Release auto-follow based on position (the user has left the near-bottom zone)
instead of scroll-delta direction. The old `currentTop < previousTop` check
treated the tiny scrollTop clamp the browser applies when the composer grows —
which keeps you at the bottom — as a user scroll-up and released follow, so
content finishing loading then drifted the view backward.
Also always return to the bottom on session switch, dropping the saved-ratio
restore: it had a low success rate and, by landing 'released' partway up,
produced the same visible backward jump as content finished loading.
overflow-anchor is already disabled on the chat scroll container, so no
delta-threshold workaround is needed; this is a net simplification.
Bohdan Triapitsyn
f2d4a7833d