Three separate causes, all measurable with a MutationObserver over a turn.
The reply vanished and came back. On completion the code cleared
`streaming` and only then wrote the turn, trimmed the transcript and read
it back -- three awaits during which the message was unmounted, the typing
dots took its place, and the log jumped by the message's height. The write
now happens first and the placeholder is swapped for the committed turn in
one render.
The dots blinked again underneath the finished reply, because `busy`
stayed set until the finally block: one render with the turn committed and
the pending slot still mounted with `streaming` back to null. All three
state changes are now in the same batch.
The log shook while streaming. scrollIntoView() scrolls every scrollable
ancestor, so each token nudged the page as well as the log, and it fired
unconditionally, so scrolling up to re-read something was undone by the
next token. It now writes scrollTop on the log alone, once per animation
frame, and only when the reader is already within 80px of the bottom.
Also: the pending bubble is one keyed node rather than two sibling
conditionals, so waiting-to-streaming no longer remounts; and the
transcript is parsed once per change instead of on every render -- every
keystroke in the composer was re-parsing up to 26 messages of blocks.
Measured before: 1 bubble-count drop, 2 log-height shrinks, page scrolled
per token. After: 0, 0, and the page never moves.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
c93ea657f9