Move SelectedItem setting for TabbedCommandBar to mitigate binding timing issue

Author: michael-hawker

Microsoft.Toolkit.Uwp.UI.Controls.Core/TabbedCommandBar/TabbedCommandBar.cs

@@ -40,6 +40,7 @@ public TabbedCommandBar()
             DefaultStyleResourceUri = new System.Uri("ms-appx:///Microsoft.Toolkit.Uwp.UI.Controls.Core/Themes/Generic.xaml");
 
             SelectionChanged += SelectedItemChanged;
+            Loaded += TabbedCommandBar_Loaded;
         }
 
         /// <inheritdoc/>
@@ -57,6 +58,15 @@ protected override void OnApplyTemplate()
             _tabbedCommandBarContentBorder = GetTemplateChild("PART_TabbedCommandBarContentBorder") as Border;
             _tabChangedStoryboard = GetTemplateChild("TabChangedStoryboard") as Storyboard;
 
+            // TODO: We could maybe optimize and use a lower-level Loaded event for what's hosting the MenuItems
+            // to set SelectedItem, but then we may have to pull in another template part, so think we're OK
+            // to do the Loaded event at the top level.
+        }
+
+        private void TabbedCommandBar_Loaded(object sender, RoutedEventArgs e)
+        {
+            // We need to select the item after the template is realized, otherwise the SelectedItem's
+            // DataTemplate bindings don't properly navigate the visual tree.
             SelectedItem = MenuItems.FirstOrDefault();
         }