Opus advisor on stage-302 (#1732 verification Q5) flagged that
_lastScrollTop is module-global and persists across chat switches. When
the user switches sessions, the new chat's first user scroll compares
against the previous chat's last scrollTop. If the previous was deep-
scrolled (e.g. 5000) and the new chat starts at top=0, scrolling down
to 100 would evaluate as movedUp=true → false-unpin, blocking auto-
scroll on the new chat's first incoming token.
Fix: expose _resetScrollDirectionTracker() from static/ui.js on window
so static/sessions.js loadSession() can reset _lastScrollTop=null when
S.session is reassigned. The scroll listener's existing _lastScrollTop!==null
guard then handles the first sample after reset correctly (no false-trigger
on the very first scroll event in the new chat).
Absorb-in-release per Opus stage-302 verdict — small, defensive, ≤20 LOC.
93f30ecfda
