Virtualization supports Beginning|End|None anchor modes for fixed-height items

[ilonatommy]

Summary

Adds a new AnchorMode parameter to the Virtualize<TItem> component that controls how the viewport behaves at list edges when items are added dynamically.

Out of scope

This PR adds tests for both: fixed and variable height items but focuses on supporting only the former. Variable height support for prepended items scenario turns out to be more time consuming than expected and is out of scope for this PR.
The exact scroll compensation is unknown until the new items are rendered and measured in the DOM, so keeping the viewport perfectly stable requires an asynchronous, race-free post-layout correction rather than a deterministic precomputed shift (which is sufficient for fixed height scenarios).

New API

[Flags]
public enum VirtualizeAnchorMode
{
    None = 0,      // No edge pinning — viewport stays at current scroll position
    Beginning = 1, // Pins to top — new items at the beginning keep viewport at the top (news feed UX)
    End = 2,       // Pins to bottom — new items at the end auto-scroll to show them (chat/log UX)
}

Virtualize<TItem>.AnchorMode — default is Beginning (matches .NET 10 behavior at the top edge).

Modes can be combined: Beginning | End pins both edges. Combining None with other modes is supported but does not change the combined value -> since None = 0 the combination None | Beginning is just Beginning.

Behavior matrix

Scenario	None	Beginning	End
Prepend at top	Viewport stable (scroll compensated)	Stays at top showing new items	Viewport stable
Append at bottom	Viewport stable	Viewport stable	Follows to new bottom
Mid-list changes	Viewport stable	Viewport stable	Viewport stable
Home/End keys	Still works	Works	Works

Changes

C# (Virtualize.cs, VirtualizeJsInterop.cs)

• New AnchorMode component parameter with [Parameter]
• _pendingScrollToBottom gated on (AnchorMode & End) != 0 instead of != None
• Dynamic AnchorMode updates: detects changes in OnAfterRenderAsync, pushes to JS via SetAnchorModeAsync
• None-mode prepend compensation: sets data-scroll-compensate attribute, JS adjusts scrollTop

JS (Virtualize.ts)

• anchorMode parameter flows into init(), gating convergence logic:
  • Top convergence (onSpacerBeforeVisible): gated on anchorMode & 1 (Beginning)
  • Bottom convergence (onSpacerAfterVisible): gated on anchorMode & 2 (End)
• setAnchorMode export for dynamic updates
• scrollEventTarget fix: uses scrollContainer ?? window for scroll unlock handler (fixes window-scrolling mode)
• Prepend compensation: scrollElement.scrollTop += spacerBefore.offsetHeight (was =, broke window-scrolling)

Contributes to #65742.

[Copilot]

Pull request overview

Adds an opt-in anchoring API to Virtualize<TItem> so apps can control whether the viewport “pins” to the beginning and/or end of a virtualized list as items are added, enabling chat/log/feed-style UX patterns.

Changes:

• Introduces VirtualizeAnchorMode (None|Beginning|End) and exposes it as Virtualize<TItem>.AnchorMode (default: Beginning).
• Extends Virtualize JS interop to accept anchorMode, gating convergence behavior and adding a None-mode prepend compensation path.
• Adds a BasicTestApp test page plus new E2E coverage for anchor mode scenarios.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
src/Components/Web/src/Virtualization/VirtualizeJsInterop.cs	Extends JS init interop to pass anchorMode (and root margin).
src/Components/Web/src/Virtualization/VirtualizeAnchorMode.cs	Adds new public [Flags] enum defining anchoring modes.
src/Components/Web/src/Virtualization/Virtualize.cs	Adds AnchorMode parameter and C#/render hooks for None-mode prepend compensation + bottom convergence suppression in None.
src/Components/Web/src/PublicAPI.Unshipped.txt	Declares the new public API surface.
src/Components/Web.JS/src/Virtualize.ts	Implements anchor-mode gated convergence and None-mode prepend compensation + suppression logic.
src/Components/test/testassets/BasicTestApp/VirtualizationAnchorMode.razor	Adds a manual test page for switching anchor mode and prepending/appending items.
src/Components/test/testassets/BasicTestApp/Index.razor	Adds navigation entry for the new test page.
src/Components/test/E2ETest/Tests/VirtualizationTest.cs	Adds E2E tests validating behavior for None/Beginning/End modes.

[ilonatommy]

All 5 variableHeight: True E2E failures are expected and will be blocked before merging, threated as "out-of-scope" for now.

[ilonatommy]

Closing in favor of #66262.