Fix AnchorMode_End_PrependAtTop_ViewportStaysStable

[ilonatommy]

Summary

Fixes a flaky failure in ServerVirtualizationTest.AnchorMode_End_PrependAtTop_ViewportStaysStable(useItemsProvider: true). The test intermittently failed with the viewport jumping to the prepended items instead of staying stable:

End mode at top: viewport should stay stable after prepend (index before: 0, after: -10, relTop before: 1, after: 1, scrollTop: 0, tolerance: 5)

Root cause

This is a Blazor Server timing race, not a logic error (the anchor mechanism is correct).

When items are prepended while the viewport is at the very top (scrollTop == 0), Virtualize preserves the pre-shift anchor snapshot and relies on RestoreAnchorAsync to reposition the scroll so the previously-visible row stays at its place. On Server, RestoreAnchorAsync is a SignalR round-trip. Because the grown spacerBefore is already visible at scrollTop == 0, its IntersectionObserver fires and dispatches OnSpacerBeforeVisible to the server, loading the prepended rows (-10..-1) into the viewport before the restore round-trip completes. An async ItemsProvider with delay widens the window, making the race more probable to happen. WebAssembly/synchronous paths don't race because restore is effectively synchronous.

Fix

In the snapshot-preserve branch of updateAnchorSnapshot, set suppressSpacerCallbacks = true only when the pin originated at the very top (existing.scrollTop < 1) and the anchor mode is End. This blocks the racing spacerBefore callback from dispatching a load to the server for the one render cycle until the anchor restore runs. The suppression is cleared on the next user scroll (handleScroll -> reobserveSpacers), so the prepended rows remain fully reachable when the user scrolls up.

The existing.scrollTop < 1 gate is required: an unconditional suppress would re-suppress the spacer every time the user scrolled back to the top, permanently preventing the above-viewport rows from loading and breaking the "prepended items become reachable" assertion.

[Copilot]

Pull request overview

Fixes a timing race in Blazor Server virtualization that could cause the viewport to jump when items are prepended at the very top in End anchor mode, leading to flakiness in ServerVirtualizationTest.AnchorMode_End_PrependAtTop_ViewportStaysStable(useItemsProvider: true).

Changes:

• When updateAnchorSnapshot() preserves the pre-shift snapshot, it now suppresses spacer IntersectionObserver callbacks only if the preserved snapshot originated at the very top (existing.scrollTop < 1).
• This prevents spacerBefore visibility callbacks from racing ahead of the anchor-restore round-trip specifically in the “pinned at top” scenario, while avoiding suppression in cases that would break reachability of prepended rows in other modes.