[release/11.0] Stabilize Virtualize initial item positioning

[github-actions]

Backport of #68886 to release/11.0

/cc @ilonatommy

Stabilize Virtualize initial item positioning

Prevent viewport oscillation while applying InitialItemIndex.

Description

When Virtualize or a virtualized QuickGrid initially positions a large list near its end, rendered-item measurements can repeatedly change the spacer geometry while the requested item is still being aligned. This creates a feedback loop:

measure rows -> resize spacers -> realign item -> measure rows -> ...

Depending on rendering and network timing, users can see the viewport repeatedly jump, temporarily become empty, or finish at the wrong item.

This change keeps spacer geometry fixed while the initial viewport is being filled. Once real items cover the viewport, the accumulated measured item size is applied once and the requested item is realigned against the resulting spacer geometry.

The positioning process is split into two explicit phases:

• FillingViewport: grow the rendered window and collect measurements without changing spacer geometry.
• ApplyingMeasuredGeometry: apply the accumulated measurement once and perform the final alignment.

The existing running average is preserved. The change also corrects the E2E viewport-coverage helper to compare against the scroll container's scaled content viewport instead of its outer border box, and unquarantines the existing QuickGrid regression test.

Fixes #68724

Customer Impact

Customers using InitialItemIndex with Virtualize or virtualized QuickGrid, particularly with tall containers and rendered rows whose size differs from the initial estimate, can see severe initial viewport instability (a few seconds of jumps).

The reported behavior includes repeated visible jumps, blank viewport frames, and the requested item being lost under slower Server-side rendering or network conditions. Avoiding InitialItemIndex or disabling virtualization are the only practical application-level workarounds.

Regression?

• Yes
• No

InitialItemIndex is new functionality in net11.

Risk

• High
• Medium
• Low

The change is limited to InitialItemIndex positioning and does not modify public API or normal user-driven virtualization behavior. However, it changes the internal measurement, spacer, and alignment lifecycle used by both Virtualize and QuickGrid.

Verification

• Manual (required)
• Automated

Manual verification was performed with Server-side and WebAssembly QuickGrid scenarios using provider delay, CPU throttling, and network throttling. Before the fix, the viewport visibly oscillated and could lose the target item. After the fix, the requested item remained stable while the viewport filled.

Packaging changes reviewed?

• Yes
• No
• N/A