), PlainLine("C")];
+        model.NewlyCommittedRows = [];
+        renderer.Paint(model); // frame 2: _lastRestingRow = 1 → stepsUp = 1
+
+        string output = writer.ToString();
+        int eraseIdx = output.IndexOf(_eraseEos, StringComparison.Ordinal);
+        Assert.True(eraseIdx >= 0, "ESC[0J must be present");
+        string beforeErase = output[..eraseIdx];
+        // Exactly ESC[1A, not ESC[2A.
+        Assert.Contains("\x1b[1A", beforeErase, StringComparison.Ordinal);
+        Assert.DoesNotContain("\x1b[2A", beforeErase, StringComparison.Ordinal);
+        Assert.Contains("A", output, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void SecondFrameWithHiddenCursorMovesUpByHeightMinusOne()
+    {
+        // Hidden cursor: ParkCursor does not reposition, so cursor rests on the last row.
+        // _lastRestingRow = totalRows - 1 = 2. Frame 2 must emit ESC[2A.
+        var (renderer, writer, model) = MakeRenderer();
+        model.LiveWindowRows = [PlainLine("r0"), PlainLine("r1"), PlainLine("r2")];
+        model.IsCursorVisible = false;
+
+        renderer.Paint(model); // frame 1: cursor on last row (hidden)
+        writer.GetStringBuilder().Clear();
+
+        model.LiveWindowRows = [PlainLine("A"), PlainLine("B"), PlainLine("C")];
+        model.NewlyCommittedRows = [];
+        renderer.Paint(model); // frame 2
+
+        string output = writer.ToString();
+        int eraseIdx = output.IndexOf(_eraseEos, StringComparison.Ordinal);
+        Assert.True(eraseIdx >= 0, "ESC[0J must be present");
+        string beforeErase = output[..eraseIdx];
+        Assert.Contains("\x1b[2A", beforeErase, StringComparison.Ordinal);
+        Assert.Contains("A", output, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void SecondFrameContainsEraseEosAndNewContent()
+    {
+        var (renderer, writer, model) = MakeRenderer();
+        model.LiveWindowRows = [PlainLine("old-row0"), PlainLine("old-row1")];
+        renderer.Paint(model);
+        writer.GetStringBuilder().Clear();
+
+        model.LiveWindowRows = [PlainLine("new-row0"), PlainLine("new-row1")];
+        renderer.Paint(model);
+
+        string output = writer.ToString();
+        // Clear sequence present.
+        Assert.Contains(_eraseEos, output, StringComparison.Ordinal);
+        // New content present.
+        Assert.Contains("new-row0", output, StringComparison.Ordinal);
+        Assert.Contains("new-row1", output, StringComparison.Ordinal);
+        // Old content not in this frame's output.
+        Assert.DoesNotContain("old-row", output, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void CommitRowsEmittedAboveRegionWithTrailingNewline()
+    {
+        // When NewlyCommittedRows is non-empty the committed rows must be emitted
+        // (as frozen native-scrollback lines) before the ESC[0J clear.
+        var (renderer, writer, model) = MakeRenderer();
+        model.LiveWindowRows = [PlainLine("live0"), PlainLine("live1")];
+        renderer.Paint(model); // frame 1
+        writer.GetStringBuilder().Clear();
+
+        // Frame 2: commit one row.
+        model.NewlyCommittedRows = [PlainLine("committed-row")];
+        model.LiveWindowRows = [PlainLine("live1-updated")];
+        renderer.Paint(model);
+
+        string output = writer.ToString();
+
+        // Committed row text must appear.
+        Assert.Contains("committed-row", output, StringComparison.Ordinal);
+
+        // committed-row must appear BEFORE the ESC[0J clear.
+        int commitIdx = output.IndexOf("committed-row", StringComparison.Ordinal);
+        int clearIdx = output.IndexOf(_eraseEos, StringComparison.Ordinal);
+        Assert.True(commitIdx < clearIdx,
+            "committed row must be emitted before the region clear (ESC[0J)");
+
+        // The live content follows after the clear.
+        int liveIdx = output.IndexOf("live1-updated", StringComparison.Ordinal);
+        Assert.True(liveIdx > clearIdx,
+            "live content must follow the region clear");
+    }
+
+    [Fact]
+    public void FrozenRowNeverRewrittenInSubsequentFrames()
+    {
+        // After a row is committed in frame 2, frames 3+ must not emit its text again.
+        var (renderer, writer, model) = MakeRenderer();
+        model.LiveWindowRows = [PlainLine("live0"), PlainLine("live1")];
+        renderer.Paint(model); // frame 1
+
+        // Frame 2: commit "live0".
+        model.NewlyCommittedRows = [PlainLine("live0")];
+        model.LiveWindowRows = [PlainLine("live1")];
+        renderer.Paint(model);
+
+        // Frame 3: committed list cleared; model only has live1.
+        model.NewlyCommittedRows = [];
+        model.LiveWindowRows = [PlainLine("live1"), PlainLine("live2")];
+        writer.GetStringBuilder().Clear(); // only capture frame-3 output
+
+        renderer.Paint(model);
+        string output = writer.ToString();
+
+        // "live0" must not appear in frame 3's output — it is frozen.
+        Assert.DoesNotContain("live0", output, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void AnchorMovesDownByCommittedCount()
+    {
+        // Frame 1 paints 3 rows → cursor rests 2 rows below anchor.
+        // Frame 2 commits 1 row → it emits ESC[nA (move to anchor), prints committed
+        // row + \n (pushing anchor down by 1), then ESC[0J, then 2 live rows.
+        // Frame 3: previous painted height = 2 (frame 2 painted 2 live rows),
+        // move-up = 2-1 = 1. Verify frame 3 emits ESC[1A (not ESC[2A).
+        var (renderer, writer, model) = MakeRenderer();
+        model.LiveWindowRows = [PlainLine("r0"), PlainLine("r1"), PlainLine("r2")];
+        renderer.Paint(model); // frame 1 — _lastPaintedHeight = 3
+
+        model.NewlyCommittedRows = [PlainLine("frozen0")];
+        model.LiveWindowRows = [PlainLine("r1"), PlainLine("r2")];
+        renderer.Paint(model); // frame 2 — _lastPaintedHeight = 2
+        writer.GetStringBuilder().Clear();
+
+        model.NewlyCommittedRows = [];
+        model.LiveWindowRows = [PlainLine("r1-v2"), PlainLine("r2-v2")];
+        renderer.Paint(model); // frame 3
+
+        string output = writer.ToString();
+        // Frame 3: previous region had 2 rows → move up 1.
+        Assert.Contains(CursorUp(1), output, StringComparison.Ordinal);
+        // Must NOT move up 2 (that would intrude into frozen area).
+        Assert.DoesNotContain(CursorUp(2), output, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void RegionGrowsCorrectlyMoveUpReflectsNewHeight()
+    {
+        // Frame 1: 1 row.  Frame 2: 3 rows.  Frame 3: verify move-up = 2 (3-1).
+        var (renderer, writer, model) = MakeRenderer();
+        model.LiveWindowRows = [PlainLine("only")];
+        renderer.Paint(model); // frame 1 — _lastPaintedHeight = 1
+
+        model.LiveWindowRows = [PlainLine("r0"), PlainLine("r1"), PlainLine("r2")];
+        renderer.Paint(model); // frame 2 — _lastPaintedHeight = 3
+        writer.GetStringBuilder().Clear();
+
+        model.LiveWindowRows = [PlainLine("x0"), PlainLine("x1"), PlainLine("x2")];
+        renderer.Paint(model); // frame 3
+
+        string output = writer.ToString();
+        // Frame 3 should move up 2 (3 rows in frame 2 → 3-1 = 2 steps up).
