[release/11.0-preview4] Virtualization AnchorMode with for variable-height support#66521
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wtgodbe merged 29 commits intodotnet:release/11.0-preview4from Apr 29, 2026
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…ight support Adds a new AnchorMode parameter to the Virtualize component that controls how the viewport behaves at list edges when items are added dynamically. New API: - VirtualizeAnchorMode enum: None (0), Beginning (1), End (2) [Flags] - Virtualize<TItem>.AnchorMode parameter (default: Beginning) Anchor snapshot/restore mechanism: - JS lazily captures the first visible item's position on every IO callback - After a render that shifts content (prepend/append/redistribution), JS restores the anchored item to its original viewport position using measured DOM positions instead of average-height estimates - Works correctly for both fixed and variable height items - Stale IO callbacks are suppressed after anchor restore to prevent undo Behavior matrix: | Scenario | None | Beginning | End | |---------------------|-------------------|-------------------|-------------------| | Prepend at top | Viewport stable | Shows new items | Viewport stable | | Append at bottom | Viewport stable | Auto-scrolls | Auto-scrolls | | Mid-list changes | Viewport stable | Viewport stable | Viewport stable | | Home/End keys | Works | Works | Works | Contributes to dotnet#65742, dotnet#26943. Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
…tability fixes. - Fix IO suppression race: unified suppressSpacerCallbacks flag persists until next user scroll, preventing IntersectionObserver re-entry from undoing anchor restore scroll compensation. - Fix End+variable-height large append: JS-side wasAtBottom detection in refreshObservedElements starts scroll-to-bottom convergence directly, avoiding unreliable C# race conditions. - Remove stale C# _pendingScrollToBottom branch from RefreshDataCoreAsync that caused false re-engage when user scrolled away from bottom. - Fix prepend ordering: items now appear -10,-9,...,-1 instead of -1,-2,...-10. - Fix LargePrependAtTop test to check for -100 (first item after Reverse). Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
…d properly on expansion.
…for container scroll.
…m the edge + remove drift correction because it was causing sub-pixel shifts.
…llection outside of the item provider call.
…mode should converge for them.
…herwise it's working.
…ems when new param is not provided.
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Pull request overview
Backports Virtualize AnchorMode support (including variable-height stability) to improve viewport behavior when items are dynamically prepended/appended, and adds coverage/tests and tooling to encourage correct usage with ItemsProvider.
Changes:
- Adds new
VirtualizeAnchorModeAPI plusVirtualize<TItem>.AnchorMode, and JS interop support for anchor snapshot/restore. - Updates
Virtualize<TItem>logic for prepend/append detection and viewport stabilization; adds extensive E2E and unit test coverage. - Introduces a Roslyn analyzer warning when
VirtualizeusesItemsProviderwithoutItemComparer, and adds BasicTestApp pages for manual testing.
Reviewed changes
Copilot reviewed 15 out of 15 changed files in this pull request and generated 3 comments.
Show a summary per file
| File | Description |
|---|---|
| src/Components/test/testassets/BasicTestApp/VirtualizationAnchorModeWindowScroll.razor | New manual test page for anchor mode behavior with window scrolling. |
| src/Components/test/testassets/BasicTestApp/VirtualizationAnchorMode.razor | New manual test page for anchor mode behavior in a scroll container. |
| src/Components/test/testassets/BasicTestApp/Index.razor | Adds the new manual test pages to the BasicTestApp selector list. |
| src/Components/test/E2ETest/Tests/VirtualizationTest.cs | Adds/adjusts E2E coverage for anchor modes, prepend/append stability, and window scroll scenarios. |
| src/Components/Web/test/Virtualization/VirtualizeTest.cs | Adds unit tests and updates helpers to validate new anchor mode behaviors. |
| src/Components/Web/src/Virtualization/VirtualizeJsInterop.cs | Extends interop to pass anchor mode on init and expose set/restore APIs. |
| src/Components/Web/src/Virtualization/VirtualizeAnchorMode.cs | Adds the new public enum for anchoring behavior. |
| src/Components/Web/src/Virtualization/Virtualize.cs | Implements AnchorMode/ItemComparer parameters and anchor restore logic. |
| src/Components/Web/src/PublicAPI.Unshipped.txt | Declares new public APIs (AnchorMode, ItemComparer, VirtualizeAnchorMode). |
| src/Components/Web.JS/test/Virtualize.test.ts | Extends export tests to include setAnchorMode and restoreAnchor. |
| src/Components/Web.JS/src/Virtualize.ts | Implements anchor snapshot/restore mechanism and anchor-mode-aware convergence/suppression. |
| src/Components/Analyzers/test/VirtualizeItemComparerAnalyzerTest.cs | Adds analyzer tests for ItemsProvider usage without ItemComparer. |
| src/Components/Analyzers/src/VirtualizeItemComparerAnalyzer.cs | Adds analyzer that warns when ItemsProvider is used without ItemComparer. |
| src/Components/Analyzers/src/Resources.resx | Adds localized strings for the new analyzer diagnostic. |
| src/Components/Analyzers/src/DiagnosticDescriptors.cs | Registers new diagnostic descriptor BL0011. |
…nd detection with a fix for it.
…netcore into backport/pr-66262-to-release/11.0-preview4
This was referenced Apr 29, 2026
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/ba-g template and IIS test failures unrelated - we don't have Virtualization in templates and this PR didn't touch that part |
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Hi @ilonatommy. This PR was just approved to be included in the upcoming servicing release. Somebody from the @dotnet/aspnet-build team will get it merged when the branches are open. Until then, please make sure all the CI checks pass and the PR is reviewed. |
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Backport of #66262 to release/11.0-preview4
/cc @ilonatommy
Virtualization
AnchorModewith variable-height supportAdds
VirtualizeAnchorModeparameter and anchor snapshot/restore mechanism for variable-height items.Description
Backport of PR #66262 which adds a new
AnchorModeparameter to theVirtualizecomponent that controls viewport behavior at list edges when items are added dynamically. Uses a measure-based anchor snapshot/restore mechanism that works correctly for both fixed and variable-height items.Fixes #65742
Customer Impact
Enables stable viewport behavior for dynamically updated virtualized lists (chat UIs, news feeds, logs) with variable-height items.
Regression?
Risk
New feature with E2E test coverage.
Verification
Packaging changes reviewed?