fix(web): stop mid-turn timeline overlap on streaming tool groups

[macodev00]

What

• Keep streaming tool-group / live work rows from overlapping mid-turn by measuring expandable agent-spawn work-live rows (unpinned fixed size) and remasuring when spawn expand changes.

Why

Fixes #12094

Redo of closed #12734 addressing Macroscope Not-approved / Medium: mechanical LegendList sizing + expandedSpawnEntryIds in list extraData; avoid oversized layout policy rewrite.

UI

Desktop chat timeline: live agent-spawn expand should not overlap the following Working row.

Checklist

• I read CONTRIBUTING.md and kept this small / bug-only
• What + why explained above
• Followed AGENTS.md documentation rules (no new docs)
• No unrelated changes
• CLA / AI-reviewed standing override for macodev00 auto-contrib

Summary by CodeRabbit

• Bug Fixes
  • Improved chat timeline behavior when new user messages appear during an active turn.
  • Stabilized live activity group placement and identity across timeline updates.
  • Improved list refresh and row measurement when live groups or expanded agent activity change.
  • Preserved accurate sizing for working, live activity, user, and expanded detail rows.

[macroscopeapp]

Approvability

Verdict: Approved at a58096c

Macroscope's review found this PR approvable — The PR makes a localized fix to existing web timeline virtualization: it preserves live-row identity and remasures variable-height streaming groups to prevent overlap. It adds focused regression tests without changing product defaults, APIs, security-sensitive code, or static-analysis settings.

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Repository: pingdotgg/t3code/.coderabbit.yaml

Review profile: CHILL

Plan: Advanced

Run ID: 7436088e-d1ec-4a9b-ab43-12660be25d88

📥 Commits

Reviewing files that changed from the base of the PR and between 1de563c and a58096c.

📒 Files selected for processing (3)

• apps/web/src/components/chat/MessagesTimeline.logic.test.ts
• apps/web/src/components/chat/MessagesTimeline.logic.ts
• apps/web/src/components/chat/MessagesTimeline.tsx

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📝 Walkthrough

Walkthrough

The timeline keeps live activity rows stable across mid-turn steers. LegendList now uses row-specific fixed sizes and height signatures that include expanded activity and spawn content.

Changes

Timeline overlap and row sizing

Layer / File(s)	Summary
Live activity stability apps/web/src/components/chat/MessagesTimeline.logic.ts	Activity groups remain active when only user messages follow. Active live rows receive a stable identity.
Row sizing and height signatures apps/web/src/components/chat/MessagesTimeline.logic.ts, apps/web/src/components/chat/MessagesTimeline.tsx	The timeline adds fixed heights for eligible rows. Expanded activity and agent-spawn rows remain measured. LegendList extra data now tracks expanded content and spawn expansion state.
Timeline regression coverage apps/web/src/components/chat/MessagesTimeline.logic.test.ts	Tests cover stable live-row identity, expanded-group changes, fixed-size selection, and spawn expansion updates.

Priority: ⬇️ Low

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix · Severity of issue fixed: Low

Suggested reviewers: maria-rcks

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 38.46% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 13 functions across 3 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly identifies the web fix and the specific mid-turn timeline overlap problem during streaming tool groups.
Description check	✅ Passed	The description explains what changed, why the change is needed, the affected UI behavior, and the related issue. It is focused and provides checklist details. It does not include screenshots or a vid…
Linked Issues check	✅ Passed	The changes satisfy the coding requirements in [#12094] and [#12734]. assignLiveActivityRowId preserves the live row id across a mid-turn user message. remainderIsOnlyUserMessages keeps the live a…
Out of Scope Changes check	✅ Passed	The reviewed changes are limited to desktop chat timeline placement and measurement. They modify timeline derivation, LegendList sizing and refresh inputs, and focused timeline logic tests. The change…

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