[Components][E2E] Stabilize virtualization test setup after scroll

[ilonatommy]

Summary

• stabilize the setup phase for the server virtualization prepend/append E2E tests
• keep the actual viewport-preservation assertions unchanged
• wait until the virtualized window has really advanced past overscan before starting the real check

Why

The intermittent failure has been happening in the preparation stage, where the test sets scrollTop = 5000 and then immediately expects the rendered range to have moved beyond the overscan window. In server mode that can race with the async virtualization update, so the test can fail before it even reaches the real prepend assertion.

Test

dotnet test .\src\Components\test\E2ETest\Microsoft.AspNetCore.Components.E2ETests.csproj --no-build --no-restore --filter "FullyQualifiedName~ServerVirtualizationTest.DynamicContent_"

[Copilot]

Pull request overview

Stabilizes the server-side virtualization dynamic content E2E tests by making the “scroll past overscan” setup phase wait for the rendered window to actually advance before running the viewport-preservation assertions.

Changes:

• Added a shared helper to scroll to a target offset and poll until the first rendered item index advances beyond a threshold, then wait for scroll stabilization.
• Updated the prepend/append dynamic content virtualization tests to use the new stabilized scroll helper (keeping the core assertions unchanged).