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feat(virtual-core): add resetItemSize to forget one item's measured size - #1381
Implements the proposal in #1362: a public way to say "the size you have for this item is no longer valid", for content that changes shape while its key stays the same.
resetItemSize(index):
Item not rendered: removes its entry from itemSizeCache and lays it out at estimateSize(index). The change (measured size to estimate) is compensated like a first measurement, so it also holds while scrolling backward. The item's next measurement counts as a first one, and takeSnapshot() leaves it out until then.
Item rendered: reads the element right away, so a rendered row never shows the estimate. The default measureElement answers from the cache when it has the key, so the key is taken out while the element is read.
With useCachedMeasurements there is no element read, so the item is always reset to its estimate.
It shares resizeItem's path (now _resizeItem, with size: null meaning "forget"): same pendingMin / version invalidation, scroll compensation, anchorTo: 'end' handling and notify. resizeItem itself behaves exactly as before.
One change from the discussion: a rendered item is anchored as a re-measurement, not as a first measurement. On screen, a row spanning the fold that expands would otherwise pull the viewport down with it on every expand, which is the case #1218 fixed for re-measurements. The first-measurement rule matters for the item that is not rendered, where the re-measurement rule would skip the correction while scrolling backward and the row would jump when it comes back into view.
I kept the index-based signature to match resizeItem and the rest of the API.
Tests (measurement.test.ts):
An item that is not rendered returns to its estimate, later offsets use it, and takeSnapshot() leaves it out but keeps the other measured items (the regression check suggested in the discussion).
An item measured at exactly its estimate is still removed from the snapshot.
A reset below an index that resizeItem dirtied since the last read is not lost (the pendingMin case from the discussion).
A rendered element is read instead of its cached size.
A rendered item spanning the fold is re-measured without moving the scroll position.
While scrolling backward, the reset of an item above the viewport and its next measurement are both compensated.
Each test fails when the line it guards is removed.
Not included: Marko's <virtualizer> / <window-virtualizer> tag variables list their methods explicitly, so resetItemSize is not reachable there yet. I can add it here or in a follow-up. The Angular proxy forwards methods it does not list, so it works there as is.
I have tested this code locally with pnpm run test:pr.
Everything passed except @tanstack/react-virtual:test:e2e, which did not start: its webServer command uses POSIX VAR=value syntax that cmd.exe on Windows cannot run. I ran the same build and preview from bash and the suite then passed (44 tests).
🚀 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
New Features
Added the ability to reset an individual item’s measured size. The item uses its estimated size until measured again, and rendered items are remeasured immediately.
Scroll position is corrected when a reset item is measured again.
Documentation
Updated the virtualizer API documentation with details on resetting item sizes and the effects on snapshots and scroll correction.
No actionable comments were generated in the recent review.
Walkthrough
The virtualizer adds resetItemSize(index) to forget an item’s measured size. The item uses its estimated size until measured again. The changes also update cache handling, scroll anchoring, API documentation, and measurement tests.
Changes: Reset item size
Layer / File(s)
Summary
Reset API and measurement behavior packages/virtual-core/src/index.ts, docs/api/virtualizer.md, .changeset/reset-item-size.md
Adds resetItemSize(index). The virtualizer removes the cached size and uses estimateSize until remeasurement. A connected item is remeasured immediately unless cached measurements are enabled. The API documentation and changeset describe the reset behavior.
sequenceDiagram
participant Caller
participant Virtualizer
participant Element
participant ResizeObserver
Caller->>Virtualizer: resetItemSize(index)
Virtualizer->>Element: reread size if connected
Element->>Virtualizer: current measured size
ResizeObserver->>Virtualizer: report later size
Virtualizer->>Virtualizer: update measurements and scroll anchoring
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Possibly related PRs
TanStack/virtual#1363: Updates offset reads to use current measurements after size changes, which is relevant to scroll anchoring when an item is reset.
Suggested reviewers:piecyk
Merge Risk: Low · up to c794d
The new resetItemSize API works as described for normal use. One edge case remains: an item re-measured at exactly its estimate stays marked as unmeasured, which can affect snapshots and anchoring in rare cases. Resetting very large numbers of items in a loop is also costly. Neither blocks merging.
