fix: follow content growth from first item measurements at the end

[chrisnojima]

Problem

A list pinned to its end drifts short after a prepend, and never recovers.

updateItemSizes asks for a re-pin from an item layout only when the row already had a known size that moved more than 5px:

// Check if we should maintain scroll at end
if (prevSizeKnown !== undefined && Math.abs(prevSizeKnown - size) > 5) {
    shouldMaintainScrollAtEnd = true;
}

Rows measured for the first time take the undefined branch, so they never request one and maintainScrollAtEnd.onItemLayout cannot fire for them.

Prepending is entirely first measurements. The sequence:

1. Data changes → onDataChange re-pins against the content size at that instant.
2. The newly inserted rows measure for the first time and grow the content underneath.
3. No re-pin follows that growth.
4. The residue ends up smaller than maintainScrollAtEndThreshold, so isWithinMaintainScrollAtEndThreshold and isAtEnd both read true and the list considers itself finished — parked a couple hundred pixels short of the last item.

Step 4 is what makes it stick: the list believes it is already at the end, so nothing later corrects it.

Evidence

Instrumented updateItemSizes and doMaintainScrollAtEnd in a running web list and ran the failing flow:

measurements:   285 total  →  first=true: 285,  first=false: 0
maintain calls: 3         →  last was the onDataChange re-pin
after it:       ~25 further first measurements moved totalSize
                scroll frozen, no maintain call
final:          gap = +57px  (pinned baseline is -116px)

Every measurement in the flow is a first measurement, so the item-layout path is unreachable end to end.

With the condition flipped, the same flow:

maintain calls: 9
trailing measurements coalesce via pendingMaintainScrollAtEnd
final:          gap = -116px   (pinned)

Fix

Treat a first measurement as maintain-worthy too. doMaintainScrollAtEnd still gates on isWithinMaintainScrollAtEndThreshold and didContainersLayout, so this only follows growth for a list already sitting at its end, and concurrent requests still coalesce through pendingMaintainScrollAtEnd.

Test

Every existing test in updateItemSize.test.ts pre-seeds sizesKnown, so none covered a first measurement — which is why this path had no coverage. The added test asserts doMaintainScrollAtEnd is called when sizesKnown has no entry for the item; it fails on main.

Verified: bun run lint, bun run tsc:src, bun test (1554 pass / 0 fail), bun run build.

Related: #518, #519 (independent 3.3.x issues).

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[jmeistrich]

Thanks for all the PRs!

[chrisnojima]

Thanks @jmeistrich for the library and all the recent work, awesome stuff