Keep Chat anchored as rows grow past the pane

The transcript stack could shrink to the pane height, so the observer never
saw rows grow once the transcript overflowed. Growth now follows only a
reader resting on the last line, and rows that arrived behind a hidden Chat
do not fade in when it opens. A browser test covers both reading positions.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>