Fixes #3673 and resolves issues with some samples not working correctly

Microsoft.Toolkit.Uwp.SampleApp/Pages/SampleController.xaml.cs

@@ -195,8 +195,8 @@ namespace Microsoft.Toolkit.Uwp.SampleApp
                 {
                     try
                     {
-                        var pageInstance = Activator.CreateInstance(CurrentSample.PageType);
-                        SampleContent.Content = pageInstance;
+                        SamplePage = Activator.CreateInstance(CurrentSample.PageType) as Page;
+                        SampleContent.Content = SamplePage;
 
                         // Some samples use the OnNavigatedTo and OnNavigatedFrom
                         // Can't use Frame here because some samples depend on the current Frame
@@ -206,7 +206,7 @@ namespace Microsoft.Toolkit.Uwp.SampleApp
 
                         if (method != null)
                         {
-                            method.Invoke(pageInstance, new object[] { e });
+                            method.Invoke(SamplePage, new object[] { e });
                         }
                     }
                     catch
@@ -348,6 +348,8 @@ namespace Microsoft.Toolkit.Uwp.SampleApp
 
         private void SamplePage_Loaded(object sender, RoutedEventArgs e)
         {
+            SamplePage.Loaded -= SamplePage_Loaded;
+
             if (CurrentSample != null && CurrentSample.HasXAMLCode)
             {
                 _lastRenderedProperties = true;
@@ -516,17 +518,23 @@ namespace Microsoft.Toolkit.Uwp.SampleApp
                 if (CurrentSample.HasType)
                 {
                     root = SamplePage?.FindDescendantByName("XamlRoot");
-                }
 
-                if (root is Panel)
-                {
-                    // If we've defined a 'XamlRoot' element to host us as a panel, use that.
-                    (root as Panel).Children.Clear();
-                    (root as Panel).Children.Add(element);
+                    if (root is Panel)
+                    {
+                        // If we've defined a 'XamlRoot' element to host us as a panel, use that.
+                        (root as Panel).Children.Clear();
+                        (root as Panel).Children.Add(element);
+                    }
+                    else
+                    {
+                        // if we didn't find a XamlRoot host, then we replace the entire content of
+                        // the provided sample page with the XAML.
+                        SamplePage.Content = element;
+                    }
                 }
                 else
                 {
-                    // Otherwise, just replace the entire page's content
+                    // Otherwise, just replace our entire presenter's content
                     SampleContent.Content = element;
                 }
 
@@ -675,7 +683,8 @@ namespace Microsoft.Toolkit.Uwp.SampleApp
             }
         }
 
-        private Page SamplePage => SampleContent.Content as Page;
+        // The Loaded Instance of the backing .xaml.cs Page (if any)
+        private Page SamplePage { get; set; }
 
         private bool CanChangePaneState => !_onlyDocumentation;