+        Assert.Contains(CursorUp(2), output, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void SingleRowRegionSecondFrameNoMoveUp()
+    {
+        // When _lastPaintedHeight == 1 the cursor is already at the anchor row.
+        // ESC[0A is never emitted; the move-up is skipped (stepsUp == 0).
+        var (renderer, writer, model) = MakeRenderer();
+        model.LiveWindowRows = [PlainLine("one")];
+        renderer.Paint(model); // frame 1
+        writer.GetStringBuilder().Clear();
+
+        model.LiveWindowRows = [PlainLine("two")];
+        renderer.Paint(model); // frame 2
+
+        string output = writer.ToString();
+        // No cursor-up sequence — stepsUp = 1 - 1 = 0.
+        Assert.DoesNotContain("\x1b[0A", output, StringComparison.Ordinal);
+        Assert.DoesNotContain("\x1b[1A", output, StringComparison.Ordinal);
+        // But ESC[0J clear must still be present.
+        Assert.Contains(_eraseEos, output, StringComparison.Ordinal);
+        Assert.Contains("two", output, StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void MultipleCommittedRowsEmittedInOrder()
+    {
+        // Two rows committed in one frame must both appear before ESC[0J, in order.
+        var (renderer, writer, model) = MakeRenderer();
+        model.LiveWindowRows = [PlainLine("a"), PlainLine("b"), PlainLine("c")];
+        renderer.Paint(model);
+        writer.GetStringBuilder().Clear();
+
+        model.NewlyCommittedRows = [PlainLine("first-commit"), PlainLine("second-commit")];
+        model.LiveWindowRows = [PlainLine("c")];
+        renderer.Paint(model);
+
+        string output = writer.ToString();
+        int firstIdx = output.IndexOf("first-commit", StringComparison.Ordinal);
+        int secondIdx = output.IndexOf("second-commit", StringComparison.Ordinal);
+        int clearIdx = output.IndexOf(_eraseEos, StringComparison.Ordinal);
+
+        Assert.True(firstIdx >= 0, "first-commit not found");
+        Assert.True(secondIdx > firstIdx, "second-commit must follow first-commit");
+        Assert.True(clearIdx > secondIdx, "ESC[0J must follow committed rows");
+    }
+
+    // ── Utility ───────────────────────────────────────────────────────────────
+
+    private static int CountOccurrences(string haystack, string needle)
+    {
+        int count = 0;
+        int idx = 0;
+        while ((idx = haystack.IndexOf(needle, idx, StringComparison.Ordinal)) >= 0)
+        {
+            count++;
+            idx += needle.Length;
+        }
+        return count;
+    }
+
+    // ── Test double ──────────────────────────────────────────────────────────
+
+    private sealed class FixedSizeSource(int cols, int rows) : ITerminalSizeSource
+    {
+        public (int Columns, int Rows) GetSize() => (cols, rows);
+    }
+}
diff --git a/tests/Dcli.Tests/VtInputParserTests.cs b/tests/Dcli.Tests/VtInputParserTests.cs
new file mode 100644
index 0000000..5809f1a
--- /dev/null
+++ b/tests/Dcli.Tests/VtInputParserTests.cs
@@ -0,0 +1,1211 @@
+using System.Text;
+using Dcli.Internal.Input;
+using Xunit;
+
+namespace Dcli.Tests;
+
+/// <summary>
+/// Behavioural tests for Section 5 (Chunk A + B): VT input parser, event model, and decoding rules.
+/// Each test maps to a specific spec scenario or design-decision rule.
+/// </summary>
+public sealed class VtInputParserTests
+{
+    // ─── Helpers ─────────────────────────────────────────────────────────────
+
+    private static List<InputEvent> Parse(params byte[] bytes)
+    {
+        VtInputParser parser = new();
+        List<InputEvent> events = new();
+        parser.Feed(bytes, events);
+        return events;
+    }
+
+    private static List<InputEvent> ParseAndFlush(params byte[] bytes)
+    {
+        VtInputParser parser = new();
+        List<InputEvent> events = new();
+        parser.Feed(bytes, events);
+        parser.Flush(events);
+        return events;
+    }
+
+    private static KeyEvent ExpectKey(List<InputEvent> events, int index = 0)
+    {
+        Assert.True(index < events.Count, $"Expected at least {index + 1} event(s); got {events.Count}.");
+        Assert.IsType<KeyEvent>(events[index]);
+        return (KeyEvent)events[index];
+    }
+
+    /// <summary>Builds the bracketed-paste byte sequence: ESC [ 200 ~ + content + ESC [ 201 ~.</summary>
+    private static byte[] BracketedPaste(string content)
+    {
+        byte[] start = [0x1B, (byte)'[', (byte)'2', (byte)'0', (byte)'0', (byte)'~'];
+        byte[] end = [0x1B, (byte)'[', (byte)'2', (byte)'0', (byte)'1', (byte)'~'];
+        byte[] body = Encoding.UTF8.GetBytes(content);
+        byte[] result = new byte[start.Length + body.Length + end.Length];
+        start.CopyTo(result, 0);
+        body.CopyTo(result, start.Length);
+        end.CopyTo(result, start.Length + body.Length);
+        return result;
+    }
+
+    // ─── 5.2 Named-key table ─────────────────────────────────────────────────
+
+    [Fact]
+    public void TabByteProducesNamedTabWithNoModifiers()
+    {
+        // Spec scenario: byte 0x09 → Named(Tab), not Ctrl+I.
+        List<InputEvent> events = Parse(0x09);
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.Tab), ke.Code);
+        Assert.Equal(Modifiers.None, ke.Modifiers);
+    }
+
+    [Fact]
+    public void EnterByteProducesNamedEnter()
+    {
+        List<InputEvent> events = Parse(0x0D);
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.Enter), ke.Code);
+        Assert.Equal(Modifiers.None, ke.Modifiers);
+    }
+
+    [Fact]
+    public void BackspaceDelByteProducesNamedBackspace()
+    {
+        // 0x7F (DEL) is the standard backspace in raw mode.
+        List<InputEvent> events = Parse(0x7F);
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.Backspace), ke.Code);
+    }
+
+    [Fact]
+    public void BackspaceBsByteProducesNamedBackspace()
+    {
+        // 0x08 (BS) is also mapped to Backspace.
+        List<InputEvent> events = Parse(0x08);
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.Backspace), ke.Code);
+    }
+
+    // ─── 5.5 Terminal-truth rules ────────────────────────────────────────────
+
+    [Fact]
+    public void CtrlCProducesLowercaseCWithCtrlModifier()
+    {
+        // Spec scenario: 0x03 → KeyEvent(Char('c'), Ctrl). Lowercase.
+        List<InputEvent> events = Parse(0x03);
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.FromRune(new Rune('c')), ke.Code);
+        Assert.Equal(Modifiers.Ctrl, ke.Modifiers);
+    }
+
+    [Fact]
+    public void CtrlAProducesLowercaseAWithCtrlModifier()
+    {
+        // 0x01 → FromRune('a') + Ctrl.
+        List<InputEvent> events = Parse(0x01);
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.FromRune(new Rune('a')), ke.Code);
+        Assert.Equal(Modifiers.Ctrl, ke.Modifiers);
+    }
+
+    [Fact]
+    public void CtrlZProducesLowercaseZWithCtrlModifier()
+    {
+        // 0x1A → FromRune('z') + Ctrl.
