Expose InitialItemIndex parameter and ScrollToItemAsync method on QuickGrid

[ilonatommy]

Summary

Adds two public, non-experimental APIs to QuickGrid<TGridItem> that forward to its inner Virtualize component:

• InitialItemIndex parameter - scrolls to the given row on first interactive render (clamped, applied once).
• ScrollToItemAsync(int itemIndex, CancellationToken) method - programmatically scrolls to a row, aligning it to the top. Last call wins; throws InvalidOperationException when Virtualize is disabled or the grid isn't rendered yet.

These mirror the existing Virtualize.InitialItemIndex / Virtualize.ScrollToItemAsync APIs from #66328.

API surface

namespace Microsoft.AspNetCore.Components.QuickGrid;

public partial class QuickGrid<TGridItem> : ComponentBase
{
    // Applies only when Virtualize is true.
    [Parameter]
    public int InitialItemIndex { get; set; }

    public Task ScrollToItemAsync(int itemIndex, CancellationToken cancellationToken = default);
}

Usage example

<QuickGrid @ref="_grid" Items="@people" Virtualize="true" ItemSize="50" InitialItemIndex="500">
    <PropertyColumn Property="@(p => p.Name)" />
    <PropertyColumn Property="@(p => p.Age)" />
</QuickGrid>

<button @onclick="() => _grid.ScrollToItemAsync(500)">Jump to 500</button>

@code {
    QuickGrid<Person> _grid = default!;
}

[Copilot]

Pull request overview

This PR exposes InitialItemIndex and ScrollToItemAsync as public, non-experimental APIs on QuickGrid<TGridItem> by forwarding to the underlying Virtualize component, and adds new BasicTestApp + E2E coverage to validate the behavior end-to-end.

Changes:

• Added QuickGrid<TGridItem>.InitialItemIndex parameter and QuickGrid<TGridItem>.ScrollToItemAsync(...) method, forwarding to Virtualize.
• Forwarded InitialItemIndex from QuickGrid to the inner <Virtualize> instance.
• Added a new BasicTestApp component and new E2E tests covering scrolling/initial-index behaviors (including variable height, provider delay, rapid calls, and cancellation).

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/Components/test/testassets/BasicTestApp/QuickGridTest/QuickGridScrollComponent.razor	New test asset component exercising QuickGrid’s scroll APIs in a scrollable container.
src/Components/test/testassets/BasicTestApp/Index.razor	Adds the new QuickGrid scroll test component to the test asset menu.
src/Components/test/E2ETest/Tests/VirtualizationTest.cs	Adds E2E coverage for QuickGrid scroll-to-item and initial-index scenarios.
src/Components/QuickGrid/Microsoft.AspNetCore.Components.QuickGrid/src/QuickGrid.razor.cs	Adds the new public parameter and method (and validation) to QuickGrid.
src/Components/QuickGrid/Microsoft.AspNetCore.Components.QuickGrid/src/QuickGrid.razor	Forwards InitialItemIndex into the inner Virtualize component.
src/Components/QuickGrid/Microsoft.AspNetCore.Components.QuickGrid/src/PublicAPI.Unshipped.txt	Declares the newly added public API surface for API review tooling.

[dariatiurina]

Looks great!