Fix maintainVisibleContentPosition with rapid data updates (#53542)

[kulkarni-rohan]

Fixes #53542

Summary:

Problem: maintainVisibleContentPosition fails when FlatList data is updated rapidly with prepends in quick succession (e.g., chat receiving messages). After 2 prepends before native scroll drains, render window stays frozen, onViewableItemsChanged suppressed, onEndReached never fires.

Root cause: pendingScrollUpdateCount assumption is structurally unsound – it increments by 1 per prepend in getDerivedStateFromProps (0→1→2) but native scroll events are dispatched as unique events that coalesce. Verified in C++:

• ScrollViewEventEmitter.cpp:13 – onScroll dispatched with dispatchUniqueEvent("scroll", ...)
• EventQueue.cpp:29-50 – unique event whose target+type matches existing replaces in place (line 49) instead of appending
  So N scroll events emitted before queue flush deliver exactly 1 to JS. Single coalesced event drains 2→1 leaving blocked. Also leak when JS predicate fires (old key found at new index) but native declines to adjust (tag recycled, view deleted, delta ≤0.5, clamped).

Fix (1 file, 2 lines core): Clamp pending to at most 1 and drain to 0 on any scroll:

• pendingScrollUpdateCount: 1 instead of prev+1 – prevents accumulation during rapid prepends
• setState({pendingScrollUpdateCount: 0}) instead of -1 – single coalesced scroll unblocks

This turns "stuck at N" into unblocked after next scroll, fixing frozen window / viewability / onEndReached for rapid updates. For leak path where native declines (no scroll ever), flag would still be stranded at 1 – addressed by boolean rename + escape hatch in follow-up, but this PR already strictly improves and matches existing tests that drain 0→1→0.

Changelog:

[GENERAL] [FIXED] - Fix maintainVisibleContentPosition with rapid data updates (#53542)

Test Plan:

Jest (VirtualizedList):

yarn jest --watchman=false packages/virtualized-lists/Lists/__tests__/VirtualizedList-test.js --no-coverage --ci
# Before: new coalesced test fails with pending 1
# After: 83 passed (82 existing + 1 new rapid prepends with coalesced scroll), 1 skipped, 59 snapshots

New test ( regression for #53542 ):

• Simulates 2 rapid prepends WITHOUT intermediate scroll (5 + 3 items)
• Only 1 coalesced scroll event (delta 8*ITEM_HEIGHT)
• Asserts pendingScrollUpdateCount === 0 (fails on main with 1) and firstVisibleItemKey not null

Existing MVCP tests still pass:

• handles maintainVisibleContentPosition
• handles multiple rapid prepends (separate scroll per prepend)
• delta stays bounded
• minIndexForVisible >0 and inverted

Lint:

yarn lint
# Done (max-warnings 0)

Closes #53542

[meta-codesync]

@fabriziocucci has imported this pull request. If you are a Meta employee, you can view this in D115907610.