The thread feed reset following and its scroll session when the thread
changed or a message was sent. The effects listed the feed's own
handlers as dependencies, and those change whenever the parent passes a
new follow callback, so any such re-render reset them too. A reader
scrolled up through a thread could find following switched back on, and
a fold's width reflow lost its place partway through.

The resets now run as an effect event, triggered only by the thread
key and the submitted message.

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