Fifth step of the virtualization plan. Two small, coupled changes that
make the virtualized path stable without a merged-ref helper.
- Attach `rowVirtualizer.measureElement` to the existing virtualizer
wrapper div. Because the wrapper carries no padding or margin, its
bounding box matches the inner row, so the observer ref (which stays
on the inner AnimatedHeightReveal node) and the measure ref (on the
outer wrapper) address the same effective height. No merged ref
callback is needed.
- Suppress mount-based entry animation inside the virtualized path.
The virtualizer mounts and unmounts rows as the user scrolls them in
and out; without this, the "new item" fade would replay every time
an older row re-entered the viewport. `renderTimelineRow` now takes
an optional `suppressEntryAnimation` flag and forwards `isNew=false`
to both `LeadThoughtsGroupRow` and `MemoizedMessageRowWithObserver`
when set. The direct render path is unchanged.
Still dormant in this release — `viewport.virtualizationEnabled` stays
false at every call site. PR #6 adds the threshold gate, tests, and
opt-in wiring.
b9c2dd5480
