Fix Virtualize user scroll during pending ScrollToItemAsync

[ilonatommy]

Fixes a race where manual user scrolling does not reliably cancel an in-progress Virtualize<TItem>.ScrollToItemAsync operation.

When the target item is still loading, the rendered viewport can consist primarily of a large spacer. Scrolling within that spacer does not necessarily change its intersection state, so the existing IntersectionObserver may not produce another callback. Without that callback, the pending provider request is not canceled and the original programmatic scroll can later override the user's position.

Description

• Detect user input that interrupts a programmatic scroll, pending alignment, or scroll convergence.
• Stop the active convergence and clear abandoned alignment state.
• Create a fresh one-shot IntersectionObserver using the same configuration as the main observer.
• Feed its initial result through the existing spacer callback path, which cancels the pending scroll and loads the user's viewport.
• Cancel pending one-shot observation when superseded or disposed.
• Remove the quarantine from ScrollToItem_UserScrollDuringProviderFetch_UserScrollWins.

Fixes #68777

[Copilot]

🟡 Changes recommended

Four unresolved moderate findings remain in the user-interruption and observer lifecycle paths.

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Pull request overview

Fixes a race where user scrolling could be overridden by an in-progress ScrollToItemAsync.

Changes:

• Adds one-shot spacer observation for interrupted programmatic scrolling.
• Cleans up observers and pending alignment state.
• Removes quarantine from the regression E2E test.

File summaries

File	Summary and final review comments
src/Components/Web.JS/src/Virtualize.ts	Implements user-scroll interruption handling. Four unresolved moderate findings concern cancellation when spacers are outside the viewport (2 votes), stale pendingScrollCorrection (1 vote), uncleared pendingCallbacks (1 vote), and duplicate observers for Home/End paths (1 vote).
src/Components/test/E2ETest/Tests/VirtualizationTest.cs	Re-enables and documents the regression E2E test.

Review details

Suppressed comments (3)

src/Components/Web.JS/src/Virtualize.ts:561

• When interrupting RestoreSnapshot, restoreAnchorForShift may already have set pendingScrollCorrection. Leaving that flag set lets the next refreshObservedElements apply the old correction to the user's new position, so the abandoned self-scroll can still move the viewport. Clear pendingScrollCorrection here along with the other pending alignment state.

    pendingAlignLocalIndex = null;

src/Components/Web.JS/src/Virtualize.ts:558

• Queued entries from the main observer are left in pendingCallbacks when switching to the one-shot observer. If user input arrives during the 50 ms throttle window, the old programmatic entry is flushed later and treated as a current viewport-fill event, potentially starting another provider refresh for the abandoned target. Clear both spacer entries here, as beginProgrammaticScroll already does.

    stopConvergenceObserving();

src/Components/Web.JS/src/Virtualize.ts:562

• When End or Home interrupts an active programmatic scroll, this call creates a one-shot observer, but those handlers immediately call reobserveSpacers() and start their own jump/convergence. Both observers can then deliver the same initial spacer state as UserScroll, causing duplicate spacer callbacks and provider-refresh churn. Avoid scheduling the one-shot for these explicit reobserve paths, or cancel it before reobserving.

    observeSpacersAfterUserScroll();

• Files reviewed: 2/2 changed files
• Comments generated: 1
• Review effort level: Lite

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