Fix app title bar gap beneath the caption buttons

WinUIGallery/MainWindow.xaml.cs

@@ -32,12 +32,46 @@ public NavigationView NavigationView
 
     public Action NavigationViewLoaded { get; set; }
 
+#nullable enable
+    private OverlappedPresenter? WindowPresenter { get; }
+#nullable disable
+
+    private OverlappedPresenterState CurrentWindowState { get; set; }
+
     public MainWindow()
     {
         this.InitializeComponent();
         SetWindowProperties();
         RootGrid.ActualThemeChanged += (_, _) => TitleBarHelper.ApplySystemThemeToCaptionButtons(this, RootGrid.ActualTheme);
         dispatcherQueue = Microsoft.UI.Dispatching.DispatcherQueue.GetForCurrentThread();
+
+        // Workaround for WinUI issue #9934:
+        // https://github.com/microsoft/microsoft-ui-xaml/issues/9934.
+        // See `AdjustNavigationViewMargin()` for implementation details.
+        if (AppWindow.Presenter is OverlappedPresenter windowPresenter)
+        {
+            WindowPresenter = windowPresenter;
+            CurrentWindowState = WindowPresenter.State;
+            AdjustNavigationViewMargin(force: true);
+            AppWindow.Changed += (_, _) => AdjustNavigationViewMargin();
+        }
+    }
+
+    // Adjusts the NavigationView margin based on the window state
+    // to fix the gap between the caption buttons and the NavigationView.
+    // Use `force = true` to ensure adjustment on the first call.
+    private void AdjustNavigationViewMargin(bool? force = null)
+    {
+        if (WindowPresenter is null ||
+            (WindowPresenter.State == CurrentWindowState && force is not true))
+        {
+            return;
+        }
+
+        NavigationView.Margin = WindowPresenter.State == OverlappedPresenterState.Maximized
+            ? new Thickness(0, -1, 0, 0)
+            : new Thickness(0, -2, 0, 0);
+        CurrentWindowState = WindowPresenter.State;
     }
 
     private void RootGrid_Loaded(object sender, RoutedEventArgs e)