Fix PowerOCR white flash on overlay startup and close

Use WinUIEx TransparentTintBackdrop for a truly transparent window
background, and toggle RootGrid visibility so content is hidden until
the screenshot is loaded and re-hidden before the window closes.

Co-Authored-By: Claude Opus 4 <[email protected]>

Author: Yu Leng

src/modules/PowerOCR/PowerOCR/OCROverlay.xaml

@@ -4,7 +4,7 @@
     xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
     Title="TextExtractor">
 
-    <Grid x:Name="RootGrid">
+    <Grid x:Name="RootGrid" Visibility="Collapsed">
         <Viewbox>
             <Image x:Name="BackgroundImage" Stretch="UniformToFill" />
         </Viewbox>

src/modules/PowerOCR/PowerOCR/OCROverlay.xaml.cs

@@ -122,6 +122,9 @@ public OCROverlay(RectInt32 screenRectParam, double rasterizationScaleParam)
 
         InitializeComponent();
 
+        // Transparent window background to prevent white flash during load and close
+        this.SystemBackdrop = new WinUIEx.TransparentTintBackdrop();
+
         userSettings = new UserSettings(new ThrottledActionInvoker());
 
         // Configure transparent overlay window
@@ -191,6 +194,7 @@ private async void Window_Loaded(object sender, RoutedEventArgs e)
             _capturedScreenshotBytes = null;
         }
 
+        RootGrid.Visibility = Visibility.Visible;
         DarkOverlayPath.Opacity = ActiveOpacity;
         TopButtonsStackPanel.Visibility = Visibility.Visible;
 
@@ -208,6 +212,7 @@ private async void Window_Loaded(object sender, RoutedEventArgs e)
 
     private void Window_Closed(object sender, WindowEventArgs args)
     {
+        RootGrid.Visibility = Visibility.Collapsed;
         BackgroundImage.Source = null;
         App.UntrackOverlay(this);
     }

src/modules/PowerOCR/PowerOCR/PowerOCR.csproj

@@ -31,6 +31,7 @@
     <PackageReference Include="Microsoft.WindowsAppSDK" />
     <PackageReference Include="Microsoft.Windows.SDK.BuildTools" />
     <PackageReference Include="System.Drawing.Common" />
+    <PackageReference Include="WinUIEx" />
   </ItemGroup>
 
   <ItemGroup>