Scrolling a long thread to the end with the composer at rest landed flush
against the short composer. The expansion that followed then covered the
last rows, because the timeline reserves only the live overlay height and
does not move for footer growth.

The timeline now keeps the expanded composer's height clear while the
composer rests, so expanding it again changes nothing above the composer.
The composer reports its resting flag from a layout effect and publishes a
fresh overlay height whenever that flag changes, so the reservation is
always computed from a height that belongs to the same layout.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>