Fix FlatList initialScrollIndex behavior when short list fits viewport

[shameemkpofficial-git]

Summary:

This PR fixes a bug in VirtualizedList (and thus FlatList) where setting initialScrollIndex to a non-zero value prevents items before it from rendering if the entire list fits within the viewport.

The issue was that VirtualizedList blocks render window updates when there is a pendingScrollUpdateCount > 0. However, when the content fits the screen, a scroll event is never triggered to clear that pending update, leaving preceding items unrendered. This fix ensures that updates are only blocked if the content is large enough to scroll (contentLength > visibleLength).

Fixes #56237

Changelog:

[ANDROID] [FIXED] - Fix FlatList initialScrollIndex behavior when short list fits viewport

Test Plan:

1. Use the provided reproducer: https://snack.expo.dev/@andyhoover/flatlist-initialscrollindex-bug-2026
2. Verify that on Android, items before the initialScrollIndex (e.g., Index 0, 1) are now correctly rendered when the list is short.
3. Verified with existing unit tests: yarn jest packages/virtualized-lists/Lists/VirtualizedList-test.js (Snapshots updated).

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