[Known Build Error] VirtualizationTest.QuickGrid_InitialIndex_TallContainer_NearEnd_FillsViewportWithoutUserScroll — top rendered index was -1

[lewing]

Build Information

Build: https://dev.azure.com/dnceng-public/public/_build/results?buildId=1563047
Build error leg or test failing: aspnetcore-components-e2e / Test: Blazor E2E tests on Linux (Mono and CoreCLR) / Microsoft.AspNetCore.Components.E2ETests
Pull request: #68704 (unrelated, docs-only — failure also reproduces on main)

Error Message

{
  "ErrorMessage": "should remain aligned with the viewport top once the last item has loaded, but top rendered index was -1",
  "BuildRetry": false,
  "ExcludeConsoleLog": false
}

Description

VirtualizationTest.QuickGrid_InitialIndex_TallContainer_NearEnd_FillsViewportWithoutUserScroll fails intermittently on the Blazor E2E legs. The failure is not confined to one render mode or one leg:

• Both theory cases (useProvider: true and useProvider: false) fail.
• Both the WebAssembly variant (Tests.VirtualizationTest) and the Interactive Server variant (ServerExecutionTests.ServerVirtualizationTest) fail.
• It has been observed on both the Mono and CoreCLR legs.

The test mounts QuickGridScrollComponent, sizes the scroll container to 2000px, switches to variable item heights, unloads the list, sets InitialItemIndex = 950, and reloads. It then waits for the top rendered index to settle on 950. When it fails, no items are rendered at all — the top rendered index is reported as -1 with scrollTop=0, i.e. the list never repopulated after the reload rather than landing at the wrong offset.

Assertion:

OpenQA.Selenium.BrowserAssertFailedException : Xunit.Sdk.TrueException: Item 950 should remain aligned with the viewport top once the last item has loaded, but top rendered index was -1, scrollTop=0.

Stack trace:

   at Microsoft.AspNetCore.E2ETesting.WaitAssert.WaitAssertCore[TResult](IWebDriver driver, Func`1 assertion, TimeSpan timeout) in src/Shared/E2ETesting/WaitAssert.cs:line 160
   at Microsoft.AspNetCore.E2ETesting.WaitAssert.True(IWebDriver driver, Func`1 actual, String message) in src/Shared/E2ETesting/WaitAssert.cs:line 43
   at Microsoft.AspNetCore.Components.E2ETest.Tests.VirtualizationTest.QuickGrid_InitialIndex_TallContainer_NearEnd_FillsViewportWithoutUserScroll(Boolean useProvider) in src/Components/test/E2ETest/Tests/VirtualizationTest.cs:line 5767

Origin: the test was added on 2026-08-21 by fde48e9 — "Fix InitialItemIndex viewport underfill for small items in big container or on window resize" (#67936). Every observed failure post-dates that commit.

Observed occurrences

Surveyed the 12 most recent failed aspnetcore-components-e2e builds (all 2026-08-21). 6 of 12 matched this signature, across 5 distinct branches/PRs:

Build	Branch / PR	Leg	Failing variant
1562769	PR #68691	CoreCLR	Tests.VirtualizationTest, both useProvider
1562788	PR #68700	Mono	ServerVirtualizationTest, useProvider: False
1562908	refs/heads/main	Mono	—
1562986	PR #68663	Mono	ServerVirtualizationTest, useProvider: True
1563047	PR #68704 (docs-only)	Mono	Tests.VirtualizationTest
1563228	PR #68701	Mono	ServerVirtualizationTest, both useProvider

Passing counter-example: 1563027 (refs/heads/main) — the test ran and passed, flagged Long Running at 33s.

The remaining failed builds in the window failed on unrelated tests (VirtualizationTest.CancelsOutdatedRefreshes_Async, QuickGridInteractiveCompatTest.PaginatorCorrectItemsPerPage, BlazorWebJsInitializersTest.InitializersRunsModernCallbacksByDefaultWhenPresent, and the already-tracked QuickGrid_ScrollToItem_NegativeIndex_ScrollsToTop from #68559).

It reproduces on main and on unrelated PRs, so this is not caused by any single PR in flight. It passes on some runs, so it is intermittent rather than a hard break — but at roughly a 50% hit rate on failing builds it is frequent enough to red-fail the required aspnetcore-components-e2e check on unrelated PRs. Build Analysis currently reports it as an unmatched failure (Bash exited with code '1' on Run E2E tests), which is why this Known Build Error is being filed.

The chosen ErrorMessage also matches the sibling InitialIndex_TallContainer_NearEnd_FillsViewportWithoutUserScroll test, which shares the same scenario and assertion text and is expected to have the same root cause.

