fix(web): prevent composer label measurement update loops

[jorda0mega]

Why the change

Scrolling can collapse the composer and trigger React error #185 because split branch names are measured differently when their labels collapse, so this keeps their full measured width stable.

Special things to note

• Reproduced the original crash in the local web client with a synthetic eight-turn thread and the page constrained to 960px; the same layout and scrolling remain stable with this fix.
• 115 focused tests pass, including expanded, intermediate, and collapsed label widths; web typecheck, targeted lint, and formatting pass.
• Shared web/desktop toolbar logic only; no provider, mobile, or wire-contract changes.

Change outline

 BranchToolbar → useLabelsOverflow → measure label text
-  maximum descendant scrollWidth
+  MiddleTruncate: sum head and tail scrollWidth
+  other descendants: keep existing maximum
   compare full width with available space
   keep compact / expanded labels stable

Before: composer collapse crashes	After: compact composer stays stable

Before recording:

before.mp4

After recording:

after.mp4

Summary by CodeRabbit

• Bug Fixes

  • Improved branch toolbar width calculations for long or middle-truncated branch names.
  • Branch labels now account for the full rendered text width, helping prevent incorrect clipping and overflow behavior.

• Tests

  • Added coverage for branch labels with varying available widths, including narrow and fully hidden text areas.

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📥 Commits

Reviewing files that changed from the base of the PR and between f25a8e4 and 83d2fe8.

📒 Files selected for processing (4)

• apps/web/src/components/BranchToolbar.logic.test.ts
• apps/web/src/components/BranchToolbar.logic.ts
• apps/web/src/components/BranchToolbar.tsx
• apps/web/src/components/ui/middle-truncate.tsx

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📝 Walkthrough

Walkthrough

The change extracts context strip label-width measurement into measureContextStripLabelWidth. It handles middle-truncated content by summing child widths. MiddleTruncate adds a identifying data attribute, and the branch toolbar uses the helper.

Changes

Context strip width measurement

Layer / File(s)	Summary
Width measurement helper and validation apps/web/src/components/BranchToolbar.logic.ts, apps/web/src/components/ui/middle-truncate.tsx, apps/web/src/components/BranchToolbar.logic.test.ts	The new helper measures maximum rendered widths and sums child widths for middle-truncated elements. Tests cover label widths of 200, 100, and 0, with an expected result of 200. MiddleTruncate exposes the data-slot="middle-truncate" attribute.
Branch toolbar integration apps/web/src/components/BranchToolbar.tsx	useLabelsOverflow now calls measureContextStripLabelWidth instead of scanning the label subtree inline.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix

Suggested reviewers: juliusmarminge, maria-rcks

Merge Risk: ⚪ Minimal · up to 83d2f

The reported scrolling measurement loop is addressed, with focused coverage and no identified regression.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 25.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 4 functions across 4 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title clearly identifies the main fix: preventing composer label measurement update loops. It matches the pull request objectives and changes.
Description check	✅ Passed	The description explains the problem, the measurement approach, validation results, and UI behavior. It includes screenshots and recordings for the UI change. It does not use the template's exact "Wha…

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[juliusmarminge]

Superseded by #13555 (merged) — same composer/BranchToolbar label measurement loop fix (sum MiddleTruncate text widths instead of max scrollWidth).