Pre-merge checks: 5
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Title check
The title clearly and concisely identifies the main change: adding resetItemSize to forget an item's measured size.
Description check
The description follows the required template, explains the implementation and motivation, records testing results, and includes the changeset checklist. It also documents the Windows E2E limitation a…
Docstring Coverage
Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentat…
Linked Issues check
Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check
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Full details: Description check
Explanation
The description follows the required template, explains the implementation and motivation, records testing results, and includes the changeset checklist. It also documents the Windows E2E limitation and the successful bash workaround.
Full details: Docstring Coverage
Explanation
Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentation-only edits and deletions within surviving functions are included; unchanged diff context and fully deleted functions are excluded. Analyzed 1 functions across 2 files.
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packages/virtual-core/tests/measurement.test.ts
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Actionable comments posted: 2
Fix comments: Fix CodeRabbit comments on this PR
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Inline comments:
Review comments at @packages/virtual-core/src/index.ts:
- Line 2109: Update the delta calculation using itemSize so a reset immediately
establishes the reset estimate as the baseline, or ensure pending measurements
are rebuilt before another reset or measurement uses that baseline. Preserve the
effective estimated size across resets when onChange does not read measurements,
and add coverage for that callback behavior.
- Around line 2203-2205: Update the size === null branch to retain the numeric
measurement in itemSizeCache without triggering an unnecessary layout
notification, so takeSnapshot recognizes the item as measured and later growth
does not cause an incorrect scroll adjustment. Add coverage for reset, rebuild,
measurement at the estimate, and subsequent growth.
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.changeset/reset-item-size.md
docs/api/virtualizer.md
packages/virtual-core/src/index.ts
packages/virtual-core/tests/measurement.test.ts
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With the key gone from itemSizeCache, the layout was the only record of
the item's size, and it still held the size the item was reset from
until the next read. A second reset, or a measurement, before that read
started from the old size: the scroll correction was applied twice and
the next measurement was compensated from the wrong baseline.
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🎯 Changes
Implements the proposal in #1362: a public way to say "the size you have for this item is no longer valid", for content that changes shape while its key stays the same.
resetItemSize(index):itemSizeCacheand lays it out atestimateSize(index). The change (measured size to estimate) is compensated like a first measurement, so it also holds while scrolling backward. The item's next measurement counts as a first one, andtakeSnapshot()leaves it out until then.measureElementanswers from the cache when it has the key, so the key is taken out while the element is read.useCachedMeasurementsthere is no element read, so the item is always reset to its estimate.It shares
resizeItem's path (now_resizeItem, withsize: nullmeaning "forget"): samependingMin/ version invalidation, scroll compensation,anchorTo: 'end'handling and notify.resizeItemitself behaves exactly as before.One change from the discussion: a rendered item is anchored as a re-measurement, not as a first measurement. On screen, a row spanning the fold that expands would otherwise pull the viewport down with it on every expand, which is the case #1218 fixed for re-measurements. The first-measurement rule matters for the item that is not rendered, where the re-measurement rule would skip the correction while scrolling backward and the row would jump when it comes back into view.
I kept the index-based signature to match
resizeItemand the rest of the API.Tests (
measurement.test.ts):takeSnapshot()leaves it out but keeps the other measured items (the regression check suggested in the discussion).resizeItemdirtied since the last read is not lost (thependingMincase from the discussion).Each test fails when the line it guards is removed.
Not included: Marko's
<virtualizer>/<window-virtualizer>tag variables list their methods explicitly, soresetItemSizeis not reachable there yet. I can add it here or in a follow-up. The Angular proxy forwards methods it does not list, so it works there as is.✅ Checklist
pnpm run test:pr.Everything passed except
@tanstack/react-virtual:test:e2e, which did not start: itswebServercommand uses POSIXVAR=valuesyntax that cmd.exe on Windows cannot run. I ran the same build and preview from bash and the suite then passed (44 tests).🚀 Release Impact
Summary by CodeRabbit