[chrispader]

ScrollAdjust places its anchor view 10,000,000 points down, so native maintainVisibleContentPosition anchors to it. Native layout stores positions as 32-bit floats, and at that size a float only holds whole points. Each scroll adjustment was rounded by up to half a point, while the items above the viewport moved by the exact amount, so the visible content shifted by the difference, about 1 to 2 px on Android. With a bias of 1,000,000, the anchor's position stays precise to 1/16 of a point.

// An item above the viewport measures 131.9 points smaller than estimated.
Math.fround(10_000_000 - 131.9); // 9999868: the anchor moves by 132 points
Math.fround(1_000_000 - 131.9); // 999868.125: the anchor moves by 131.875 points

The bias also sets how far the list can scroll, because native MVCP only anchors to the view while it sits below the scroll offset. Each adjustment moves the anchor and the scroll offset together, so prepended content doesn't use up the range. Only content that is scrolled through without an adjustment does, such as the initial data and appended items. 1,000,000 points is about 10,000 rows of 100 points. If you'd rather trade some precision for range, 2,097,152 doubles it and still rounds to a quarter point.

We hit this in Expensify/App#100733, which moves the chat message list to LegendList. On Android, a chat that opens at a linked message moved up by 1 to 2 px when the rows above it were measured. Expensify currently carries this fix as a patch.

Reproduction

1. On Android, render a list whose items render smaller than estimatedItemSize, with maintainVisibleContentPosition enabled.
2. Scroll to the middle of the list.
3. Wait until the items above the viewport are measured.
4. Verify the items on screen don't move.

🤖 Generated with Claude Code

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