Various virtualization improvements. Fixes #25535 (#25260)

* Use ItemSize if it's close to the calculated item size.

* Update WeatherForecastService.cs

* Improved item size calculation.

* Always use calculated item size

* Disable overflow anchoring on scroll containers (except the document itself)

* Update JS files following rebase

* Apply overflow anchor fix to document element too

* Add OverscanCount parameter

Co-authored-by: Steve Sanderson <SteveSandersonMS@users.noreply.github.com>

src/Components/Web.JS/src/Virtualize.ts

@@ -5,7 +5,7 @@ export const Virtualize = {
 
 const observersByDotNetId = {};
 
-function findClosestScrollContainer(element: Element | null): Element | null {
+function findClosestScrollContainer(element: HTMLElement | null): HTMLElement | null {
   if (!element) {
     return null;
   }
@@ -20,8 +20,14 @@ function findClosestScrollContainer(element: Element | null): Element | null {
 }
 
 function init(dotNetHelper: any, spacerBefore: HTMLElement, spacerAfter: HTMLElement, rootMargin = 50): void {
+  // Overflow anchoring can cause an ongoing scroll loop, because when we resize the spacers, the browser
+  // would update the scroll position to compensate. Then the spacer would remain visible and we'd keep on
+  // trying to resize it.
+  const scrollContainer = findClosestScrollContainer(spacerBefore);
+  (scrollContainer || document.documentElement).style.overflowAnchor = 'none';
+
   const intersectionObserver = new IntersectionObserver(intersectionCallback, {
-    root: findClosestScrollContainer(spacerBefore),
+    root: scrollContainer,
     rootMargin: `${rootMargin}px`,
   });
 
@@ -57,7 +63,9 @@ function init(dotNetHelper: any, spacerBefore: HTMLElement, spacerAfter: HTMLEle
         return;
       }
 
-      const spacerSeparation = spacerAfter.offsetTop - (spacerBefore.offsetTop + spacerBefore.offsetHeight);
+      const spacerBeforeRect = spacerBefore.getBoundingClientRect();
+      const spacerAfterRect = spacerAfter.getBoundingClientRect();
+      const spacerSeparation = spacerAfterRect.top - spacerBeforeRect.bottom;
       const containerSize = entry.rootBounds?.height;
 
       if (entry.target === spacerBefore) {

src/Components/Web/src/Virtualization/Virtualize.cs

@@ -88,6 +88,15 @@ namespace Microsoft.AspNetCore.Components.Web.Virtualization
         [Parameter]
         public ICollection<TItem>? Items { get; set; }
 
+        /// <summary>
+        /// Gets or sets a value that determines how many additional items will be rendered
+        /// before and after the visible region. This help to reduce the frequency of rendering
+        /// during scrolling. However, higher values mean that more elements will be present
+        /// in the page.
+        /// </summary>
+        [Parameter]
+        public int OverscanCount { get; set; } = 3;
+
         /// <inheritdoc />
         protected override void OnParametersSet()
         {
@@ -251,8 +260,8 @@ namespace Microsoft.AspNetCore.Components.Web.Virtualization
                 _itemSize = ItemSize;
             }
 
-            itemsInSpacer = Math.Max(0, (int)Math.Floor(spacerSize / _itemSize) - 1);
-            visibleItemCapacity = (int)Math.Ceiling(containerSize / _itemSize) + 2;
+            itemsInSpacer = Math.Max(0, (int)Math.Floor(spacerSize / _itemSize) - OverscanCount);
+            visibleItemCapacity = (int)Math.Ceiling(containerSize / _itemSize) + 2 * OverscanCount;
         }
 
         private void UpdateItemDistribution(int itemsBefore, int visibleItemCapacity)