Make Virtualize recover from having incorrect ItemSize info. Fixes #25915 (#26933)

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

@@ -253,6 +253,15 @@ namespace Microsoft.AspNetCore.Components.Web.Virtualization
         {
             CalcualteItemDistribution(spacerSize, spacerSeparation, containerSize, out var itemsBefore, out var visibleItemCapacity);
 
+            // Since we know the before spacer is now visible, we absolutely have to slide the window up
+            // by at least one element. If we're not doing that, the previous item size info we had must
+            // have been wrong, so just move along by one in that case to trigger an update and apply the
+            // new size info.
+            if (itemsBefore == _itemsBefore && itemsBefore > 0)
+            {
+                itemsBefore--;
+            }
+
             UpdateItemDistribution(itemsBefore, visibleItemCapacity);
         }
 
@@ -262,6 +271,15 @@ namespace Microsoft.AspNetCore.Components.Web.Virtualization
 
             var itemsBefore = Math.Max(0, _itemCount - itemsAfter - visibleItemCapacity);
 
+            // Since we know the after spacer is now visible, we absolutely have to slide the window down
+            // by at least one element. If we're not doing that, the previous item size info we had must
+            // have been wrong, so just move along by one in that case to trigger an update and apply the
+            // new size info.
+            if (itemsBefore == _itemsBefore && itemsBefore < _itemCount - visibleItemCapacity)
+            {
+                itemsBefore++;
+            }
+
             UpdateItemDistribution(itemsBefore, visibleItemCapacity);
         }
 

src/Components/test/E2ETest/Tests/VirtualizationTest.cs

@@ -2,6 +2,7 @@
 // Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
 
 using System.Linq;
+using System.Threading.Tasks;
 using BasicTestApp;
 using Microsoft.AspNetCore.Components.E2ETest.Infrastructure;
 using Microsoft.AspNetCore.Components.E2ETest.Infrastructure.ServerFixtures;
@@ -211,6 +212,33 @@ namespace Microsoft.AspNetCore.Components.E2ETest.Tests
             Browser.NotEqual(expectedInitialSpacerStyle, () => topSpacer.GetAttribute("style"));
         }
 
+        [Fact]
+        public async Task ToleratesIncorrectItemSize()
+        {
+            Browser.MountTestComponent<VirtualizationComponent>();
+            var topSpacer = Browser.Exists(By.Id("incorrect-size-container")).FindElement(By.TagName("div"));
+            var expectedInitialSpacerStyle = "height: 0px;";
+
+            // Wait until items have been rendered.
+            Browser.True(() => GetItemCount() > 0);
+            Browser.Equal(expectedInitialSpacerStyle, () => topSpacer.GetAttribute("style"));
+
+            // Scroll slowly, in increments of 50px at a time. At one point this would trigger a bug
+            // due to the incorrect item size, whereby it would not realise it's necessary to show more
+            // items because the first time the spacer became visible, the size calculation said that
+            // we're already showing all the items we need to show.
+            for (var pos = 0; pos < 1000; pos += 50)
+            {
+                Browser.ExecuteJavaScript($"document.getElementById('incorrect-size-container').scrollTop = {pos};");
+                await Task.Delay(200);
+            }
+
+            // Validate that the top spacer did change
+            Browser.NotEqual(expectedInitialSpacerStyle, () => topSpacer.GetAttribute("style"));
+
+            int GetItemCount() => Browser.FindElements(By.ClassName("incorrect-size-item")).Count;
+        }
+
         [Fact]
         public void CanMutateDataInPlace_Sync()
         {

src/Components/test/testassets/BasicTestApp/VirtualizationComponent.razor

@@ -28,6 +28,15 @@
     </div>
 </p>
 
+<p>
+    Slightly incorrect item size:<br />
+    <div id="incorrect-size-container" style="background-color: #eee; height: 500px; overflow-y: auto">
+        <Virtualize Items="@fixedItems" ItemSize="50">
+            <div @key="context" class="incorrect-size-item" style="height: 49px; background-color: rgb(@((context % 2) * 255), @((1-(context % 2)) * 255), 255);">Item @context</div>
+        </Virtualize>
+    </div>
+</p>
+
 <p id="viewport-as-root">
     Viewport as root:<br />
     <Virtualize Items="@fixedItems" ItemSize="itemSize">