+        List<InputEvent> events = Parse(0x1A);
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.FromRune(new Rune('z')), ke.Code);
+        Assert.Equal(Modifiers.Ctrl, ke.Modifiers);
+    }
+
+    [Fact]
+    public void UppercaseAProducesRuneWithNoModifiers()
+    {
+        // Spec scenario: 'A' → FromRune('A'), Modifiers.None — Shift is implicit in the rune.
+        List<InputEvent> events = Parse((byte)'A');
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.FromRune(new Rune('A')), ke.Code);
+        Assert.Equal(Modifiers.None, ke.Modifiers);
+    }
+
+    [Fact]
+    public void LowercaseAProducesRuneWithNoModifiers()
+    {
+        List<InputEvent> events = Parse((byte)'a');
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.FromRune(new Rune('a')), ke.Code);
+        Assert.Equal(Modifiers.None, ke.Modifiers);
+    }
+
+    // ─── 5.1 CSI arrow keys ──────────────────────────────────────────────────
+
+    [Fact]
+    public void CsiUpProducesNamedUp()
+    {
+        // Spec scenario: ESC [ A → Named(Up).
+        List<InputEvent> events = Parse(0x1B, (byte)'[', (byte)'A');
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.Up), ke.Code);
+        Assert.Equal(Modifiers.None, ke.Modifiers);
+    }
+
+    [Fact]
+    public void CsiDownProducesNamedDown()
+    {
+        List<InputEvent> events = Parse(0x1B, (byte)'[', (byte)'B');
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.Down), ke.Code);
+    }
+
+    [Fact]
+    public void CsiRightProducesNamedRight()
+    {
+        List<InputEvent> events = Parse(0x1B, (byte)'[', (byte)'C');
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.Right), ke.Code);
+    }
+
+    [Fact]
+    public void CsiLeftProducesNamedLeft()
+    {
+        List<InputEvent> events = Parse(0x1B, (byte)'[', (byte)'D');
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.Left), ke.Code);
+    }
+
+    // ─── SS3 arrow keys (macOS terminal reality) ─────────────────────────────
+
+    [Fact]
+    public void Ss3UpProducesNamedUp()
+    {
+        // macOS Terminal sends ESC O A for Up — MUST decode correctly.
+        List<InputEvent> events = Parse(0x1B, (byte)'O', (byte)'A');
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.Up), ke.Code);
+        Assert.Equal(Modifiers.None, ke.Modifiers);
+    }
+
+    [Fact]
+    public void Ss3DownProducesNamedDown()
+    {
+        List<InputEvent> events = Parse(0x1B, (byte)'O', (byte)'B');
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.Down), ke.Code);
+    }
+
+    [Fact]
+    public void Ss3RightProducesNamedRight()
+    {
+        List<InputEvent> events = Parse(0x1B, (byte)'O', (byte)'C');
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.Right), ke.Code);
+    }
+
+    [Fact]
+    public void Ss3LeftProducesNamedLeft()
+    {
+        List<InputEvent> events = Parse(0x1B, (byte)'O', (byte)'D');
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.Left), ke.Code);
+    }
+
+    [Fact]
+    public void Ss3HomeProducesNamedHome()
+    {
+        List<InputEvent> events = Parse(0x1B, (byte)'O', (byte)'H');
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.Home), ke.Code);
+    }
+
+    [Fact]
+    public void Ss3EndProducesNamedEnd()
+    {
+        List<InputEvent> events = Parse(0x1B, (byte)'O', (byte)'F');
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.End), ke.Code);
+    }
+
+    // ─── SS3 F1–F4 ───────────────────────────────────────────────────────────
+
+    [Fact]
+    public void Ss3F1ProducesNamedF1()
+    {
+        List<InputEvent> events = Parse(0x1B, (byte)'O', (byte)'P');
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.F1), ke.Code);
+    }
+
+    [Fact]
+    public void Ss3F2ProducesNamedF2()
+    {
+        List<InputEvent> events = Parse(0x1B, (byte)'O', (byte)'Q');
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.F2), ke.Code);
+    }
+
+    [Fact]
+    public void Ss3F3ProducesNamedF3()
+    {
+        List<InputEvent> events = Parse(0x1B, (byte)'O', (byte)'R');
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.F3), ke.Code);
+    }
+
+    [Fact]
+    public void Ss3F4ProducesNamedF4()
+    {
+        List<InputEvent> events = Parse(0x1B, (byte)'O', (byte)'S');
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.F4), ke.Code);
+    }
+
+    // ─── CSI Home/End ────────────────────────────────────────────────────────
+
+    [Fact]
+    public void CsiHomeProducesNamedHome()
+    {
+        List<InputEvent> events = Parse(0x1B, (byte)'[', (byte)'H');
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.Home), ke.Code);
+    }
+
+    [Fact]
+    public void CsiEndProducesNamedEnd()
+    {
+        List<InputEvent> events = Parse(0x1B, (byte)'[', (byte)'F');
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.End), ke.Code);
+    }
+
+    // ─── Tilde sequences ─────────────────────────────────────────────────────
+
+    [Fact]
+    public void CsiInsert2TildeProducesNamedInsert()
+    {
+        List<InputEvent> events = Parse(0x1B, (byte)'[', (byte)'2', (byte)'~');
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.Insert), ke.Code);
+    }
+
+    [Fact]
+    public void CsiDelete3TildeProducesNamedDelete()
+    {
+        List<InputEvent> events = Parse(0x1B, (byte)'[', (byte)'3', (byte)'~');
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.Delete), ke.Code);
+    }
+
+    [Fact]
+    public void CsiPageUp5TildeProducesNamedPageUp()
+    {
+        List<InputEvent> events = Parse(0x1B, (byte)'[', (byte)'5', (byte)'~');
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.PageUp), ke.Code);
+    }
+
+    [Fact]
+    public void CsiPageDown6TildeProducesNamedPageDown()
+    {
+        List<InputEvent> events = Parse(0x1B, (byte)'[', (byte)'6', (byte)'~');
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.PageDown), ke.Code);
+    }
+
+    [Fact]
+    public void CsiHome1TildeProducesNamedHome()
+    {
+        List<InputEvent> events = Parse(0x1B, (byte)'[', (byte)'1', (byte)'~');
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.Home), ke.Code);
+    }
+
+    [Fact]
+    public void CsiHome7TildeProducesNamedHome()
+    {
+        List<InputEvent> events = Parse(0x1B, (byte)'[', (byte)'7', (byte)'~');
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.Home), ke.Code);
+    }
+
+    [Fact]
+    public void CsiEnd4TildeProducesNamedEnd()
+    {
+        List<InputEvent> events = Parse(0x1B, (byte)'[', (byte)'4', (byte)'~');
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.End), ke.Code);
+    }
+
+    [Fact]
+    public void CsiEnd8TildeProducesNamedEnd()
+    {
+        List<InputEvent> events = Parse(0x1B, (byte)'[', (byte)'8', (byte)'~');
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.End), ke.Code);
+    }
+
+    // ─── F-key tilde sequences ────────────────────────────────────────────────
+
+    [Fact]
+    public void CsiF5Via15TildeProducesNamedF5()
+    {
+        List<InputEvent> events = Parse(0x1B, (byte)'[', (byte)'1', (byte)'5', (byte)'~');
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.F5), ke.Code);
+    }
+
+    [Fact]
+    public void CsiF6Via17TildeProducesNamedF6()
+    {
+        List<InputEvent> events = Parse(0x1B, (byte)'[', (byte)'1', (byte)'7', (byte)'~');
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.F6), ke.Code);
+    }
+
+    [Fact]
+    public void CsiF12Via24TildeProducesNamedF12()
+    {
+        List<InputEvent> events = Parse(0x1B, (byte)'[', (byte)'2', (byte)'4', (byte)'~');
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.F12), ke.Code);
+    }
+
+    // ─── BackTab ─────────────────────────────────────────────────────────────
+
+    [Fact]
+    public void CsiZProducesBackTabWithShiftModifier()
+    {
+        List<InputEvent> events = Parse(0x1B, (byte)'[', (byte)'Z');
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.BackTab), ke.Code);
+        Assert.Equal(Modifiers.Shift, ke.Modifiers);
+    }
+
+    // ─── 5.3 ESC disambiguation ──────────────────────────────────────────────
+
+    [Fact]
+    public void LoneEscAfterFlushProducesNamedEscape()
+    {
+        // Spec scenario: lone ESC + no follow-up → Flush → Named(Escape).
