When older history is prepended while the viewport is pinned to the bottom, the
timeline controller wrote the re-pin manually (scrollTop += delta). That write
is not flagged as programmatic, so useChatAutoFollow's scroll handler treated it
as movement and issued its own correcting scroll — a redundant up/down move on
every prepend. On most setups it settles after one move, but with different
virtualizer measurement/timing it never converges, producing the reported
infinite up/down scroll glitch.
When pinned, delegate the prepend re-pin to auto-follow's goToBottom('instant'):
a single authoritative write to the bottom that IS marked programmatic, so
auto-follow ignores it instead of fighting it. The released case (user reading
back through history) is unchanged and still preserves the read position.
This also covers the on-open history auto-load (loadEarlierIfPinnedViewport-
Underfilled), which only runs while pinned, so its prepends now go through the
single writer too.
Bohdan Triapitsyn
8c551c40aa