Final step of the virtualization plan. Turns the virtualized render
path on in production behind a row-count threshold, and adds regression
tests covering every gate.
- `VIRTUALIZATION_ROW_THRESHOLD = 60`. Short lists stay on the direct
render path (no wrapper, no position: absolute, no measurement
churn). Above the threshold the virtualizer takes over. Threshold is
sized so conversations under ~one session of activity don't pay the
virtualization cost; it activates once scrolling through a longer
history.
- `shouldVirtualize` now requires `renderRows.length >= threshold` in
addition to the existing opt-in and scroll-ref checks.
- `MessagesPanel` opts into virtualization for every layout it wires
(inline / sidebar / bottom-sheet). The internal threshold then
decides when to actually enable it, so callers don't need per-layout
heuristics.
- Tests: adds a new `ActivityTimeline virtualization threshold` block
covering (a) below-threshold list stays on the direct path,
(b) no viewport → direct path regardless of count, (c) above
threshold + viewport with `virtualizationEnabled` flips to the
virtualized render path (simulated by clicking "show all" past
pagination).
With this in, #70 → #74 combine to deliver:
- correct IntersectionObserver roots in scroll containers
- atomic render rows with stable keys
- windowed rendering with DOM-measured scrollMargin and measureElement
- auto-on when the cost of direct rendering actually shows up
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.