+        VtInputParser parser = new();
+        List<InputEvent> events = new();
+        parser.Feed([0x1B], events);
+        Assert.Empty(events); // still pending
+        parser.Flush(events);
+        Assert.Single(events);
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.Escape), ke.Code);
+        Assert.Equal(Modifiers.None, ke.Modifiers);
+    }
+
+    [Fact]
+    public void EscFollowedByBracketAProducesUpNotEscape()
+    {
+        // Spec scenario: ESC then [ A → Up, not Escape.
+        List<InputEvent> events = ParseAndFlush(0x1B, (byte)'[', (byte)'A');
+        Assert.Single(events);
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.Up), ke.Code);
+    }
+
+    [Fact]
+    public void AltPrefixEscThenAProducesAltModifiedA()
+    {
+        // ESC followed by printable 'a' → FromRune('a') + Alt.
+        List<InputEvent> events = ParseAndFlush(0x1B, (byte)'a');
+        Assert.Single(events);
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.FromRune(new Rune('a')), ke.Code);
+        Assert.Equal(Modifiers.Alt, ke.Modifiers);
+    }
+
+    [Fact]
+    public void AltPrefixEscThenCtrlCProducesAltCtrlC()
+    {
+        // ESC + 0x03 → FromRune('c') + Alt|Ctrl.
+        List<InputEvent> events = ParseAndFlush(0x1B, 0x03);
+        Assert.Single(events);
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.FromRune(new Rune('c')), ke.Code);
+        Assert.True((ke.Modifiers & Modifiers.Alt) != 0);
+        Assert.True((ke.Modifiers & Modifiers.Ctrl) != 0);
+    }
+
+    // ─── Modifier-encoded sequences ──────────────────────────────────────────
+
+    [Fact]
+    public void CsiShiftUpProducesShiftModifier()
+    {
+        // ESC [ 1 ; 2 A → Up + Shift. m=2 → bitmask=1 → Shift.
+        List<InputEvent> events = Parse(0x1B, (byte)'[', (byte)'1', (byte)';', (byte)'2', (byte)'A');
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.Up), ke.Code);
+        Assert.Equal(Modifiers.Shift, ke.Modifiers);
+    }
+
+    [Fact]
+    public void CsiCtrlUpProducesCtrlModifier()
+    {
+        // ESC [ 1 ; 5 A → Up + Ctrl. m=5 → bitmask=4 → Ctrl.
+        List<InputEvent> events = Parse(0x1B, (byte)'[', (byte)'1', (byte)';', (byte)'5', (byte)'A');
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.Up), ke.Code);
+        Assert.Equal(Modifiers.Ctrl, ke.Modifiers);
+    }
+
+    [Fact]
+    public void CsiShiftCtrlUpStripsShiftUnderCtrl()
+    {
+        // ESC [ 1 ; 6 A → Up + Shift+Ctrl on the wire. Terminal-truth rule: Ctrl strips Shift.
+        List<InputEvent> events = Parse(0x1B, (byte)'[', (byte)'1', (byte)';', (byte)'6', (byte)'A');
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.Up), ke.Code);
+        Assert.Equal(Modifiers.Ctrl, ke.Modifiers);
+        Assert.Equal(0, (int)(ke.Modifiers & Modifiers.Shift));
+    }
+
+    [Fact]
+    public void CsiAltUpProducesAltModifier()
+    {
+        // ESC [ 1 ; 3 A → Up + Alt. m=3 → bitmask=2 → Alt.
+        List<InputEvent> events = Parse(0x1B, (byte)'[', (byte)'1', (byte)';', (byte)'3', (byte)'A');
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.Up), ke.Code);
+        Assert.Equal(Modifiers.Alt, ke.Modifiers);
+    }
+
+    // ─── Unicode / UTF-8 ─────────────────────────────────────────────────────
+
+    [Fact]
+    public void TwoByteUtf8ScalarDecodesCorrectly()
+    {
+        // Spec scenario: multi-byte UTF-8 char → KeyEvent carrying that scalar.
+        // U+00E9 (é) = 0xC3 0xA9
+        List<InputEvent> events = Parse(0xC3, 0xA9);
+        Assert.Single(events);
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.FromRune(new Rune('é')), ke.Code);
+        Assert.Equal(Modifiers.None, ke.Modifiers);
+    }
+
+    [Fact]
+    public void ThreeByteUtf8CjkScalarDecodes()
+    {
+        // U+4E2D (中) = 0xE4 0xB8 0xAD
+        List<InputEvent> events = Parse(0xE4, 0xB8, 0xAD);
+        Assert.Single(events);
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.FromRune(new Rune('中')), ke.Code);
+    }
+
+    [Fact]
+    public void FourByteUtf8EmojiDecodes()
+    {
+        // U+1F600 (😀) = 0xF0 0x9F 0x98 0x80
+        List<InputEvent> events = Parse(0xF0, 0x9F, 0x98, 0x80);
+        Assert.Single(events);
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.FromRune(new Rune(0x1F600)), ke.Code);
+    }
+
+    [Fact]
+    public void Utf8ScalarSplitAcrossFeedsDecodesCorrectly()
+    {
+        // U+00E9 split: first byte in one Feed call, second in another.
+        VtInputParser parser = new();
+        List<InputEvent> events = new();
+        parser.Feed([0xC3], events);
+        Assert.Empty(events); // incomplete
+        parser.Feed([0xA9], events);
+        Assert.Single(events);
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.FromRune(new Rune('é')), ke.Code);
+    }
+
+    [Fact]
+    public void InvalidUtf8ContinuationInGroundEmitsReplacementChar()
+    {
+        // 0x80 alone (continuation byte with no lead) → U+FFFD.
+        List<InputEvent> events = Parse(0x80);
+        Assert.Single(events);
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.FromRune(Rune.ReplacementChar), ke.Code);
+    }
+
+    [Fact]
+    public void InvalidUtf8TruncatedSequenceEmitsReplacementThenNextChar()
+    {
+        // Lead byte 0xC3 followed by non-continuation 0x41 ('A') — truncated sequence.
+        // Should emit U+FFFD then FromRune('A').
