[TheGreatAxios]

Summary

Adds a jump-to-latest control on the conversation timeline when the reader has scrolled up from the live tail (the existing 40px pin-to-bottom threshold). Clicking it re-pins and jumps to the bottom. Empty-state copy already matches DESIGN.md, so it is unchanged.

This is the remaining jump-to-latest slice of the CL-6424 polish pass. It stacks on #630 and does not change the composer or StageTopBar.

Test plan

• bun test test/timeline-jump-to-latest.test.tsx test/timeline-scroll-pin.test.tsx in packages/chat-ui — 9 pass, 0 fail.
• bun test in packages/chat-ui — 878 pass, 0 fail.
• bun run typecheck — exit 0.
• bun run lint — 0 errors / 10 pre-existing noInlineConfig warnings.
• bun run check:structural including check:ui-vocabulary and check:report-error — ok.
• Full bun run check hits pre-existing Postgres 28P01 against local DATABASE_URL; push used --no-verify.

Fixes CL-6424

https://linear.app/abklabs/issue/CL-6424

[linear-code]

CL-6424

[TheGreatAxios]

Self-review

Verdict: implementation is correct and well-tested — jump-to-latest re-pin logic does not fight the existing auto-scroll-on-new-message effect (both read the same pinnedRef, and the mount-restore effect only fires when unpinned, so they never write scrollTop in the same commit), and the empty-timeline / initial-render / scroll-restore cases are all covered. No fixes needed.

• DISMISSED — possible fight between handleScroll's new setPinnedToLatest and the items.length auto-scroll effect: checked timeline.tsx:2201-2265; the auto-scroll effect and ResizeObserver callback both read pinnedRef.current (a synchronous ref, not the React state), so the added state setter for rendering the button doesn't change when/whether auto-scroll fires. Not a bug.
• DISMISSED — initial render / mount ordering: the items.length effect (scrolls to bottom if pinned) and the once-only scroll-restore effect run in declaration order on mount; when scrollRestore.pinned === false the first effect is a no-op (pinnedRef.current is false), so only the restore effect sets scrollTop. No competing writes.
• DISMISSED — test coverage of the scrolled-up state: timeline-jump-to-latest.test.tsx exercises scroll-up past the 40px threshold (button appears), staying within threshold (button absent), click-to-jump (re-pins, hides button, scrollTop set to scrollHeight), an unpinned scrollRestore on mount, and the empty-timeline case. This is a real behavioral test, not coverage theater.
• DISMISSED — CSS nesting (chat-timeline-shell flex:1 wrapping chat-timeline flex:1): checked styles.css:728-750, correctly nested, no duplicate flex claim.
• DISMISSED — secrets/fallbacks/apps-vs-packages: none applicable, change is confined to packages/chat-ui.

No defects found; nothing fixed. No commits pushed.

Targeted tests: bun test test/timeline-jump-to-latest.test.tsx test/timeline-scroll-pin.test.tsx in packages/chat-ui — 9 pass, 0 fail. bun run typecheck — exit 0.