The list was pinned per streaming delta, but the last things to land arrive
after that final pin: the stats/rating footer, which renders only once
msg.done is set, and the copy buttons attached to code blocks after render.
Nothing re-clamped afterwards — the length watcher never fires (the message
stays in the array) and the landing loop only runs when a session opens — so
the view was left short of the bottom, looking like it had scrolled up.

Re-clamp through the same settle window the landing uses when streaming goes
true -> false, unless the user has scrolled up or a session is loading.
This addresses the late-layout half of the problem; the row is still remounted
when msg.id becomes h_<n>, which is the other source of a jump.