PR #1669 added DOM virtualization to renderSessionListFromCache() with two issues
for lists below the virtualization threshold (≤80 rows):
1. The unconditional scroll listener triggered renderSessionListFromCache() on
every rAF, rebuilding the entire list DOM on every scroll event.
2. After each rebuild, scrollTop was only restored when virtualWindow.virtualized
was true (i.e. total > 80). For lists ≤ 80 rows, scrollTop dropped to 0 on
every scroll event, producing a 'scroll keeps jumping back' feel.
Fix:
- Always restore scrollTop after re-render when listScrollTopBeforeRender > 0
(regardless of virtualized flag).
- Short-circuit _scheduleSessionVirtualizedRender when total <=
SESSION_VIRTUAL_THRESHOLD_ROWS (saves wasteful rebuild on small lists).
Live verified on a 56-session sidebar: scrollTop holds across animation frames.
3 regression tests pin the fix shape.
4e9ec6f191
