The transcript force-scrolled to the bottom on every streamed delta because
the auto-scroll effect ran unconditionally whenever the messages array identity
changed. Scrolling up to read earlier messages was impossible — each token
yanked the view back down.
Implement a "stick to bottom" pattern in MessageList:
- track whether the viewport is pinned to the bottom via a scroll listener
(pinnedToBottomRef, BOTTOM_THRESHOLD = 40px);
- only auto-scroll while pinned; a freshly sent user message always re-pins;
- attach the scroll listener via a [hasScrollArea] dependency so a brand-new
empty chat (whose ScrollArea mounts only after the first message) wires it up;
- guard the effect's own scrollTop write (programmaticScrollRef) so it is not
misread as a user scroll.
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