+        List<InputEvent> events = Parse(0xC3, (byte)'A');
+        Assert.Equal(2, events.Count);
+        KeyEvent ke0 = ExpectKey(events, 0);
+        KeyEvent ke1 = ExpectKey(events, 1);
+        Assert.Equal(KeyCode.FromRune(Rune.ReplacementChar), ke0.Code);
+        Assert.Equal(KeyCode.FromRune(new Rune('A')), ke1.Code);
+    }
+
+    // ─── Split CSI across feeds ───────────────────────────────────────────────
+
+    [Fact]
+    public void CsiUpSplitAcrossFeedsDecodesCorrectly()
+    {
+        VtInputParser parser = new();
+        List<InputEvent> events = new();
+        parser.Feed([0x1B], events);
+        Assert.Empty(events);
+        parser.Feed([(byte)'['], events);
+        Assert.Empty(events);
+        parser.Feed([(byte)'A'], events);
+        Assert.Single(events);
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.Up), ke.Code);
+    }
+
+    // ─── Multiple events in one Feed ─────────────────────────────────────────
+
+    [Fact]
+    public void MultipleKeysInSingleFeedAllDecoded()
+    {
+        // 'a' then 'b' — two events.
+        List<InputEvent> events = Parse((byte)'a', (byte)'b');
+        Assert.Equal(2, events.Count);
+        Assert.Equal(KeyCode.FromRune(new Rune('a')), ((KeyEvent)events[0]).Code);
+        Assert.Equal(KeyCode.FromRune(new Rune('b')), ((KeyEvent)events[1]).Code);
+    }
+
+    [Fact]
+    public void ArrowKeyThenPrintableCharBothDecoded()
+    {
+        // SS3 Up followed by 'x'.
+        List<InputEvent> events = Parse(0x1B, (byte)'O', (byte)'A', (byte)'x');
+        Assert.Equal(2, events.Count);
+        Assert.Equal(KeyCode.Named(NamedKey.Up), ((KeyEvent)events[0]).Code);
+        Assert.Equal(KeyCode.FromRune(new Rune('x')), ((KeyEvent)events[1]).Code);
+    }
+
+    // ─── Unrecognised sequences ───────────────────────────────────────────────
+
+    [Fact]
+    public void UnrecognisedCsiSequenceIsDroppedWithNoDesync()
+    {
+        // ESC [ 9 9 m (unrecognised) followed by 'a' — only 'a' should come out.
+        List<InputEvent> events = Parse(0x1B, (byte)'[', (byte)'9', (byte)'9', (byte)'m', (byte)'a');
+        Assert.Single(events);
+        Assert.Equal(KeyCode.FromRune(new Rune('a')), ((KeyEvent)events[0]).Code);
+    }
+
+    [Fact]
+    public void UnrecognisedSs3FinalIsDroppedWithNoDesync()
+    {
+        // ESC O X (unrecognised) followed by 'b'.
+        List<InputEvent> events = Parse(0x1B, (byte)'O', (byte)'X', (byte)'b');
+        Assert.Single(events);
+        Assert.Equal(KeyCode.FromRune(new Rune('b')), ((KeyEvent)events[0]).Code);
+    }
+
+    [Fact]
+    public void UnrecognisedTildeCodeIsDroppedWithNoDesync()
+    {
+        // ESC [ 9 9 ~ (unrecognised tilde) followed by 'z'.
+        List<InputEvent> events = Parse(0x1B, (byte)'[', (byte)'9', (byte)'9', (byte)'~', (byte)'z');
+        Assert.Single(events);
+        Assert.Equal(KeyCode.FromRune(new Rune('z')), ((KeyEvent)events[0]).Code);
+    }
+
+    // ─── Flush no-op when not pending ────────────────────────────────────────
+
+    [Fact]
+    public void FlushWhenNothingPendingIsNoop()
+    {
+        VtInputParser parser = new();
+        List<InputEvent> events = new();
+        parser.Flush(events);
+        Assert.Empty(events);
+    }
+
+    // ─── Event model construction (Decision 12 / test-harness spec) ──────────
+
+    [Fact]
+    public void KeyEventIsPubliclyConstructible()
+    {
+        KeyEvent ke = new(KeyCode.Named(NamedKey.Enter), Modifiers.None);
+        Assert.Equal(KeyCode.Named(NamedKey.Enter), ke.Code);
+        Assert.Equal(Modifiers.None, ke.Modifiers);
+    }
+
+    [Fact]
+    public void PasteEventIsPubliclyConstructible()
+    {
+        PasteEvent pe = new("hello");
+        Assert.Equal("hello", pe.Text);
+    }
+
+    [Fact]
+    public void ResizeEventIsPubliclyConstructible()
+    {
+        ResizeEvent re = new(80, 24);
+        Assert.Equal(80, re.Columns);
+        Assert.Equal(24, re.Rows);
+    }
+
+    [Fact]
+    public void KeyCodeFromRuneRoundTrips()
+    {
+        KeyCode kc = KeyCode.FromRune(new Rune('x'));
+        Assert.Equal(KeyCode.KeyCodeKind.UnicodeScalar, kc.Kind);
+        Assert.Equal(new Rune('x'), kc.RuneValue);
+    }
+
+    [Fact]
+    public void KeyCodeNamedRoundTrips()
+    {
+        KeyCode kc = KeyCode.Named(NamedKey.Delete);
+        Assert.Equal(KeyCode.KeyCodeKind.Named, kc.Kind);
+        Assert.Equal(NamedKey.Delete, kc.NamedValue);
+    }
+
+    [Fact]
+    public void KeyCodeNamedAccessorOnScalarKindThrows()
+    {
+        KeyCode kc = KeyCode.FromRune(new Rune('a'));
+        Assert.Throws<InvalidOperationException>(() => _ = kc.NamedValue);
+    }
+
+    [Fact]
+    public void KeyCodeRuneAccessorOnNamedKindThrows()
+    {
+        KeyCode kc = KeyCode.Named(NamedKey.Up);
+        Assert.Throws<InvalidOperationException>(() => _ = kc.RuneValue);
+    }
+
+    [Fact]
+    public void ModifiersIsFlagsEnumAndCombinesCorrectly()
+    {
+        Modifiers combined = Modifiers.Ctrl | Modifiers.Alt;
+        Assert.True((combined & Modifiers.Ctrl) != 0);
+        Assert.True((combined & Modifiers.Alt) != 0);
+        Assert.False((combined & Modifiers.Shift) != 0);
+    }
+
+    // ─── InputEvent is abstract record hierarchy ──────────────────────────────
+
+    [Fact]
+    public void InputEventPatternMatchWorksOnAllSubtypes()
+    {
+        InputEvent[] events =
+        [
+            new KeyEvent(KeyCode.Named(NamedKey.Tab), Modifiers.None),
+            new PasteEvent("text"),
+            new ResizeEvent(120, 40),
+        ];
+
+        int keys = 0, pastes = 0, resizes = 0;
+        foreach (InputEvent ev in events)
+        {
+            switch (ev)
+            {
+                case KeyEvent: keys++; break;
+                case PasteEvent: pastes++; break;
+                case ResizeEvent: resizes++; break;
+            }
+        }
+
+        Assert.Equal(1, keys);
+        Assert.Equal(1, pastes);
+        Assert.Equal(1, resizes);
+    }
+
+    // ═══════════════════════════════════════════════════════════════════════════
+    // Chunk B — Section 5.4: Bracketed paste
+    // ═══════════════════════════════════════════════════════════════════════════
+
+    [Fact]
+    public void BracketedPasteSimpleTextEmitsSinglePasteEvent()
+    {
+        // ESC [ 200 ~ hello ESC [ 201 ~ → one PasteEvent("hello")
+        List<InputEvent> events = Parse(BracketedPaste("hello"));
+        Assert.Single(events);
+        PasteEvent pe = Assert.IsType<PasteEvent>(events[0]);
+        Assert.Equal("hello", pe.Text);
+    }
+
+    [Fact]
+    public void BracketedPasteEmptyBodyEmitsPasteEventWithEmptyString()
+    {
+        // ESC [ 200 ~ immediately followed by ESC [ 201 ~ → PasteEvent("")
+        List<InputEvent> events = Parse(BracketedPaste(string.Empty));
+        Assert.Single(events);
+        PasteEvent pe = Assert.IsType<PasteEvent>(events[0]);
+        Assert.Equal(string.Empty, pe.Text);
+    }
+
+    [Fact]
+    public void BracketedPasteMultilinePreservesNewline()
+    {
+        // A pasted newline must NOT become an Enter key event — it is literal text.
