Expose experimental AnchorMode and ItemComparer on QuickGrid

[ilonatommy]

Adds two experimental parameters to QuickGrid<TGridItem> that let virtualized grids control viewport anchoring when data changes at the edges of the list:

• AnchorMode (VirtualizeAnchorMode, default Start) - controls how the viewport behaves when new items arrive during virtualization (mirrors the existing Virtualize.AnchorMode).
• ItemComparer (IEqualityComparer<TGridItem>?) - useful for reference types, lets the grid detect whether items were prepended or appended between data loads. Without it, anchoring across data changes might not work properly.

Both are marked [Experimental("ASP0030")], so consumers must explicitly opt in.

Virtualize already supports edge anchoring via AnchorMode/ItemComparer, but QuickGrid (which wraps Virtualize internally) only exposed the default Start behavior and provided no way to enable anchoring for ItemsProvider-backed grids. This closes that gap for scenarios like chat/log tails and infinite-scroll grids.

API surface

namespace Microsoft.AspNetCore.Components.QuickGrid;

public partial class QuickGrid<TGridItem> : ComponentBase
{
    [Parameter]
    [Experimental("ASP0030", UrlFormat = "https://aka.ms/aspnet/analyzer/{0}")]
    public VirtualizeAnchorMode AnchorMode { get; set; } = VirtualizeAnchorMode.Start;

    [Parameter]
    [Experimental("ASP0030", UrlFormat = "https://aka.ms/aspnet/analyzer/{0}")]
    public IEqualityComparer<TGridItem>? ItemComparer { get; set; }
}

Usage example

@* Chat-style log: pin to the bottom so newly appended rows auto-scroll into view,
   but disengage once the user scrolls up to read history. *@
<QuickGrid Items="@_messages" Virtualize="true" ItemSize="50"
           AnchorMode="VirtualizeAnchorMode.End"
           ItemComparer="_byId">
    <PropertyColumn Property="@(m => m.Timestamp)" />
    <PropertyColumn Property="@(m => m.Text)" />
</QuickGrid>

@code {
    IQueryable<Message> _messages = default!;

    // Provider returns fresh instances each load, so compare by a stable key
    // to correctly tell prepends from appends.
    static readonly IEqualityComparer<Message> _byId =
        EqualityComparer<Message>.Create((a, b) => a!.Id == b!.Id, m => m.Id.GetHashCode());
}

Test coverage summary

where 🆕 is a added by this PR, ❌ is failing currently. Tests on 5 runs, result is failures/total:

Op · Position · Source	None	Start	End
⬆️ · Exact top · Items	V ✅ · QG 🆕 ✅ 0/5	V ✅ · QG 🆕 ✅ 0/5	V ✅ · QG 🆕 ✅ 0/5
⬆️ · Exact top · Prov	V ✅ · QG 🆕 ❌ 1/5	V ✅ · QG 🆕 ✅ 0/5	V ✅ · QG 🆕 ❌ 1/5
⬆️ · Exact top · Prov+delay	V n/a · QG 🆕 ❌ 4/5	V n/a · QG 🆕 ✅ 0/5	V n/a · QG 🆕 ❌ 4/5
⬆️ · Near-top ~200px · Items	V ✅ · QG 🆕 ✅ 0/5	V ✅ · QG 🆕 ✅ 0/5	V ✅ · QG 🆕 ✅ 0/5
⬆️ · Near-top ~200px · Prov	V ✅ · QG 🆕 ❌ 1/5	V ✅ · QG 🆕 ❌ 2/5	V ✅ · QG 🆕 ❌ 1/5
⬆️ · Near-top ~200px · Prov+delay	V n/a · QG 🆕 ❌ 5/5	V n/a · QG 🆕 ❌ 3/5	V n/a · QG 🆕 ❌ 4/5
⬆️ · Mid-list · Items	V ✅ · QG 🆕 ✅ 0/5	V ✅ · QG 🆕 ✅ 0/5	V ✅ · QG 🆕 ✅ 0/5
⬆️ · Mid-list · Prov	V ✅ · QG 🆕 ❌ 2/5	V ✅ · QG 🆕 ❌ 2/5	V ✅ · QG 🆕 ❌ 4/5
⬆️ · Mid-list · Prov+delay	V ✅ · QG 🆕 ❌ 5/5 (None_AsyncProvider [Fact])	—	—
⬆️ · Bottom · Items	V ✅ · QG 🆕 ✅ 0/5	V ✅ · QG 🆕 ✅ 0/5	V ✅ · QG 🆕 ✅ 0/5
⬆️ · Bottom · Prov	V ✅ · QG 🆕 ❌ 2/5	V ✅ · QG 🆕 ❌ 4/5	V ✅ · QG 🆕 ❌ 1/5
⬆️ · After leaving top · Items	—	V ✅ · QG 🆕 ✅ 0/5	—
⬆️ · After leaving top · Prov	—	V ✅ · QG 🆕 ✅ 0/5	—
⬇️ · Near-top · Items	V ✅ · QG 🆕 ✅ 0/5	V ✅ · QG 🆕 ✅ 0/5	V ✅ · QG 🆕 ✅ 0/5
⬇️ · Near-top · Prov	V ✅ · QG 🆕 ✅ 0/5	V ✅ · QG 🆕 ✅ 0/5	V ✅ · QG 🆕 ✅ 0/5
⬇️ · Top · Items	V ✅ · QG 🆕 ✅ 0/5	V ✅ · QG 🆕 ✅ 0/5	V ✅ · QG 🆕 ✅ 0/5
⬇️ · Top · Prov	V ✅ · QG 🆕 ✅ 0/5	V ✅ · QG 🆕 ✅ 0/5	V ✅ · QG 🆕 ❌ 1/6
⬇️ · Mid-list · Items	V ✅ · QG 🆕 ✅ 0/5	V ✅ · QG 🆕 ✅ 0/5	V ✅ · QG 🆕 ✅ 0/5
⬇️ · Mid-list · Prov	V ✅ · QG 🆕 ✅ 0/5	V ✅ · QG 🆕 ✅ 0/5	V ✅ · QG 🆕 ✅ 0/5
⬇️ · Bottom · Items	V ✅ · QG 🆕 ✅ 0/5	V ✅ · QG 🆕 ✅ 0/5	V ✅ · QG 🆕 ✅ 0/5
⬇️ · Bottom · Prov	V ✅ · QG 🆕 ✅ 0/5	V ✅ · QG 🆕 ✅ 0/5	V ✅ · QG 🆕 ✅ 0/5
⬇️ · After leaving bottom · Items	—	—	V ✅ · QG 🆕 ✅ 0/5
⬇️ · After leaving bottom · Prov	—	—	V ✅ · QG 🆕 ✅ 0/5
⬇️ LARGE (100) · Bottom · Items	—	V ✅ · QG 🆕 ❌ WASM	V ✅ · QG 🆕 ✅ 0/5
⬇️ LARGE (100) · Bottom · Prov	—	V ✅ · QG 🆕 ✅ 0/5	V ✅ · QG 🆕 ✅ 0/5
🔄 (fast, continuous) · no backward flash · Prov+delay	V ✅ · QG 🆕 ✅ 0/5	—	—
⌨️ Home key → top · Items/Prov	— · QG 🆕 ✅ 0/5	— · QG 🆕 ✅ 0/5	— · QG 🆕 ✅ 0/5
⌨️ End key → bottom · Items/Prov	— · QG 🆕 ✅ 0/5	— · QG 🆕 ✅ 0/5	— · QG 🆕 ❌ 1/6 (CI/Mono)

Behavioral changes in comparison to net10 - no regression of the default `Start` mode without `ItemComparer`

