Add E2E test showing we can pass properties to child components and auto re-render them on change

test/Microsoft.AspNetCore.Blazor.E2ETest/Tests/ComponentRenderingTest.cs

@@ -113,6 +113,22 @@ namespace Microsoft.AspNetCore.Blazor.E2ETest.Tests
                 childComponentWrapper.FindElement(By.TagName("p")).Text);
         }
 
+        [Fact]
+        public void ChildComponentsRerenderWhenPropertiesChanged()
+        {
+            var appElement = MountTestComponent<CounterComponentUsingChild>();
+
+            Assert.Equal(
+                "Current count: 0",
+                appElement.FindElement(By.TagName("p")).Text);
+
+            appElement.FindElement(By.TagName("button")).Click();
+
+            Assert.Equal(
+                "Current count: 1",
+                appElement.FindElement(By.TagName("p")).Text);
+        }
+
         private IWebElement MountTestComponent<TComponent>() where TComponent: IComponent
         {
             var componentTypeName = typeof(TComponent).FullName;

test/testapps/BasicTestApp/CounterComponentUsingChild.cshtml

@@ -0,0 +1,12 @@
+<h1>Counter</h1>
+<p>Current count: <c:MessageComponent Message="@currentCount.ToString()" /></p>
+<button @onclick(IncrementCount)>Click me</button>
+
+@functions {
+    int currentCount = 0;
+
+    void IncrementCount()
+    {
+        currentCount++;
+    }
+}

test/testapps/BasicTestApp/MessageComponent.cshtml

@@ -0,0 +1,4 @@
+<span style="color: red">@Message</span>
+@functions {
+    public string Message { get; set; }
+}