Codex regression gate caught: the guard predicate keyed on a raw messagesEl.scrollTop>0,
so a pinned reader transiently at bottomDistance>250 mid-render (assistant body grows
before the anchor scene re-renders) got flipped to _messageUserUnpinned=true/_scrollPinned=false
and lost auto-follow. Require an explicit non-follow signal (_messageUserUnpinned || _scrollPinned===false)
instead. Updated the existing guard test to a genuine unpinned reader + added a pinned-large-growth
regression. Co-authored-by: franksong2702
franksong2702: preserve the reader's semantic viewport anchor during mid-stream transcript rebuilds
before raw scrollTop fallback; remount virtualized anchors before restore w/ _programmaticScroll
scoped to the remount frame; treat captured userUnpinned/pinned snapshot as authoritative so
distance-inference can't re-pin a reader who scrolled away during a live reply. Browser-only scroll
state. Co-authored-by: franksong2702 <franksong2702@users.noreply.github.com>
