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fix(virtual-core): keep a full range when the scroll offset is past the end of the list - #1351
calculateRange hands the scroll offset to calculateRangeImpl as it is. When the offset is past the start of the last row, the range is that row alone. That happens while the virtualizer holds an offset the element no longer has: the list shrank (filtering, rows re-measured smaller), the element clamped its scrollTop, and the scroll event that reports it hasn't arrived yet. Until it does, the viewport shows the last row and blank space above it.
50 rows of 30px in a 300px viewport, scrolled to the end, then count drops to 20:
constoptions={estimateSize: ()=>30,overscan: 0,initialRect: {width: 300,height: 300},initialOffset: 1200,getScrollElement: ()=>null,scrollToFn: vi.fn(),observeElementRect: vi.fn(),observeElementOffset: vi.fn(),}constv=newVirtualizer({ ...options,count: 50})v.setOptions({ ...options,count: 20})v.getVirtualIndexes()// [19] on main, [10, ..., 19] with this change
The change caps the offset at the last position the list can be scrolled to before computing the range. The cap has to be in the offset's own space. scrollOffset is measured in the scroll element and in layout coordinates, while getTotalSize() starts at the list and is shifted by a held correction, so the cap is _getLayoutTotalSize() + scrollMargin - outerSize. #1340 fixed the same mix of spaces in getVirtualDistanceFromEnd; here there was no cap at all. A cap against getTotalSize() alone would pin any list whose scrollMargin is larger than its own height to its first rows: I've carried this fix as a patch in OpenChamber, and its first version did exactly that to sections lower down in a shared scroller. The second test covers it. The list end is in the memo deps, so a change of paddingEnd or of the hold recomputes the range.
The cap only moves the offset down, so the new range is the old one plus rows above it, never fewer. For a list followed by other content in the same scroller, scrolled below the list, this renders a viewport's worth of rows off screen where main renders one.
Snapshot.scroll-trace moves in 29 configurations, 197 of 1800 traces. In each, the first divergence is a render while the element is past an edge (a drag that rubber-bands, a settle from a bounce): more rows are rendered and measured there, and later steps follow from those measurements. Across all traces, settled frames whose first rendered row starts below the element's position go from 22 to 12.
Tests. Two cases in range.test.ts: the shrink above, without and with scrollMargin: 1000. Both get [19] on main and pass with the change.
I have tested this code locally with pnpm run test:pr.
I ran the test:pr targets except test:e2e: test:lib, test:types, test:eslint and build for all 10 packages, test:build for virtual-core, and test:sherif, test:knip, test:docs at the root. I didn't run the Playwright e2e.
🚀 Release Impact
This change affects published code, and I have generated a changeset.
This change is docs/CI/dev-only (no release).
Summary by CodeRabbit
Bug Fixes
Fixed visible-range calculations when a list shrinks while scrolled to the end. The displayed rows now update to the new end of the list instead of temporarily retaining an outdated range.
This correction applies both with and without a scroll margin, including while the scroll element is reporting its adjusted position.
calculateRange now clamps scroll offsets to the maximum reachable position, accounting for scrollMargin. Tests cover shrinking a list while the scroll offset remains at its previous end position.
Changes: Range calculation
Layer / File(s)
Summary
Clamp the range offset and verify shrink behavior packages/virtual-core/src/index.ts, packages/virtual-core/tests/range.test.ts, .changeset/clamp-range-scroll-offset.md
calculateRange includes scrollMargin when computing the list end and clamps the offset used to calculate the visible range. Tests check indexes before and after reducing the count from 50 to 20, with and without a scroll margin. The changeset records the patch.
The clamp addresses stale offsets after list shrink. Uneven multi-lane lists incur additional range-calculation work; caching is a bounded performance follow-up rather than an established merge blocker.
Pre-merge checks: 5
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The title clearly and concisely describes the main change: preserving a full virtual range when the scroll offset exceeds the list end.
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packages/virtual-core/tests/range.test.ts
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Follow-up Comments
Cache the multi-lane layout end used by calculateRange.
1865-1865: Trivial · Performance & Scalability · Low value
Cache the multi-lane layout end used by calculateRange.
calculateRange now calls _getLayoutTotalSize() on every invocation. In multi-lane layouts, that method scans backward until each lane has an item, and it has no cache. Each scroll event can repeat this scan, so CPU work grows with the item count after the earliest-ending lane's last item.
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Review comment at @packages/virtual-core/src/index.ts at line 1865:
Cache the multi-lane layout end used by calculateRange so repeated calls do not
rescan items through _getLayoutTotalSize(). Reuse the cached result across
scroll events and invalidate or update it when layout inputs change so the range
remains correct.
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.changeset/clamp-range-scroll-offset.md
packages/virtual-core/src/index.ts
packages/virtual-core/tests/range.test.ts
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🎯 Changes
calculateRangehands the scroll offset tocalculateRangeImplas it is. When the offset is past the start of the last row, the range is that row alone. That happens while the virtualizer holds an offset the element no longer has: the list shrank (filtering, rows re-measured smaller), the element clamped itsscrollTop, and the scroll event that reports it hasn't arrived yet. Until it does, the viewport shows the last row and blank space above it.50 rows of 30px in a 300px viewport, scrolled to the end, then
countdrops to 20:The change caps the offset at the last position the list can be scrolled to before computing the range. The cap has to be in the offset's own space.
scrollOffsetis measured in the scroll element and in layout coordinates, whilegetTotalSize()starts at the list and is shifted by a held correction, so the cap is_getLayoutTotalSize() + scrollMargin - outerSize. #1340 fixed the same mix of spaces ingetVirtualDistanceFromEnd; here there was no cap at all. A cap againstgetTotalSize()alone would pin any list whosescrollMarginis larger than its own height to its first rows: I've carried this fix as a patch in OpenChamber, and its first version did exactly that to sections lower down in a shared scroller. The second test covers it. The list end is in the memo deps, so a change ofpaddingEndor of the hold recomputes the range.The cap only moves the offset down, so the new range is the old one plus rows above it, never fewer. For a list followed by other content in the same scroller, scrolled below the list, this renders a viewport's worth of rows off screen where
mainrenders one.Snapshot.
scroll-tracemoves in 29 configurations, 197 of 1800 traces. In each, the first divergence is a render while the element is past an edge (a drag that rubber-bands, a settle from a bounce): more rows are rendered and measured there, and later steps follow from those measurements. Across all traces, settled frames whose first rendered row starts below the element's position go from 22 to 12.Tests. Two cases in
range.test.ts: the shrink above, without and withscrollMargin: 1000. Both get[19]onmainand pass with the change.✅ Checklist
pnpm run test:pr.I ran the
test:prtargets excepttest:e2e:test:lib,test:types,test:eslintandbuildfor all 10 packages,test:buildforvirtual-core, andtest:sherif,test:knip,test:docsat the root. I didn't run the Playwright e2e.🚀 Release Impact
Summary by CodeRabbit