[release/11.0-preview4] Virtualization AnchorMode with variable-height support by ilonatommy · Pull Request #66521 · dotnet/aspnetcore

ilonatommy · 2026-04-28T19:47:36Z

Backport of #66262 to release/11.0-preview4

Virtualization `AnchorMode` with variable-height support

Adds VirtualizeAnchorMode parameter and anchor snapshot/restore mechanism for variable-height items.

Description

Backport of PR #66262 which adds a new AnchorMode parameter to the Virtualize component 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?

Yes
No

Risk

High
Medium
Low

New feature with E2E test coverage.

Verification

Manual (required)
Automated

Packaging changes reviewed?

Yes
No
N/A

…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.

Copilot

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 VirtualizeAnchorMode API plus Virtualize<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 Virtualize uses ItemsProvider without ItemComparer, 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

ilonatommy · 2026-04-29T14:40:52Z

/ba-g template and IIS test failures unrelated - we don't have Virtualization in templates and this PR didn't touch that part

wtgodbe · 2026-04-29T19:04:37Z

Approved over email

dotnet-policy-service · 2026-04-29T19:05:14Z

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.

guardrex · 2026-05-19T21:58:33Z

@ilonatommy ... Would you please edit the title of this because I'd like to link it in the Release Notes?

Is "with" the word you want? ...

- [release/11.0-preview4] Virtualization AnchorMode with for variable-height support
+ [release/11.0-preview4] Virtualization AnchorMode with variable-height support

…ragment field - The "What's new in ASP.NET Core in .NET 11" release notes state that Virtualize no longer assumes every item has the same height and adapts to item sizes measured at runtime (dotnet/aspnetcore#66521). Limit the same-height rule to .NET 8 through 10 and keep the single-vertical-stack and spacer rules for every version. - Rename the private RenderFragment field in the example to camelCase (logLine) to match the other private fields in the guidance.

ilonatommy and others added 26 commits April 28, 2026 21:44

Fix tests.

8580655

Unit tests were missing anchorMode.

21b4f19

Small cleanup.

f8cd56e

New test: make sure that before scroll happens the scrol is positione…

2f1c3ff

…d properly on expansion.

Indeces.

a45ddf3

Fix provider case with ItemKey approach.

32d174f

ItemProvider support + more tests for window scroll, same as we have …

3bcdb85

…for container scroll.

Refactoring.

ad52722

Make test deterministic

d14dad3

Feedback.

2f97b7b

Warning is too silent, throw instead.

9677323

Analyzer instead of exception.

49929f7

Update analyzer text.

dd4e6fa

Fix in-DOM auto-follow + do not re-engage after user scrolls away fro…

57d5c88

…m the edge + remove drift correction because it was causing sub-pixel shifts.

Test item provider loading with delay. Fix tests to not modify the co…

e47092b

…llection outside of the item provider call.

Test for shost lists without scroll that grow to have a scroll - end …

b23d5a7

…mode should converge for them.

Short list fix was too broad - update.

f06c521

Missing change for the previous commit.

68e7b0c

Document deletion behavior - above the viewport will cause a jump, ot…

c477199

…herwise it's working.

Change ItemKey -> ItemComparer to not break non-ref comparison it…

c355522

…ems when new param is not provided.

This test should be mirroring end-testing reengage test + add logging.

e9bb141

Missing renames.

3cf4c5c

Variables can be merged into one.

f7b80b9

Feedback: align test infra.

badca25

ilonatommy added this to the 11.0-preview4 milestone Apr 28, 2026

ilonatommy self-assigned this Apr 28, 2026

Copilot AI review requested due to automatic review settings April 28, 2026 19:47

ilonatommy requested a review from a team as a code owner April 28, 2026 19:47

ilonatommy added area-blazor Includes: Blazor, Razor Components feature-blazor-virtualization This issue is related to the Blazor Virtualize component labels Apr 28, 2026

Copilot started reviewing on behalf of ilonatommy April 28, 2026 19:48 View session

ilonatommy added the Servicing-consider Shiproom approval is required for the issue label Apr 28, 2026

Copilot AI reviewed Apr 28, 2026

View reviewed changes

Comment thread src/Components/Web/src/Virtualization/Virtualize.cs

Comment thread src/Components/Web/src/Virtualization/Virtualize.cs

Comment thread src/Components/Web/src/Virtualization/Virtualize.cs Outdated

ilonatommy added 2 commits April 28, 2026 22:54

Feedback: default value for comparer + test for in-memory items prepe…

2b343f6

…nd detection with a fix for it.

Merge branch 'release/11.0-preview4' of https://github.com/dotnet/asp…

9ae62d6

…netcore into backport/pr-66262-to-release/11.0-preview4

ilonatommy requested a review from lewing April 28, 2026 20:58

Stabilize preparations to test actions.

7f2e271

wtgodbe approved these changes Apr 29, 2026

View reviewed changes

wtgodbe added Servicing-approved Shiproom has approved the issue and removed Servicing-consider Shiproom approval is required for the issue labels Apr 29, 2026

wtgodbe merged commit de89542 into dotnet:release/11.0-preview4 Apr 29, 2026
27 of 30 checks passed

dotnet-policy-service Bot modified the milestone: 11.0-preview4 Apr 29, 2026

dotnet-maestro Bot mentioned this pull request Apr 29, 2026

[release/11.0.1xx-preview4] Source code updates from dotnet/aspnetcore dotnet/dotnet#6379

Merged

This was referenced Apr 30, 2026

[release-notes] ASP.NET Core in .NET 11 Preview 4 dotnet/core#10389

Merged

Virtualize: viewport stability on prepend regresses when items are inserted while the user is still within the initial render window (_itemsBefore == 0) #66541

Closed

This was referenced May 6, 2026

Blazor .NET 11 Tracking Issue dotnet/AspNetCore.Docs#36448

Open

[11.0 P4] Virtualization AnchorMode with for variable-height support dotnet/AspNetCore.Docs#37108

Closed

guardrex mentioned this pull request May 19, 2026

Improve 11.0 P4 coverage dotnet/AspNetCore.Docs#37160

Merged

ilonatommy changed the title ~~[release/11.0-preview4] Virtualization AnchorMode with for variable-height support~~ [release/11.0-preview4] Virtualization AnchorMode with variable-height support May 20, 2026

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[release/11.0-preview4] Virtualization AnchorMode with variable-height support - #66521

[release/11.0-preview4] Virtualization AnchorMode with variable-height support#66521
wtgodbe merged 29 commits into
dotnet:release/11.0-preview4from
ilonatommy:backport/pr-66262-to-release/11.0-preview4

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Virtualization AnchorMode with variable-height support

Description

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Pull request overview

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ilonatommy commented Apr 29, 2026

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wtgodbe commented Apr 29, 2026

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ilonatommy commented Apr 28, 2026 •

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Virtualization `AnchorMode` with variable-height support