Related work in this area:

• Quarantine VirtualizationTest.ScrollToItem_AfterEndJump_LandsAtTarget #68644 — Quarantine VirtualizationTest.ScrollToItem_AfterEndJump_LandsAtTarget
• Quarantine VirtualizationTest.QuickGrid_ScrollToItem_NegativeIndex_ScrollsToTop #68559 — Quarantine VirtualizationTest.QuickGrid_ScrollToItem_NegativeIndex_ScrollsToTop
• Reject stale Blazor Virtualize viewport measurements #68691 — Reject stale Blazor Virtualize viewport measurements (open)

Given the hit rate and that it blocks unrelated PRs, quarantining the affected theory is likely warranted while the underlying issue is investigated.

Report

Build	Repository	Test	Pull Request
1564034	dotnet/aspnetcore	Microsoft.AspNetCore.Components.E2ETest.ServerExecutionTests.ServerVirtualizationTest.QuickGrid_InitialIndex_TallContainer_NearEnd_FillsViewportWithoutUserScroll(useProvider: False)	#68627
1563927	dotnet/aspnetcore	Microsoft.AspNetCore.Components.E2ETest.ServerExecutionTests.ServerVirtualizationTest.QuickGrid_InitialIndex_TallContainer_NearEnd_FillsViewportWithoutUserScroll(useProvider: True)	#68733
1563923	dotnet/aspnetcore	Microsoft.AspNetCore.Components.E2ETest.ServerExecutionTests.ServerVirtualizationTest.QuickGrid_InitialIndex_TallContainer_NearEnd_FillsViewportWithoutUserScroll(useProvider: False)	#68731
1563739	dotnet/aspnetcore	Microsoft.AspNetCore.Components.E2ETest.ServerExecutionTests.ServerVirtualizationTest.QuickGrid_InitialIndex_TallContainer_NearEnd_FillsViewportWithoutUserScroll(useProvider: False)	#68720
1563420	dotnet/aspnetcore	Microsoft.AspNetCore.Components.E2ETest.ServerExecutionTests.ServerVirtualizationTest.QuickGrid_InitialIndex_TallContainer_NearEnd_FillsViewportWithoutUserScroll(useProvider: True)
1563047	dotnet/aspnetcore	Microsoft.AspNetCore.Components.E2ETest.ServerExecutionTests.ServerVirtualizationTest.QuickGrid_InitialIndex_TallContainer_NearEnd_FillsViewportWithoutUserScroll(useProvider: False)	#68704

Summary

24-Hour Hit Count	7-Day Hit Count	1-Month Count
4	6	6

[github-actions]

Triage Summary

Area: area-blazor (QuickGrid/Virtualize E2E test failure in src/Components)
Type: Bug (reproducible intermittent test failure, confirmed on main and unrelated PRs)

Regression Info

• Previously working version: behavior prior to commit fde48e9 (PR Fix  InitialItemIndex viewport underfill for small items in big container or on window resize #67936)
• Broken since: fde48e9, merged 2026-08-21
• Prior to this commit the near-end initial-alignment underfill was the known issue being fixed; after it merged, the test now intermittently fails with no items rendered at all (top rendered index -1, scrollTop=0) rather than the list landing at the wrong offset.

Potential Duplicates

• Reject stale Blazor Virtualize viewport measurements #68691 - Reject stale Blazor Virtualize viewport measurements (similarity: high) — open PR describing a stale asynchronous viewport-measurement race introduced by the same PR Fix  InitialItemIndex viewport underfill for small items in big container or on window resize #67936, explicitly called out as a follow-up in the issue body
• Quarantine VirtualizationTest.ScrollToItem_AfterEndJump_LandsAtTarget #68644 - Quarantine VirtualizationTest.ScrollToItem_AfterEndJump_LandsAtTarget (similarity: medium) — same test class, different scenario
• Quarantine VirtualizationTest.QuickGrid_ScrollToItem_NegativeIndex_ScrollsToTop #68559 - Quarantine VirtualizationTest.QuickGrid_ScrollToItem_NegativeIndex_ScrollsToTop (similarity: medium) — same test class, different scenario

Notes

• Virtualize: InitialItemIndex is intermittently ignored on fresh page load (startup race with spacer IntersectionObserver callback) #68099 (closed, fixed by PR Fix InitialItemIndex is intermittently ignored on fresh page load  #68114) describes a related but distinct startup race where the initial scroll is discarded and the list resets to index 0 — the current failure instead shows no items rendered at all after a reload, so it is not the same root cause.

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