fix(mobile): stabilize scroll-to-end control

[Kapish14]

Problem

Follow-up to #5986, which introduced the mobile scroll-to-end button.

The mobile scroll-to-end button could briefly appear while dragging at the actual end of a thread because its visibility followed the live-follow latch alone.

The button also overlapped messages and alerts shown above the composer, making both elements harder to see and use.

Fix

Track whether the feed is actually at the end separately from whether live follow is enabled. The button now appears only when live follow is paused and the feed is away from the end.

Raise the button slightly and keep it in a fixed position above composer messages and alerts so the elements no longer overlap.

Validation

• vp test run apps/mobile/src/features/threads/thread-feed-live-follow.test.ts
• vp run --filter @t3tools/mobile typecheck
• Targeted formatting and lint checks
• Standalone Android Preview APK built and tested on an OPPO CPH2707

Before

WhatsApp.Video.2026-08-12.at.03.50.35.mp4

After

WhatsApp.Video.2026-08-12.at.03.50.41.mp4

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Note

Low Risk
UI-only scroll affordance and positioning tweaks in the mobile thread feed, with no auth, data, or backend impact.

Overview
Prevents the mobile scroll-to-end button from briefly appearing when a drag starts while already at the live edge.

ThreadFeed now reports actual at-end state separately from the live-follow latch. The button shows only when follow is paused and the feed is away from the end, with thread-switch state reset via resolveThreadFeedAtEndState.

Also raises the floating control (-top-11 → -top-24) so it no longer overlaps composer messages and alerts.

^{Reviewed by Cursor Bugbot for commit 904a5b6. Bugbot is set up for automated code reviews on this repo. Configure here.}

Note

Fix scroll-to-end button visibility in thread feed to hide when feed is already at end

• Adds shouldShowThreadFeedScrollToEnd to gate button visibility on both live follow being paused and the feed not being at the end — previously the button could appear even when already at the end.
• Tracks per-thread at-end state in ThreadDetailScreen via a new feedAtEndState keyed by thread key, reset to true on thread switch or anchor-based jumps.
• Exposes an onAtEndChange callback on ThreadFeed to propagate scroll position changes to the parent.
• Moves the scroll-to-end button container position from -top-11 to -top-24, rendering it higher above the composer.

^{Macroscope summarized 904a5b6.}

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[cursor]

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

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^{Reviewed by Cursor Bugbot for commit b69fef9. Configure here.}

[macroscopeapp]

Approvability

Verdict: Approved 904a5b6

Straightforward UI behavior fix that stabilizes scroll-to-end button visibility using simple pure functions with comprehensive unit tests. Limited scope, self-contained, no runtime risk beyond the UI control it fixes.

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