Fix Page implicit and inherited style with ControlTemplate #6657 (#6661)

* Fix Page implicit and inherited style with ControlTemplate #6657

* Update StyleTests.cs
fixes #6657

Xamarin.Forms.Core.UnitTests/StyleTests.cs

@@ -759,6 +759,73 @@ public void ReplacingResourcesDoesNotOverrideManuallySetProperties()
 			Assert.That(label0.TextColor, Is.EqualTo(Color.Pink));
 			Assert.That(label1.TextColor, Is.EqualTo(Color.Lavender));
 		}
+
+		[Test]
+		public void ImplicitInheritedStyleForTemplatedElementIsAppliedCorrectlyForContentPage()
+		{
+			var controlTemplate = new ControlTemplate(typeof(ContentPresenter));
+
+			var rd0 = new ResourceDictionary {
+				new Style (typeof(ContentPage)) {
+					Setters = {
+						new Setter {Property = TemplatedPage.ControlTemplateProperty, Value = controlTemplate}
+					},
+					ApplyToDerivedTypes = true
+				}
+			};
+
+			var mockApp = new MockApplication();
+			mockApp.Resources = rd0;
+			mockApp.MainPage = new MyPage()
+			{
+				Content = new Button()
+			};
+
+			Application.Current = mockApp;
+
+			var parentPage = (ContentPage)mockApp.MainPage;
+			var pageContent = parentPage.Content;
+			Assert.That(Equals(pageContent?.Parent, parentPage));
+		}
+
+		[Test]
+		public void ImplicitInheritedStyleForTemplatedElementIsAppliedCorrectlyForContentView()
+		{
+			var controlTemplate = new ControlTemplate(typeof(ContentPresenter));
+
+			var rd0 = new ResourceDictionary {
+				new Style (typeof(ContentView)) {
+					Setters = {
+						new Setter {Property = TemplatedView.ControlTemplateProperty, Value = controlTemplate}
+					},
+					ApplyToDerivedTypes = true
+				}
+			};
+
+			var mockApp = new MockApplication();
+			mockApp.Resources = rd0;
+			mockApp.MainPage = new ContentPage()
+			{
+				Content = new MyContentView()
+				{
+					Content = new Button()
+				}
+			};
+
+			Application.Current = mockApp;
+
+			var parentView = (ContentView)((ContentPage)mockApp.MainPage).Content;
+			var content = parentView.Content;
+			Assert.That(Equals(content?.Parent, parentView));
+		}
+
+		class MyPage : ContentPage
+		{
+		}
+
+		class MyContentView : ContentView
+		{
+		}
 
 		[Test]
 		public void MismatchTargetTypeThrowsError1()
@@ -782,6 +849,6 @@ public void MatchTargetTypeDoesntThrowError()
 			var s = new Style(typeof(View));
 			var t = new Button();
 			Assert.DoesNotThrow(() => t.Style = s);
-		}
+    	}
 	}
-}
+}

Xamarin.Forms.Core/Page.cs

@@ -58,6 +58,11 @@ public Page()
 			var toolbarItems = new ObservableCollection<ToolbarItem>();
 			toolbarItems.CollectionChanged += OnToolbarItemsCollectionChanged;
 			ToolbarItems = toolbarItems;
+
+			//if things were added in base ctor (through implicit styles), the items added aren't properly parented
+			if (InternalChildren.Count > 0)
+				InternalChildrenOnCollectionChanged(this, new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Add, InternalChildren));
+
 			InternalChildren.CollectionChanged += InternalChildrenOnCollectionChanged;
 			_platformConfigurationRegistry = new Lazy<PlatformConfigurationRegistry<Page>>(() => new PlatformConfigurationRegistry<Page>(this));
 		}