Ports main's anchor-release semantics onto the branch's timeline anchor
state: the scroll-to-end pill and a manual scroll back to the live edge
both drop the send-time anchored end space before re-enabling follow, and
the pill's scroll runs a frame later so the list measures without the
anchor space and lands on the true end. Without this the timeline could
settle into following-end with the anchor still installed — following
nothing.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>