[Bug]: Excessive transient blank space above composer at end of long conversations #4619

https://github.com/pingdotgg/t3code/issues/4619### Before submitting

• I searched existing issues and did not find a duplicate.
• I included enough detail to reproduce or investigate the problem.

Area

apps/web

Steps to reproduce

1. Open a long conversation whose timeline is substantially taller than the viewport and genuinely scrollable.
2. Scroll to the absolute end of the conversation (this may also occur when live-follow reaches the end after a turn finishes).
3. Observe the distance between the bottom of the final message and the docked composer.
4. Leave the view open. The excessive gap may later disappear after a subsequent layout/scroll update, so the issue is inconsistent and timing-sensitive.

Expected behavior

At the end of a long scrollable conversation, the final message should sit above the composer with only the normal intentional bottom inset. Large empty space can be reasonable when the whole conversation is shorter than the viewport, but not when the timeline has substantial scrollable content above it and is positioned at its end.

Actual behavior

The final message can be stranded hundreds of pixels above the composer, leaving a large blank region—sometimes close to most of the remaining viewport—even though the user is at the end of a long scrollable thread. The extra space is transient/inconsistent and later collapses on its own.

This makes the end of the conversation look as though content is missing or the scroll position is incorrect.

Impact

Minor bug or occasional failure

Version or commit

Current T3 Code build; exact version unknown.

Environment

T3 Code desktop-width chat surface, long Codex-backed conversation.

Logs or stack traces

No relevant logs observed; this appears to be a client-side layout/virtualization issue.

Screenshots, recordings, or supporting files

A screenshot shows the last assistant paragraph near the upper portion of the viewport, the composer docked at the bottom, and an abnormally large blank region between them while at the end of a long thread.

Workaround

Waiting for a later layout/scroll update eventually causes the gap to disappear.

Investigation lead

The docked composer is an overlay whose measured height is passed to MessagesTimeline as LegendList's contentInsetEndAdjustment (apps/web/src/components/ChatView.tsx). Because the gap later self-corrects, this may involve a stale composer measurement or an end-inset / scroll-position recalculation in the virtualized list. This is only a hypothesis, not a confirmed root cause.