Omit omissible frames when updating rendertree to match bind event. Fixes #24014 (#26273)

src/Components/Components/src/Rendering/RenderTreeBuilder.cs

@@ -667,15 +667,16 @@ namespace Microsoft.AspNetCore.Components.Rendering
 
         // internal because this should only be used during the post-event tree patching logic
         // It's expensive because it involves copying all the subsequent memory in the array
-        internal void InsertAttributeExpensive(int insertAtIndex, int sequence, string attributeName, object? attributeValue)
+        internal bool InsertAttributeExpensive(int insertAtIndex, int sequence, string attributeName, object? attributeValue)
         {
             // Replicate the same attribute omission logic as used elsewhere
             if ((attributeValue == null) || (attributeValue is bool boolValue && !boolValue))
             {
-                return;
+                return false;
             }
 
             _entries.InsertExpensive(insertAtIndex, RenderTreeFrame.Attribute(sequence, attributeName, attributeValue));
+            return true;
         }
 
         /// <summary>

src/Components/Components/src/Rendering/RenderTreeUpdater.cs

@@ -75,7 +75,14 @@ namespace Microsoft.AspNetCore.Components.Rendering
 
             // If we get here, we didn't find the desired attribute, so we have to insert a new frame for it
             var insertAtIndex = elementFrameIndex + 1;
-            renderTreeBuilder.InsertAttributeExpensive(insertAtIndex, RenderTreeDiffBuilder.SystemAddedAttributeSequenceNumber, attributeName, attributeValue);
+            var didInsertFrame = renderTreeBuilder.InsertAttributeExpensive(insertAtIndex, RenderTreeDiffBuilder.SystemAddedAttributeSequenceNumber, attributeName, attributeValue);
+            if (!didInsertFrame)
+            {
+                // The builder decided to omit the new frame, e.g., because it's a false-valued bool
+                // In this case there's nothing else to update
+                return;
+            }
+
             framesArray = renderTreeBuilder.GetFrames().Array; // Refresh in case it mutated due to the expansion
 
             // Update subtree length for this and all ancestor containers

src/Components/Components/test/RenderTreeUpdaterTest.cs

@@ -127,6 +127,30 @@ namespace Microsoft.AspNetCore.Components.Test
                 frame => AssertFrame.Attribute(frame, "eventname", v => Assert.IsType<Action>(v), 1));
         }
 
+        [Fact]
+        public void OmitsAttributeIfNotFoundButValueIsOmissible()
+        {
+            // Arrange
+            var valuePropName = "testprop";
+            var renderer = new TestRenderer();
+            var builder = new RenderTreeBuilder();
+            builder.OpenElement(0, "elem");
+            builder.AddAttribute(1, "eventname", (Action)(() => { }));
+            builder.SetUpdatesAttributeName(valuePropName);
+            builder.CloseElement();
+            var frames = builder.GetFrames();
+            frames.Array[1] = frames.Array[1].WithAttributeEventHandlerId(123);
+
+            // Act
+            RenderTreeUpdater.UpdateToMatchClientState(builder, 123, false);
+            frames = builder.GetFrames();
+
+            // Assert
+            Assert.Collection(frames.AsEnumerable(),
+                frame => AssertFrame.Element(frame, "elem", 2, 0),
+                frame => AssertFrame.Attribute(frame, "eventname", v => Assert.IsType<Action>(v), 1));
+        }
+
         [Fact]
         public void ExpandsAllAncestorsWhenAddingAttribute()
         {