+        List<InputEvent> events = Parse(BracketedPaste("line1\nline2"));
+        Assert.Single(events);
+        PasteEvent pe = Assert.IsType<PasteEvent>(events[0]);
+        Assert.Equal("line1\nline2", pe.Text);
+    }
+
+    [Fact]
+    public void BracketedPasteNewlineIsNotSubmittedAsEnter()
+    {
+        // Verify no KeyEvent(Enter) is produced alongside the paste event.
+        List<InputEvent> events = Parse(BracketedPaste("a\nb"));
+        Assert.Single(events);
+        Assert.IsType<PasteEvent>(events[0]);
+    }
+
+    [Fact]
+    public void BracketedPasteSplitAcrossFeeds()
+    {
+        // Full paste sequence split arbitrarily across two Feed calls.
+        byte[] full = BracketedPaste("abc");
+        byte[] first = full[..^3];
+        byte[] second = full[^3..];
+
+        VtInputParser parser = new();
+        List<InputEvent> events = new();
+        parser.Feed(first, events);
+        Assert.Empty(events); // not yet complete
+        parser.Feed(second, events);
+        Assert.Single(events);
+        PasteEvent pe = Assert.IsType<PasteEvent>(events[0]);
+        Assert.Equal("abc", pe.Text);
+    }
+
+    [Fact]
+    public void BracketedPasteEndMarkerSplitAcrossFeeds()
+    {
+        // End marker ESC [ 2 0 1 ~ split so that "ESC [ 2 0" arrives in first feed
+        // and "1 ~" in the second.
+        byte[] start = [0x1B, (byte)'[', (byte)'2', (byte)'0', (byte)'0', (byte)'~'];
+        byte[] end = [0x1B, (byte)'[', (byte)'2', (byte)'0', (byte)'1', (byte)'~'];
+        byte[] content = "xy"u8.ToArray();
+
+        // First feed: paste-start + content + first 4 bytes of end-marker (ESC [ 2 0)
+        byte[] feed1 = [.. start, .. content, end[0], end[1], end[2], end[3]];
+        // Second feed: remaining 2 bytes of end-marker (1 ~)
+        byte[] feed2 = [end[4], end[5]];
+
+        VtInputParser parser = new();
+        List<InputEvent> events = new();
+        parser.Feed(feed1, events);
+        Assert.Empty(events);
+        parser.Feed(feed2, events);
+        Assert.Single(events);
+        PasteEvent pe = Assert.IsType<PasteEvent>(events[0]);
+        Assert.Equal("xy", pe.Text);
+    }
+
+    [Fact]
+    public void BracketedPasteStrayEscNotEndMarkerIsLiteralText()
+    {
+        // A lone ESC inside paste that is NOT followed by [ 2 0 1 ~ is literal.
+        // We send: ESC [ 200 ~ "a" ESC "b" ESC [ 201 ~
+        // The stray ESC + "b" should appear verbatim in the pasted text.
+        byte[] start = [0x1B, (byte)'[', (byte)'2', (byte)'0', (byte)'0', (byte)'~'];
+        byte[] end = [0x1B, (byte)'[', (byte)'2', (byte)'0', (byte)'1', (byte)'~'];
+        // body = 'a' ESC 'b'  — the ESC is not followed by the end marker
+        byte[] body = [(byte)'a', 0x1B, (byte)'b'];
+
+        byte[] input = [.. start, .. body, .. end];
+        List<InputEvent> events = Parse(input);
+        Assert.Single(events);
+        PasteEvent pe = Assert.IsType<PasteEvent>(events[0]);
+        // The stray ESC should appear as U+001B in the decoded string: "a\u001Bb" (3 chars).
+        Assert.Equal("a\u001Bb", pe.Text);
+    }
+
+    [Fact]
+    public void BracketedPasteStrayEscFollowedByNonEndMarkerSplitAcrossFeeds()
+    {
+        // Like the test above but the stray ESC arrives at the end of one feed call
+        // and the non-matching byte arrives in the next feed.
+        byte[] start = [0x1B, (byte)'[', (byte)'2', (byte)'0', (byte)'0', (byte)'~'];
+        byte[] end = [0x1B, (byte)'[', (byte)'2', (byte)'0', (byte)'1', (byte)'~'];
+
+        // Feed 1: paste-start + "a" + ESC  (partial candidate: matched 1 byte)
+        byte[] feed1 = [.. start, (byte)'a', 0x1B];
+        // Feed 2: "x" (diverges → ESC becomes literal) + end marker
+        byte[] feed2 = [(byte)'x', .. end];
+
+        VtInputParser parser = new();
+        List<InputEvent> events = new();
+        parser.Feed(feed1, events);
+        Assert.Empty(events);
+        parser.Feed(feed2, events);
+        Assert.Single(events);
+        PasteEvent pe = Assert.IsType<PasteEvent>(events[0]);
+        Assert.Equal("a\x1Bx", pe.Text);
+    }
+
+    [Fact]
+    public void BracketedPasteContainsUtf8MultibyteCjk()
+    {
+        // Paste containing 中 (U+4E2D, 3-byte UTF-8) is decoded correctly.
+        List<InputEvent> events = Parse(BracketedPaste("中"));
+        Assert.Single(events);
+        PasteEvent pe = Assert.IsType<PasteEvent>(events[0]);
+        Assert.Equal("中", pe.Text);
+    }
+
+    [Fact]
+    public void BracketedPasteContainsUtf8FourByteEmoji()
+    {
+        // Paste containing 😀 (U+1F600, 4-byte UTF-8) is decoded correctly.
+        List<InputEvent> events = Parse(BracketedPaste("😀"));
+        Assert.Single(events);
+        PasteEvent pe = Assert.IsType<PasteEvent>(events[0]);
+        Assert.Equal("😀", pe.Text);
+    }
+
+    [Fact]
+    public void BracketedPasteFollowedByRegularKeyBothParsed()
+    {
+        // After the paste, a subsequent regular key should still parse.
+        byte[] input = [.. BracketedPaste("hi"), (byte)'a'];
+        List<InputEvent> events = Parse(input);
+        Assert.Equal(2, events.Count);
+        Assert.IsType<PasteEvent>(events[0]);
+        KeyEvent ke = ExpectKey(events, 1);
+        Assert.Equal(KeyCode.FromRune(new Rune('a')), ke.Code);
+    }
+
+    [Fact]
+    public void BracketedPasteEndMarkerOverlapDivergesAndRestarts()
+    {
+        // N1: overlap / re-start — body bytes partially match the end-marker prefix then diverge.
+        // Body = [ESC, '[', '3']: ESC matches marker[0], '[' matches marker[1], but '3' ≠ marker[2]('2').
+        // On diverge, the two matched bytes (ESC '[') are flushed as literal to _pasteBuf,
+        // then '3' is stored as ordinary content. The real end marker ESC[201~ then fires.
+        // Expected PasteEvent("\x1B[3") — one ESC, one '[', one '3'.
