Fix null check crash in MultiscriptsLayoutDelegate intrinsic layout

[passte-llc]

Summary

Fixes #1.

MultiscriptsLayoutDelegate.performHorizontalIntrinsicLayout and
performVerticalIntrinsicLayout (lib/src/render/layout/multiscripts.dart)
use a non-null assertion (childrenWidths[_ScriptPos.base]!,
childrenHeights[_ScriptPos.base]!, childrenBaselines[_ScriptPos.base]!)
on map lookups that are not guaranteed to succeed during intrinsic-size
layout probing. When the base script's size/baseline hasn't been recorded
in the map yet at the time intrinsic layout runs, the assertion throws
Null check operator used on a null value (_CastError) inside
performLayout(). Because Flutter's rendering pipeline does not catch
layout exceptions, this crashes the entire widget subtree and freezes the
UI — there is no graceful degradation.

Reproduction

We hit this consistently in production with expressions that mix subscript
characters inside \mathrm{}, e.g.:

\mathrm{CL_r = CL_{tot} \times f_e}

rendered via Math.tex(...) inside a layout that triggers intrinsic width
computation (e.g. content that may need to wrap). Issue #1 reports the same
stack trace (multiscripts.dart:136) triggered via \displaystyle{...}
inside a RichText-hosted Math.tex widget, with a workaround of removing
displaystyle — but the underlying null-check panic in
MultiscriptsLayoutDelegate was never fixed upstream.

Fix

Replace the three non-null assertions on _ScriptPos.base lookups with a
?? 0.0 fallback:

• performHorizontalIntrinsicLayout: childrenWidths[_ScriptPos.base] ?? 0.0
• performVerticalIntrinsicLayout: childrenHeights[_ScriptPos.base] ?? 0.0
  and childrenBaselines[_ScriptPos.base] ?? 0.0

When the base entry is present (the expected/common case), behavior is
byte-for-byte identical to before — ?? 0.0 is a no-op unless the map
lookup is null. When the base entry is absent (the previously-crashing
case), layout now degrades gracefully instead of throwing: the widget
continues to render (with a fallback intrinsic size of 0.0 for the
missing entry) instead of freezing the whole app.

Backward compatibility

• No public API / signature changes.
• No behavior change in the non-crashing path.
• Only affects intrinsic-size probing; final paint layout is untouched.

Testing

• dart analyze lib/src/render/layout/multiscripts.dart: no issues.
• Verified against the exact expression from our production repro
  (\mathrm{CL_r = CL_{tot} \times f_e}) rendered inside a layout that
  exercises intrinsic width computation — previously threw
  Null check operator used on a null value synchronously on
  performLayout(), now renders without throwing. We have been shipping
  this exact 3-line patch in a production Flutter app (iOS, App Store)
  since 2026-05 with no regressions in math rendering.

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[coderabbitai]

📝 Walkthrough

Walkthrough

This change modifies MultiscriptsLayoutDelegate in multiscripts.dart to prevent a null check crash during intrinsic layout. Force-unwrapped map lookups for _ScriptPos.base in horizontal and vertical intrinsic layout methods are replaced with null-coalescing fallbacks defaulting to 0.0.

Changes

Intrinsic Layout Null-Safety Fix

Layer / File(s)	Summary
Fallback for missing base script position lib/src/render/layout/multiscripts.dart	Replaces force-unwrapped childrenWidths[_ScriptPos.base], childrenHeights[_ScriptPos.base], and childrenBaselines[_ScriptPos.base] lookups with ?? 0.0 fallbacks in performHorizontalIntrinsicLayout and performVerticalIntrinsicLayout, adding explanatory comments to avoid a null check crash during intrinsic-size probing.

Estimated code review effort: 1 (Trivial) | ~5 minutes

Related issues: Fixes a _CastError/null check operator crash ("Null check operator used on a null value") thrown during performLayout() in MultiscriptsLayoutDelegate.performHorizontalIntrinsicLayout when intrinsic layout is triggered (e.g., via RichText).

Suggested labels: bug, fix

Suggested reviewers: znjameswu

🐰 A tiny hop through code so tight,
No more null checks crashing in the night,
Base scripts missing? Zero will do,
Intrinsic layouts now see it through!

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title accurately summarizes the main change: preventing a null-check crash in MultiscriptsLayoutDelegate intrinsic layout.
Description check	✅ Passed	The description directly matches the code change and explains the crash fix and its impact.
Linked Issues check	✅ Passed	The patch addresses issue #1 by replacing the failing base-script null assertions in intrinsic layout with safe fallbacks.
Out of Scope Changes check	✅ Passed	The changes stay confined to the reported crash fix in multiscripts.dart and add no unrelated behavior or API changes.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[coderabbitai]

🧹 Nitpick comments (1)

lib/src/render/layout/multiscripts.dart (1)

136-142: 📐 Maintainability & Code Quality | 🔵 Trivial | ⚡ Quick win

Consider adding a regression test.

This fixes a reported crash (Null check operator used on a null value at Line 136). Since base is a required, always-present child in Multiscripts's widget tree, the missing map entry is specific to CustomLayout's intrinsic-probing pass — worth a targeted golden/unit test (e.g. an intrinsic-width query on a nested subscript/superscript expression) to guard against regressing back to the unsafe ! lookup.

Also applies to: 184-192

🤖 Prompt for AI Agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

In `@lib/src/render/layout/multiscripts.dart` around lines 136 - 142, Add a
regression test for Multiscripts to cover the intrinsic-probing path that can
omit _ScriptPos.base from childrenWidths. Use the Multiscripts/CustomLayout flow
exercised by nested subscript/superscript expressions and assert an
intrinsic-width query no longer throws the null check exception. Place the test
alongside the Multiscripts layout logic so it protects the baseSize lookup from
regressing back to an unsafe ! access.

🤖 Prompt for all review comments with AI agents

Verify each finding against current code. Fix only still-valid issues, skip the
rest with a brief reason, keep changes minimal, and validate.

Nitpick comments:
In `@lib/src/render/layout/multiscripts.dart`:
- Around line 136-142: Add a regression test for Multiscripts to cover the
intrinsic-probing path that can omit _ScriptPos.base from childrenWidths. Use
the Multiscripts/CustomLayout flow exercised by nested subscript/superscript
expressions and assert an intrinsic-width query no longer throws the null check
exception. Place the test alongside the Multiscripts layout logic so it protects
the baseSize lookup from regressing back to an unsafe ! access.

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 6539cd66-1895-411e-b696-b9f8e933a0bc

📥 Commits

Reviewing files that changed from the base of the PR and between 75a6f61 and 036bb03.

📒 Files selected for processing (1)

• lib/src/render/layout/multiscripts.dart