fix(web): keep the last message visible when the resting composer expands

[t3dotgg]

When the composer was at rest and I scrolled a long thread to the bottom, the page was slightly shorter than it is with the composer expanded. Reaching the end expands the composer, and the expansion covered the last message.

The cause: the timeline reserves space at its end equal to the live composer overlay height. A resting composer is about 94px shorter, so the scroll range shrank with it. The timeline deliberately does not move for footer growth, so the expansion had nothing to push against and covered the last rows.

The fix keeps the reserved space constant. While the composer rests, the timeline holds the expanded height (or the resting height plus the empty-composer delta when no expanded height is known yet). The visible composer still shrinks. Only the reserved space above it stays the same, so expanding again changes nothing behind it. The composer reports its overlay height together with the resting state so the two never get paired with the wrong layout, and switching threads rebuilds the reservation from that thread's overlay.

The scroll-to-end pill and the preview mini player still track the live overlay height. Mobile and the collapse-off settings are unchanged.

📹 screen recording

Created with Claude Fable 5.1 in Claude Code.

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Note

Low Risk
Localized chat scroll and composer layout behavior; scroll-to-end pill and mini player still use live overlay height.

Overview
Fixes a scroll bug where the resting composer was shorter than the expanded one, so the timeline reserved too little space at the bottom and expanding after scrolling to the end covered the last message.

ChatView now drives timeline end padding, anchor metrics, and overflow checks from a composerTimelineInset instead of the live overlay height. While the composer is expanded, the inset matches the measured overlay; while it rests, it stays at the last expanded reservation (or resting height plus a 94px minimum expansion allowance via new resolveComposerTimelineInset in composerFooterLayout.ts). Switching threads resets and remeasures that reservation so another thread’s tall draft does not leak padding.

ChatComposer reports onRestingChange from a layout effect (before the parent measures) and republishes overlay height when resting changes without a collapse transition; resting transition code publishes destination overlay height earlier in the layout pass.

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Note

Keep last message visible when resting composer expands in ChatViewContent

• The chat timeline now reserves resolved expanded-composer space at its end, using the larger known reservation or the minimum expansion allowance while the composer rests; when expanded, the current overlay height becomes authoritative
• Adds a resting-state ref and callback passed into ChatComposer, and a layout effect that rebuilds the reservation when the active thread or overlay element changes in ChatView.tsx
• End anchoring and overflow calculations in MessagesTimeline now use the resolved timeline inset instead of the raw overlay height

^{Macroscope summarized 166dc41.}

[github-actions]

Thread transfer impact

✅ Thread transfer remains within every enforced ceiling.

Provider	Metric	Main baseline	This PR	Impact	PR ceiling
Codex	Total thread wire	13.6 KiB	13.6 KiB	+34 B (+0.2%)	15.1 KiB	✅
Codex	Thread snapshot wire	7.0 KiB	7.0 KiB	0 B (0.0%)	7.3 KiB	✅
Codex	Live turn WebSocket wire	6.5 KiB	6.6 KiB	+34 B (+0.5%)	7.8 KiB	✅
Codex	Live turn WebSocket decoded	57.0 KiB	57.1 KiB	+44 B (+0.1%)	66.4 KiB	✅
Codex	Live turn messages	9	10	+1 (+11.1%)	21	✅
Claude	Total thread wire	13.6 KiB	13.6 KiB	−13 B (−0.1%)	15.1 KiB	✅
Claude	Thread snapshot wire	7.0 KiB	7.0 KiB	+5 B (+0.1%)	7.3 KiB	✅
Claude	Live turn WebSocket wire	6.6 KiB	6.5 KiB	−18 B (−0.3%)	7.8 KiB	✅
Claude	Live turn WebSocket decoded	57.9 KiB	57.8 KiB	−44 B (−0.1%)	66.4 KiB	✅
Claude	Live turn messages	10	9	−1 (−10.0%)	21	✅

Baseline: 617edab · PR result: 166dc41 · Source CI: success

Scenario and decoded snapshot size

10 historical turns, 5 command tools per turn, 878.9 KiB retained MCP result per historical turn, and a 1.05 MiB retained result in the measured turn.

• Codex decoded thread snapshot: 113.8 KiB
• Claude decoded thread snapshot: 114.5 KiB

Updated in place by a trusted workflow. PR artifacts are strictly validated and never executed.

[cursor]

Cursor Bugbot has reviewed your changes using default effort and found 1 potential issue.

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^{Reviewed by Cursor Bugbot for commit 6d6c567. Configure here.}

[macroscopeapp]

Approvability

Verdict: Approved at 166dc41

Macroscope's review found this PR approvable — This is a localized web UI bug fix that keeps the timeline’s expanded-composer space reserved while the composer rests, while preserving live geometry for the mini-player and scroll controls. The added state coordination is confined to existing composer/timeline layout paths and includes focused unit coverage.

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[shivamhwp]

ty. was just about to make this pr.