+        byte[] startMarker = [0x1B, (byte)'[', (byte)'2', (byte)'0', (byte)'0', (byte)'~'];
+        byte[] endMarker = [0x1B, (byte)'[', (byte)'2', (byte)'0', (byte)'1', (byte)'~'];
+        byte[] body = [0x1B, (byte)'[', (byte)'3'];
+
+        byte[] input = [.. startMarker, .. body, .. endMarker];
+
+        List<InputEvent> events = Parse(input);
+        Assert.Single(events);
+        PasteEvent pe = Assert.IsType<PasteEvent>(events[0]);
+        Assert.Equal("\x1B[3", pe.Text);
+    }
+
+    [Fact]
+    public void BracketedPasteEndMarkerDivergingEscReseedsCandidate()
+    {
+        // N1b: diverging byte is itself an ESC — it must re-seed a fresh candidate, not be dropped.
+        // Body = [ESC, '[', '2']: matches end-marker template through index 2 (candidate len 3).
+        // Next byte = 0x1B (the real end-marker's first byte): diverges at template index 3 ('0').
+        // The matched prefix [ESC '[' '2'] flushes as literal text ("\x1B[2") into _pasteBuf.
+        // The diverging ESC re-seeds _pasteEndCandidateLen to 1; the remaining [' ' 2 0 1 ~]
+        // completes the end marker → PasteEvent("[2").
+        // NOTE: "\x1B[2" has ESC as first char; PasteEvent.Text = "[2" (only the non-ESC body
+        // bytes that made it to _pasteBuf before the candidate was started at ESC).
+        // Trace: start marker consumed → in paste mode.
+        //   ESC  → candidate[0] match, candidateLen=1
+        //   '['  → candidate[1] match, candidateLen=2
+        //   '2'  → candidate[2] match, candidateLen=3  (end-marker is ESC [ 2 0 1 ~)
+        //   0x1B → candidate[3] expected '0', got ESC → flush [ESC '[' '2'] to _pasteBuf, re-seed: candidateLen=1
+        //   '['  → candidate[1] match, candidateLen=2
+        //   '2'  → candidate[2] match, candidateLen=3
+        //   '0'  → candidate[3] match, candidateLen=4
+        //   '1'  → candidate[4] match, candidateLen=5
+        //   '~'  → candidate[5] match → end marker complete → emit PasteEvent("[2")
+        // Wait — the flushed prefix is ESC '[' '2', giving _pasteBuf = [ESC, '[', '2'].
+        // But the start-marker was already consumed; _pasteBuf only holds post-start content.
+        // So Text = "\x1B[2". Assert that, then assert a key after proves no desync.
+        byte[] startMarker = [0x1B, (byte)'[', (byte)'2', (byte)'0', (byte)'0', (byte)'~'];
+        byte[] endMarker = [0x1B, (byte)'[', (byte)'2', (byte)'0', (byte)'1', (byte)'~'];
+        // Body: ESC '[' '2' — matches first 3 bytes of end-marker, then the real end marker follows.
+        // The real end marker's leading ESC is the diverging byte that re-seeds the candidate.
+        byte[] input = [.. startMarker, 0x1B, (byte)'[', (byte)'2', .. endMarker, (byte)'a'];
+
+        VtInputParser parser = new();
+        List<InputEvent> events = new();
+        parser.Feed(input, events);
+
+        // Exactly two events: one PasteEvent then one KeyEvent('a').
+        Assert.Equal(2, events.Count);
+        PasteEvent pe = Assert.IsType<PasteEvent>(events[0]);
+        Assert.Equal("\x1B[2", pe.Text);
+        // The 'a' after the paste bracket parses correctly — no desync.
+        KeyEvent ke = Assert.IsType<KeyEvent>(events[1]);
+        Assert.Equal(KeyCode.FromRune(new Rune('a')), ke.Code);
+    }
+
+    [Fact]
+    public void BracketedPasteUtf8ScalarStraddlingFeedBoundaryDecodesCorrectly()
+    {
+        // N2: a multibyte UTF-8 scalar split across two Feed calls inside a paste body.
+        // U+4E2D (中) = 0xE4 0xB8 0xAD (3 bytes).
+        // Feed 1: start marker + first byte of 中 (0xE4).
+        // Feed 2: remaining two bytes of 中 (0xB8 0xAD) + end marker.
+        // The parser accumulates raw bytes and UTF-8 decodes at the end marker;
+        // feed-call boundaries inside paste are irrelevant.
+        byte[] startMarker = [0x1B, (byte)'[', (byte)'2', (byte)'0', (byte)'0', (byte)'~'];
+        byte[] endMarker = [0x1B, (byte)'[', (byte)'2', (byte)'0', (byte)'1', (byte)'~'];
+
+        byte[] feed1 = [.. startMarker, 0xE4];
+        byte[] feed2 = [0xB8, 0xAD, .. endMarker];
+
+        VtInputParser parser = new();
+        List<InputEvent> events = new();
+        parser.Feed(feed1, events);
+        Assert.Empty(events);
+        parser.Feed(feed2, events);
+        Assert.Single(events);
+        PasteEvent pe = Assert.IsType<PasteEvent>(events[0]);
+        Assert.Equal("中", pe.Text);
+    }
+
+    // ═══════════════════════════════════════════════════════════════════════════
+    // Section 5.6: Exhaustive byte-fixture suite (collision-direction guards,
+    // modifier assertions, unrecognised-sequence resilience)
+    // ═══════════════════════════════════════════════════════════════════════════
+
+    // ── Collision-direction guards (reviewer nit 3) ──────────────────────────
+
+    [Fact]
+    public void Byte0x09IsNamedTabNotCtrlI()
+    {
+        // 0x09 must produce Named(Tab) with Modifiers.None, NOT Ctrl+I.
+        List<InputEvent> events = Parse(0x09);
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.Tab), ke.Code);
+        Assert.Equal(Modifiers.None, ke.Modifiers);
+    }
+
+    [Fact]
+    public void Byte0x0DIsNamedEnterNotCtrlM()
+    {
+        // 0x0D must produce Named(Enter) with Modifiers.None, NOT Ctrl+M.
+        List<InputEvent> events = Parse(0x0D);
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.Enter), ke.Code);
+        Assert.Equal(Modifiers.None, ke.Modifiers);
+    }
+
+    [Fact]
+    public void Byte0x08IsNamedBackspaceNotCtrlH()
+    {
+        // 0x08 must produce Named(Backspace) with Modifiers.None, NOT Ctrl+H.
+        List<InputEvent> events = Parse(0x08);
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.Backspace), ke.Code);
+        Assert.Equal(Modifiers.None, ke.Modifiers);
+    }
+
+    // ── 'A' has no Shift ─────────────────────────────────────────────────────
+
+    [Fact]
+    public void UppercaseAProducesRuneWithModifiersNoneNoShift()
+    {
+        // Spec rule: Shift is never synthesised for printable runes.
+        List<InputEvent> events = Parse((byte)'A');
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.FromRune(new Rune('A')), ke.Code);
+        Assert.Equal(Modifiers.None, ke.Modifiers);
+        Assert.Equal(0, (int)(ke.Modifiers & Modifiers.Shift));
+    }
+
+    [Fact]
+    public void CtrlLetterHasNoShiftModifier()
+    {
+        // 0x03 → Char('c') + Ctrl. Ctrl letter must not have Shift.