Test family	Source	net11 (Start)	net10 (default)	net10 fail type	Evidence
NearTop_Prepend	Items	❌	❌	SHIFT	AnchorMode 1 near top: viewport should stay stable after prepend (index before: 3, after: -7, relTop before: -44.65625, after: -44.65625, scrollTop: 240, tolerance: 2)
NearTop_Prepend	Provider	❌ (2/5)	❌	SHIFT	AnchorMode 1 near top: viewport should stay stable after prepend (index before: 3, after: -7, relTop before: -44.65625, after: -44.65625, scrollTop: 240, tolerance: 2)
NearTop_Prepend (delay)	Provider	❌ (3/5)	❌	SHIFT	AnchorMode 1 near top: viewport should stay stable after prepend (index before: 3, after: -7, relTop before: -44.65625, after: -44.65625, scrollTop: 240, tolerance: 2)
NearTop_Append	Items	✅	✅		—
NearTop_Append	Provider	✅	✅		—
Top_Append	Items	✅	✅		—
Top_Append	Provider	✅	✅		—
Bottom_Prepend	Items	✅	❌	STUCK	Visible items should be rendered after scrolling to bottom
Bottom_Prepend	Provider	❌ (4/5)	❌	STUCK	Visible items should be rendered after scrolling to bottom
Start_PrependAtTop_NewItemsVisible	Items	✅	✅		—
Start_PrependAtTop_NewItemsVisible	Provider	✅	✅		—
Start_PrependAtTop_NewItemsVisible (delay)	Provider	✅	✅		—
Start_PrependAfterLeavingTop_DoesNotReengage	Items	✅	✅		—
Start_PrependAfterLeavingTop_DoesNotReengage	Provider	✅	✅		—
Start_MidList_Prepend	Items	✅	❌	SHIFT	Start mode mid-list: viewport should stay stable after prepend (index before: 73, after: 63, relTop before: -21.53125, after: -21.53125, scrollTop: 4080, tolerance: 0)
Start_MidList_Prepend	Provider	❌ (2/5)	❌	SHIFT	Start mode mid-list: viewport should stay stable after prepend (index before: 73, after: 63, relTop before: -21.53125, after: -21.53125, scrollTop: 4080, tolerance: 0)
Start_LargeAppendAtBottom_DoesNotFollow	Items	✅	❌	STUCK	Visible items should be rendered after scrolling to bottom
Start_LargeAppendAtBottom_DoesNotFollow	Provider	✅	❌	STUCK	Visible items should be rendered after scrolling to bottom
MidList_Append	Items	✅	✅		—
MidList_Append	Provider	✅	✅		—
Bottom_Append	Items	✅	❌	STUCK	Visible items should be rendered after scrolling to bottom
Bottom_Append	Provider	✅	❌	STUCK	Visible items should be rendered after scrolling to bottom
HomeKeyJumpsToTop	Items	✅	✅		—
HomeKeyJumpsToTop	Provider	✅	✅		—
EndKeyJumpsToBottom	Items	✅	✅		—
EndKeyJumpsToBottom	Provider	✅	❌	OTHER	AnchorMode 1: End key should jump to the bottom

[Copilot]

Pull request overview

This PR exposes QuickGrid’s virtualization anchoring controls by adding two experimental parameters—AnchorMode and ItemComparer—and forwarding them to the internal Virtualize usage, enabling stable viewport behavior when items are prepended/appended during virtualized data loads.

Changes:

• Add experimental AnchorMode (VirtualizeAnchorMode) and ItemComparer (IEqualityComparer<TGridItem>?) parameters to QuickGrid<TGridItem>.
• Forward these parameters to the underlying Virtualize instance via an attribute-splat dictionary.
• Add a new BasicTestApp test component plus expanded E2E coverage for QuickGrid anchoring behaviors; document new analyzer diagnostic ASP0030.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 6 comments.

Show a summary per file

File	Description
src/Components/test/testassets/BasicTestApp/QuickGridTest/QuickGridAnchorModeComponent.razor	New test component exercising QuickGrid anchoring modes with Items and ItemsProvider.
src/Components/test/testassets/BasicTestApp/Index.razor	Adds navigation entry for the new anchoring test component.
src/Components/test/testassets/BasicTestApp/BasicTestApp.csproj	Suppresses ASP0030 for the test asset that intentionally uses experimental APIs.
src/Components/test/E2ETest/Tests/VirtualizationTest.cs	Adds E2E tests validating QuickGrid anchoring/viewport stability across modes and mutations.
src/Components/QuickGrid/Microsoft.AspNetCore.Components.QuickGrid/src/QuickGrid.razor.cs	Introduces AnchorMode/ItemComparer parameters and builds Virtualize attribute-splat values.
src/Components/QuickGrid/Microsoft.AspNetCore.Components.QuickGrid/src/QuickGrid.razor	Applies the new Virtualize attributes via @attributes.
src/Components/QuickGrid/Microsoft.AspNetCore.Components.QuickGrid/src/PublicAPI.Unshipped.txt	Declares new public API surface for the added parameters.
docs/list-of-diagnostics.md	Updates analyzer diagnostic range and adds ASP0030 description.

[dariatiurina]

I will take a look at tests later, but for now here are the notes: