Keep VirtualizedList's on-screen cells when estimates above them overlap them

[mozzius]

Summary:

computeWindowedRenderLimits finds the visible cells by binary-searching estimated cell offsets, and an unmeasured cell is estimated at the current average cell length times its index. A spacer keeps the size it was rendered with, so once the average changes, the estimates for the cells in a spacer can overlap the rendered cells after it. After a large prepend at the top under maintainVisibleContentPosition, the prepended cells rendered above the spacer change the average before native's correction arrives, the corrected offset maps into the spacer, and the cells on screen are unmounted. This takes the visible range from the mounted cells' measured frames when the search disagrees with them about where the viewport starts. When estimates and measurements agree, nothing changes.

Fixes #58939. Repro: https://github.com/mozzius/virtualizedlist-average-mismatch-repro

Changelog:

[GENERAL] [FIXED] - Fix VirtualizedList unmounting the cells on screen when the estimates for a spacer above them overlap them

Test Plan:

• New test in VirtualizedList-test.js: on main the window moves to 10..12 and unmounts cells 20-24, which are on screen; with the fix they stay. All other VirtualizedList tests pass unmodified.
• yarn jest packages/virtualized-lists, yarn flow-check, yarn lint, yarn prettier.
• Android emulator, 100 rows prepended at the top, with Fix maintainVisibleContentPosition prepend skipped when batched with a cells update #58911, Anchor VirtualizedList interior spacers to the cell before them #58916 and Fix VirtualizedList unmounting the maintainVisibleContentPosition anchor before its correction #58922 also applied: the window skipped the visible row in 12 of 32 runs without the fix and 0 of 32 with it. Flings and drags behaved the same either way.

🤖 Generated with Claude Code