Add RepositionThemeTransition to sample pages for smoother layout updates

WinUIGallery/Pages/ItemPage.xaml.cs

@@ -3,6 +3,7 @@
 
 using Microsoft.UI.Xaml;
 using Microsoft.UI.Xaml.Controls;
+using Microsoft.UI.Xaml.Media.Animation;
 using Microsoft.UI.Xaml.Navigation;
 using System;
 using System.Linq;
@@ -64,6 +65,8 @@ protected override async void OnNavigatedTo(NavigationEventArgs e)
 
                 if (pageType != null)
                 {
+                    // Attach Navigated event to add RepositionThemeTransition after navigation
+                    contentFrame.Navigated += ContentFrame_Navigated;
                     var pageName = string.IsNullOrEmpty(group.Folder) ? pageType.Name : $"{group.Folder}/{pageType.Name}";
                     pageHeader.SetControlSourceLink(WinUIBaseUrl, item.SourcePath);
                     pageHeader.SetSamplePageSourceLinks(GalleryBaseUrl, pageName);
@@ -82,6 +85,38 @@ protected override async void OnNavigatedTo(NavigationEventArgs e)
         base.OnNavigatedTo(e);
     }
 
+    private void ContentFrame_Navigated(object sender, NavigationEventArgs e)
+    {
+        if (e.Content is Page loadedPage)
+        {
+            // Add RepositionThemeTransition to the root Panel of the loaded page
+            if (loadedPage.Content is Panel panel)
+            {
+                if (panel.ChildrenTransitions == null)
+                {
+                    panel.ChildrenTransitions = new TransitionCollection();
+                }
+
+                // Insure we don't add duplicate RepositionThemeTransition
+                bool hasReposition = false;
+                foreach (var t in panel.ChildrenTransitions)
+                {
+                    if (t is RepositionThemeTransition)
+                    {
+                        hasReposition = true;
+                        break;
+                    }
+                }
+
+                if (!hasReposition)
+                {
+                    panel.ChildrenTransitions.Add(new RepositionThemeTransition());
+                }
+            }
+        }
+        contentFrame.Navigated -= ContentFrame_Navigated; // Cleanup event handler
+    }
+
     protected override void OnNavigatingFrom(NavigatingCancelEventArgs e)
     {
         SetControlExamplesTheme(ThemeHelper.ActualTheme);

WinUIGallery/Samples/ControlPages/StoragePickersPage.xaml

@@ -9,10 +9,6 @@
     mc:Ignorable="d">
 
     <StackPanel>
-        <StackPanel.ChildrenTransitions>
-            <RepositionThemeTransition/>
-        </StackPanel.ChildrenTransitions>
-
         <InfoBar IsClosable="False" IsOpen="True" Margin="0,8,0,0">
             <RichTextBlock Margin="0,12,12,12" IsTextSelectionEnabled="False">
                 <Paragraph>