+        List<InputEvent> events = Parse(0x03);
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.FromRune(new Rune('c')), ke.Code);
+        Assert.True((ke.Modifiers & Modifiers.Ctrl) != 0);
+        Assert.Equal(0, (int)(ke.Modifiers & Modifiers.Shift));
+    }
+
+    // ── Arrows: both CSI and SS3 encodings ───────────────────────────────────
+
+    [Theory]
+    [InlineData(new byte[] { 0x1B, (byte)'[', (byte)'A' }, NamedKey.Up)]
+    [InlineData(new byte[] { 0x1B, (byte)'[', (byte)'B' }, NamedKey.Down)]
+    [InlineData(new byte[] { 0x1B, (byte)'[', (byte)'C' }, NamedKey.Right)]
+    [InlineData(new byte[] { 0x1B, (byte)'[', (byte)'D' }, NamedKey.Left)]
+    public void CsiArrowProducesCorrectNamedKey(byte[] input, NamedKey expected)
+    {
+        List<InputEvent> events = Parse(input);
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(expected), ke.Code);
+        Assert.Equal(Modifiers.None, ke.Modifiers);
+    }
+
+    [Theory]
+    [InlineData(new byte[] { 0x1B, (byte)'O', (byte)'A' }, NamedKey.Up)]
+    [InlineData(new byte[] { 0x1B, (byte)'O', (byte)'B' }, NamedKey.Down)]
+    [InlineData(new byte[] { 0x1B, (byte)'O', (byte)'C' }, NamedKey.Right)]
+    [InlineData(new byte[] { 0x1B, (byte)'O', (byte)'D' }, NamedKey.Left)]
+    public void Ss3ArrowProducesCorrectNamedKey(byte[] input, NamedKey expected)
+    {
+        List<InputEvent> events = Parse(input);
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(expected), ke.Code);
+        Assert.Equal(Modifiers.None, ke.Modifiers);
+    }
+
+    // ── Unicode scalar fixture: 2-byte, 3-byte, 4-byte ───────────────────────
+
+    [Fact]
+    public void TwoByteUtf8EAccentProducesCorrectScalar()
+    {
+        // U+00E9 é = 0xC3 0xA9
+        List<InputEvent> events = Parse(0xC3, 0xA9);
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.FromRune(new Rune('é')), ke.Code);
+        Assert.Equal(Modifiers.None, ke.Modifiers);
+    }
+
+    [Fact]
+    public void ThreeByteUtf8CjkProducesCorrectScalar()
+    {
+        // U+4E2D 中 = 0xE4 0xB8 0xAD
+        List<InputEvent> events = Parse(0xE4, 0xB8, 0xAD);
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.FromRune(new Rune('中')), ke.Code);
+    }
+
+    [Fact]
+    public void FourByteUtf8EmojiProducesCorrectScalar()
+    {
+        // U+1F600 😀 = 0xF0 0x9F 0x98 0x80
+        List<InputEvent> events = Parse(0xF0, 0x9F, 0x98, 0x80);
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.FromRune(new Rune(0x1F600)), ke.Code);
+    }
+
+    [Fact]
+    public void TwoByteUtf8ScalarSplitAcrossFeedsFixture()
+    {
+        // U+00E9 é split across two feeds.
+        VtInputParser parser = new();
+        List<InputEvent> events = new();
+        parser.Feed([0xC3], events);
+        Assert.Empty(events);
+        parser.Feed([0xA9], events);
+        Assert.Single(events);
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.FromRune(new Rune('é')), ke.Code);
+    }
+
+    // ── ESC disambiguation ────────────────────────────────────────────────────
+
+    [Fact]
+    public void LoneEscPlusFlushProducesEscape()
+    {
+        VtInputParser parser = new();
+        List<InputEvent> events = new();
+        parser.Feed([0x1B], events);
+        Assert.Empty(events);
+        parser.Flush(events);
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.Escape), ke.Code);
+        Assert.Equal(Modifiers.None, ke.Modifiers);
+    }
+
+    [Fact]
+    public void EscThenBracketAProducesUpNotEscape()
+    {
+        List<InputEvent> events = ParseAndFlush(0x1B, (byte)'[', (byte)'A');
+        Assert.Single(events);
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.Up), ke.Code);
+    }
+
+    [Fact]
+    public void EscThenPrintableProducesAltKey()
+    {
+        List<InputEvent> events = ParseAndFlush(0x1B, (byte)'a');
+        Assert.Single(events);
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.FromRune(new Rune('a')), ke.Code);
+        Assert.Equal(Modifiers.Alt, ke.Modifiers);
+    }
+
+    // ── Modifier-encoded sequences ────────────────────────────────────────────
+
+    [Fact]
+    public void CsiCtrlUpFixture()
+    {
+        // ESC [ 1 ; 5 A → Named(Up) + Ctrl
+        List<InputEvent> events = Parse(0x1B, (byte)'[', (byte)'1', (byte)';', (byte)'5', (byte)'A');
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.Up), ke.Code);
+        Assert.Equal(Modifiers.Ctrl, ke.Modifiers);
+    }
+
+    [Fact]
+    public void CsiShiftUpFixture()
+    {
+        // ESC [ 1 ; 2 A → Named(Up) + Shift
+        List<InputEvent> events = Parse(0x1B, (byte)'[', (byte)'1', (byte)';', (byte)'2', (byte)'A');
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.Up), ke.Code);
+        Assert.Equal(Modifiers.Shift, ke.Modifiers);
+    }
+
+    // ── F-key tilde sequences ─────────────────────────────────────────────────
+
+    [Fact]
+    public void CsiF5Via15TildeFixture()
+    {
+        List<InputEvent> events = Parse(0x1B, (byte)'[', (byte)'1', (byte)'5', (byte)'~');
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.F5), ke.Code);
+    }
+
+    [Fact]
+    public void CsiF12Via24TildeFixture()
+    {
+        List<InputEvent> events = Parse(0x1B, (byte)'[', (byte)'2', (byte)'4', (byte)'~');
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.Named(NamedKey.F12), ke.Code);
+    }
+
+    // ── Unrecognised sequence → no desync → next key parses ──────────────────
+
+    [Fact]
+    public void UnrecognisedCsiDroppedNoDesyncFixtureSuite()
+    {
+        // ESC [ 9 9 m (unrecognised) then 'z' — only 'z' comes out.
+        List<InputEvent> events = Parse(0x1B, (byte)'[', (byte)'9', (byte)'9', (byte)'m', (byte)'z');
+        Assert.Single(events);
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.FromRune(new Rune('z')), ke.Code);
+    }
+
+    [Fact]
+    public void UnrecognisedTildeCodeDroppedNoDesyncFixtureSuite()
+    {
+        // ESC [ 9 9 ~ then 'q'
+        List<InputEvent> events = Parse(0x1B, (byte)'[', (byte)'9', (byte)'9', (byte)'~', (byte)'q');
+        Assert.Single(events);
+        KeyEvent ke = ExpectKey(events);
+        Assert.Equal(KeyCode.FromRune(new Rune('q')), ke.Code);
+    }
+
+    // ── KeyCode.ToString() consistency ───────────────────────────────────────
+
+    [Fact]
+    public void KeyCodeFromRuneToStringIsUnicodeScalarFormat()
+    {
+        KeyCode kc = KeyCode.FromRune(new Rune('A'));
+        Assert.StartsWith("UnicodeScalar(", kc.ToString(), StringComparison.Ordinal);
+    }
+
+    [Fact]
+    public void KeyCodeNamedToStringIsNamedFormat()
+    {
+        KeyCode kc = KeyCode.Named(NamedKey.Up);
+        Assert.StartsWith("Named(", kc.ToString(), StringComparison.Ordinal);
+    }
+}