Codex found that with Auto-follow OFF the #3920 ResizeObserver settle + 2s fallback
+ the non-preserve render path still force-scrolled to bottom (the setting was
incomplete vs the scroll machinery shipped in #3920). Fixes:
1. RO settle guard (ui.js:3153): bail when !_autoScrollFollow.
2. 2s static fallback guard (ui.js:3186): bail when !_autoScrollFollow.
3. non-preserve render fall-through (ui.js:8653): when Auto-follow off AND the user
has unpinned, restore the scroll snapshot instead of scrollToBottom() — also
closes the send() race (renderMessages runs before S.activeStreamId is set).
A fresh session load (not unpinned) still lands at the bottom. Explicit
user-initiated scrollToBottom() (End button) is untouched.
2bdef4166f
