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npub1223z34hd7vtwc6qj4s7flsxkj644nlre2nthu7lrrmkumhu3xddsrx9r6wandTaylor Ho d39fb399f0 fix(timeline): gate first-load bottom pin until scroll margin is measured

Kills the first-load out-of-place flash. On `isLoading: true→false` in
one commit, two sibling layout effects race: the init bottom pin
(scrollToBottom → scrollToIndex align:end) and the scrollMargin
re-measure (which only mounts once the list outer ref mounts). The pin
can fire against a STALE `scrollMargin: 0`, landing scrollMargin px short
of true bottom — rows paint at translateY(start - 0) while the list
actually sits scrollMargin lower. That's the flash. The margin then lands
and the settle pin re-anchors, catching up.

Fix: gate the init pin on a measured margin.
- useVirtualScrollMargin now returns { value, measured }. `measured` flips
  true only after a real measurement against a mounted list — a `> 0`
  test won't do, since a legitimate margin can be 0.
- The init pin bails on `isLoading || !scrollMarginReady` WITHOUT marking
  hasInitialized, so it re-fires once the margin lands and pins against a
  trustworthy offset. scrollMarginReady added to the effect deps.
- MessageTimeline threads scrollMargin.value into the virtualizer + list
  prop and scrollMargin.measured into the scroll hook.

Adds a DOM regression test: 0 end-pins while the margin is unmeasured,
exactly 1 once it lands.

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

2026-06-15 22:47:48 -07:00