perf(shell): skip reading unchanged rows when rendering

[domenkozar]

Summary

devenv shell mediates all PTY output through a libghostty VT and previously re-read every cell of every row on each frame, so rendering cost scaled with screen area. Nested full-screen TUIs (neovim, claude-code) that emit frequent updates triggered full-screen re-reads even for single-line changes, causing noticeable lag on larger terminals.

The renderer now tracks per-row dirty state via a libghostty RenderState:

• Clean rows that already have a baseline are skipped without reading their cells.
• The screen's dirty flags are consumed after each frame, so only rows changed since then are re-read.
• When the render state can't be allocated, it falls back to reading every row (and now logs that fallback so the lost optimization is observable).

Output is byte-identical; this only avoids redundant per-cell FFI reads.

Also bumps the PTY read buffer from 4 KB to 64 KB so a full-screen repaint burst feeds the VT as one frame instead of fragmenting across syscalls.

Correctness note

The optimization relies on libghostty's per-row dirty flags being a superset of all visible changes. This holds because a global Dirty::Full (palette / reverse-video / scroll-region / default-color changes) forces every per-row bit dirty in ghostty's render-state update, so reading only the per-row flags is safe. A viewport scroll marks dirtiness at the page level, which the render-state snapshot reflects (the bare per-row bit would miss it) — covered by a regression test.

Tests

Deterministic unit tests (no timing):

• A single-line repaint reads only a handful of rows.
• An unchanged frame reads no rows.
• A full-screen scroll re-reads every shifted row via the snapshot dirty flag (asserts on rows_read, so it fails if the page-level dirty path regresses — not just a substring check).

🤖 Generated with Claude Code

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View logs

[domenkozar]

I'm still not happy with this, claude scrollbar is slow

[AodhanHayter]

I took this branch for a test drive, it does feel better, but really starts to struggle as soon as I hit the bounds of a pane and some kind of scroll happens. I owe you a video as it's more clear what kind of renders it struggles with watching a session.

It's definitely not to the point that I'd willingly run nvim or any other TUI within the devenv TUI itself, there's a perfectly fine workaround with devenv --no-tui --no-reload. Although, I could see this catching new users off guard and them not understanding why their system is feeling sluggish all of the sudden. It took me a bit to narrow down devenv as the culprit myself.

I appreciate you burning some tokens for my sake. I can also live with just using devenv a bit differently than I have in the past.

[domenkozar]

I want to fix this before we get 2.2 out.