[fix] the live transcript follows the tail only when you are already at the bottom

[yui0303]

Root cause

TranscriptPanel scrolled itself to the bottom on every render pass of the transcript:

useEffect(() => {
  bottomRef.current?.scrollIntoView({ behavior: "smooth" });
}, [runs]);

runs is rebuilt from every transcript update — including interim, non-final segments — so during a meeting this fires every few seconds and always beats the user. Scrolling up to re-read something said a minute ago was impossible: the view was yanked away mid-sentence. CoachFeed had the identical unconditional pattern and the identical problem.

New behaviour

Standard follow-the-tail, like a log viewer or a chat:

• At (or within 48px of) the bottom → new content keeps pulling the view down.
• Scrolled up → the scroll position is left completely alone; new text appends below.
• Scrolling back to the bottom re-arms the follow.
• While detached, the transcript shows a small jump to latest pill (bottom-centre) that scrolls to the tail and re-arms. The coach feed gets the fix but no pill — its ask bar already sits under the feed.

How

One shared hook, src/lib/useStickToBottom.ts, used by both panes.

A scroll the hook itself starts is guarded by a ref flag, so a programmatic scroll is never mistaken for the user leaving the bottom and a smooth scroll still in flight cannot spuriously disarm the follow. The guard is released as soon as the bottom is reached, on any wheel / touchmove / keydown (the user has taken over the viewport), or after a 700ms backstop — scrollend alone is not dependable in WKWebView.

Dragging the Radix scrollbar thumb fires none of those events, and the thumb is a sibling of the viewport, so it can't be listened for there either. Since a tail-chasing scroll only ever travels toward the bottom, the handler also releases the guard when scrollTop decreases by more than 2px of jitter — that can only be a hand on the scrollbar. The rule is scoped to tail-chasing scrolls, because jump-to-timestamp legitimately travels upward and must not cancel itself.

The jump-to-timestamp effect runs through that same guard: it is neither misread as the user drifting off the tail while it travels, nor overridden by the next arriving segment, because the follow is re-derived from where the jump actually landed.

ScrollArea gains an optional viewportRef prop forwarded to ScrollAreaPrimitive.Viewport — the viewport is the element that actually scrolls, and it wasn't reachable before. The shadcn component is otherwise untouched, so it stays updatable.

The pure part, isAtBottom(metrics, threshold), is unit-tested in src/lib/useStickToBottom.test.ts: at the bottom, within the threshold, above it, non-overflowing content, and sub-pixel overshoot — alongside isUserTakeover(metrics, tolerance), which decides whether a mid-flight scroll event came from the user.

transcript.jumpToLatest added to both the zh-TW and en dictionaries.

Verification

• bunx tsc --noEmit — clean.
• bunx vitest run — 30 files, 335 tests passing.

Closes #331

🤖 Generated with Claude Code

[github-actions]

✅ SonarQube Quality Gate passed — pathorsAI_parley

0 open issues on this PR.