Update ContentPresenter when ContentTemplate changed.

src/Avalonia.Controls/Presenters/ContentPresenter.cs

@@ -89,6 +89,7 @@ namespace Avalonia.Controls.Presenters
         static ContentPresenter()
         {
             ContentProperty.Changed.AddClassHandler<ContentPresenter>(x => x.ContentChanged);
+            ContentTemplateProperty.Changed.AddClassHandler<ContentPresenter>(x => x.ContentChanged);
             TemplatedParentProperty.Changed.AddClassHandler<ContentPresenter>(x => x.TemplatedParentChanged);
         }
 

tests/Avalonia.Controls.UnitTests/Presenters/ContentPresenterTests_InTemplate.cs

@@ -164,6 +164,32 @@ namespace Avalonia.Controls.UnitTests.Presenters
             Assert.Equal("foo", target.DataContext);
         }
 
+        [Fact]
+        public void Should_Use_ContentTemplate_If_Specified()
+        {
+            var (target, _) = CreateTarget();
+
+            target.ContentTemplate = new FuncDataTemplate<string>(_ => new Canvas());
+            target.Content = "Foo";
+
+            Assert.IsType<Canvas>(target.Child);
+        }
+
+        [Fact]
+        public void Should_Update_If_ContentTemplate_Changed()
+        {
+            var (target, _) = CreateTarget();
+
+            target.Content = "Foo";
+            Assert.IsType<TextBlock>(target.Child);
+
+            target.ContentTemplate = new FuncDataTemplate<string>(_ => new Canvas());
+            Assert.IsType<Canvas>(target.Child);
+
+            target.ContentTemplate = null;
+            Assert.IsType<TextBlock>(target.Child);
+        }
+
         [Fact]
         public void Assigning_Control_To_Content_After_NonControl_Should_Clear_DataContext()
         {