Fix InitialItemIndex is intermittently ignored on fresh page load

src/Components/Web/src/Virtualization/IVirtualizeJsCallbacks.cs

@@ -5,6 +5,6 @@ namespace Microsoft.AspNetCore.Components.Web.Virtualization;
 
 internal interface IVirtualizeJsCallbacks
 {
-    void OnBeforeSpacerVisible(float spacerSize, float spacerSeparation, float containerSize);
-    void OnAfterSpacerVisible(float spacerSize, float spacerSeparation, float containerSize);
+    void OnBeforeSpacerVisible(float spacerSize, float spacerSeparation, float containerSize, SpacerVisibilityReason reason);
+    void OnAfterSpacerVisible(float spacerSize, float spacerSeparation, float containerSize, SpacerVisibilityReason reason);
 }

src/Components/Web/src/Virtualization/SpacerVisibilityReason.cs

@@ -0,0 +1,32 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace Microsoft.AspNetCore.Components.Web.Virtualization;
+
+/// <remarks>
+/// The numeric values must stay in sync with the <c>SpacerVisibilityReason</c> constant in
+/// <c>Virtualize.ts</c>.
+/// </remarks>
+internal enum SpacerVisibilityReason
+{
+    /// <summary>The user scrolled the spacer into view.</summary>
+    UserScroll = 0,
+
+    /// <summary>
+    /// A scroll the component itself performed (initial positioning, <c>ScrollToItemAsync</c>, or an
+    /// anchor restore) transiently exposed the spacer.
+    /// </summary>
+    ProgrammaticScroll = 1,
+
+    /// <summary>
+    /// The spacer is visible at rest because the rendered window does not cover the viewport, so the
+    /// window should grow toward the spacer to fill it.
+    /// </summary>
+    ViewportFill = 2,
+
+    /// <summary>
+    /// Not a real spacer-visibility event. Refines the item-size estimate from the already-rendered
+    /// window without triggering window redistribution.
+    /// </summary>
+    RenderedContentMeasurement = 3,
+}