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npub1223z34hd7vtwc6qj4s7flsxkj644nlre2nthu7lrrmkumhu3xddsrx9r6wandTaylor Ho e9ad63d880 fix(desktop): hold timeline scroll anchors through zoom-scaled reflow

Three scroll-anchor gaps surfaced by the 1.1 default text scale (all
app-side; no test tolerances changed):

- Prepend drift: after the older-history re-pin, prepended rows above
  the viewport realize their content-visibility intrinsic-size
  estimates a few frames later. No scroll event fires for that shrink
  and native scroll anchoring has no anchor node above the viewport, so
  the reading row drifted ~30px (scales with root font size — 1.1
  pushed it past the 28px e2e budget). The content ResizeObserver now
  re-pins mid-history message anchors (it only handled at-bottom), and
  onScroll swallows the echoes of our own anchor-holding writes instead
  of re-capturing the drifted position as the new anchor.

- Stranded bottom jump: the scroll-to-latest pill's smooth jump
  computes its animation target from metrics at call time; rows
  realizing their true heights mid-flight make that target stale, the
  engine settles ~52px above the real floor, and then emits no further
  scroll events — an event-driven settle guard can never issue the
  final corrective write. The settle guard is now a rAF loop outside
  the event stream: hands-off while the position is moving, corrective
  pin once it holds still short of the true floor, disarmed at the
  floor, on trusted user input, or a hard time cap.

- Anchor-blind system rows: SystemRow rendered no data-message-id, so
  in a young channel whose only rows are system events computeAnchor
  found nothing and fell through to at-bottom — a user who scrolled up
  never got the scroll-to-latest pill and was yanked down by the next
  arrival. System rows now carry their message id.

Co-authored-by: Taylor Ho <taylorkmho@gmail.com>
Signed-off-by: Taylor Ho <taylorkmho@gmail.com>

2026-07-02 12:20:14 -07:00