Fix titlebar color when using backdrop in WPF Window

src/Wpf.Ui/Appearance/WindowBackgroundManager.cs

@@ -100,6 +100,7 @@ WindowBackdropType backdrop
         }
 
         _ = WindowBackdrop.ApplyBackdrop(window, backdrop);
+
         if (applicationTheme is ApplicationTheme.Dark)
         {
             ApplyDarkThemeToWindow(window);
@@ -109,6 +110,8 @@ WindowBackdropType backdrop
             RemoveDarkThemeFromWindow(window);
         }
 
+        _ = WindowBackdrop.RemoveTitlebarBackground(window);
+
         foreach (object? subWindow in window.OwnedWindows)
         {
             if (subWindow is Window windowSubWindow)
@@ -123,6 +126,8 @@ WindowBackdropType backdrop
                 {
                     RemoveDarkThemeFromWindow(windowSubWindow);
                 }
+
+                _ = WindowBackdrop.RemoveTitlebarBackground(window);
             }
         }
     }

src/Wpf.Ui/Controls/FluentWindow/FluentWindow.cs

@@ -199,6 +199,8 @@ protected virtual void OnBackdropTypeChanged(WindowBackdropType oldValue, Window
         if (WindowBackdrop.IsSupported(newValue) && WindowBackdrop.RemoveBackground(this))
         {
             _ = WindowBackdrop.ApplyBackdrop(this, newValue);
+
+            _ = WindowBackdrop.RemoveTitlebarBackground(this);
         }
     }
 

src/Wpf.Ui/Controls/VirtualizingWrapPanel/VirtualizingWrapPanel.cs

@@ -425,7 +425,8 @@ protected override ItemRange UpdateItemRange()
                 int rowCountInCacheBefore = (int)(cacheBeforeInPixel / GetHeight(ChildSize));
                 int rowCountInCacheAfter =
                     (
-                        (int)Math.Ceiling(
+                        (int)
+                            Math.Ceiling(
                                 (offsetInPixel + viewportHeight + cacheAfterInPixel) / GetHeight(ChildSize)
                             )
                     ) - (int)Math.Ceiling((offsetInPixel + viewportHeight) / GetHeight(ChildSize));

src/Wpf.Ui/Controls/Window/WindowBackdrop.cs

@@ -38,7 +38,7 @@ public static bool IsSupported(WindowBackdropType backdropType)
     /// <param name="window">The window to which the backdrop effect will be applied.</param>
     /// <param name="backdropType">The type of backdrop effect to apply. Determines the visual appearance of the window's backdrop.</param>
     /// <returns><see langword="true"/> if the operation was successful; otherwise, <see langword="false"/>.</returns>
-    public static bool ApplyBackdrop(System.Windows.Window window, WindowBackdropType backdropType)
+    public static bool ApplyBackdrop(System.Windows.Window? window, WindowBackdropType backdropType)
     {
         if (window is null)
         {
@@ -216,6 +216,40 @@ public static bool RemoveBackground(System.Windows.Window? window)
         return true;
     }
 
+    public static bool RemoveTitlebarBackground(System.Windows.Window? window)
+    {
+        if (window is null)
+        {
+            return false;
+        }
+
+        IntPtr windowHandle = new WindowInteropHelper(window).Handle;
+
+        if (windowHandle == IntPtr.Zero)
+        {
+            return false;
+        }
+
+        HwndSource? windowSource = HwndSource.FromHwnd(windowHandle);
+
+        // Remove background from client area
+        if (windowSource?.Handle != IntPtr.Zero && windowSource?.CompositionTarget != null)
+        {
+            // NOTE: https://learn.microsoft.com/en-us/windows/win32/api/dwmapi/ne-dwmapi-dwmwindowattribute
+            // Specifying DWMWA_COLOR_DEFAULT (value 0xFFFFFFFF) for the color will reset the window back to using the system's default behavior for the caption color.
+            uint titlebarPvAttribute = 0xFFFFFFFE;
+
+            Dwmapi.DwmSetWindowAttribute(
+                windowSource.Handle,
+                Dwmapi.DWMWINDOWATTRIBUTE.DWMWA_CAPTION_COLOR,
+                ref titlebarPvAttribute,
+                Marshal.SizeOf(typeof(uint))
+            );
+        }
+
+        return true;
+    }
+
     private static bool ApplyDwmwWindowAttrubute(IntPtr hWnd, Dwmapi.DWMSBT dwmSbt)
     {
         if (hWnd == IntPtr.Zero)
@@ -228,7 +262,7 @@ private static bool ApplyDwmwWindowAttrubute(IntPtr hWnd, Dwmapi.DWMSBT dwmSbt)
             return false;
         }
 
-        var backdropPvAttribute = (int)dwmSbt;
+        int backdropPvAttribute = (int)dwmSbt;
 
         var dwmApiResult = Dwmapi.DwmSetWindowAttribute(
             hWnd,

src/Wpf.Ui/Interop/Dwmapi.cs

@@ -619,6 +619,22 @@ public static extern int DwmSetWindowAttribute(
         [In] int cbAttribute
     );
 
+    /// <summary>
+    /// Sets the value of Desktop Window Manager (DWM) non-client rendering attributes for a window.
+    /// </summary>
+    /// <param name="hWnd">The handle to the window for which the attribute value is to be set.</param>
+    /// <param name="dwAttribute">A flag describing which value to set, specified as a value of the DWMWINDOWATTRIBUTE enumeration.</param>
+    /// <param name="pvAttribute">A pointer to an object containing the attribute value to set.</param>
+    /// <param name="cbAttribute">The size, in bytes, of the attribute value being set via the <c>pvAttribute</c> parameter.</param>
+    /// <returns>If the function succeeds, it returns <c>S_OK</c>. Otherwise, it returns an <c>HRESULT</c> error code.</returns>
+    [DllImport(Libraries.Dwmapi)]
+    public static extern int DwmSetWindowAttribute(
+        [In] IntPtr hWnd,
+        [In] DWMWINDOWATTRIBUTE dwAttribute,
+        [In] ref uint pvAttribute,
+        [In] int cbAttribute
+    );
+
     /// <summary>
     /// Retrieves the current value of a specified Desktop Window Manager (DWM) attribute applied to a window. For programming guidance, and code examples, see Controlling non-client region rendering.
     /// </summary>

src/Wpf.Ui/Interop/UnsafeNativeMethods.cs

@@ -376,9 +376,7 @@ public static Color GetDwmColor()
 
                     return Color.FromArgb(255, values[2], values[1], values[0]);
                 }
-                catch
-                {
-                }
+                catch { }
             }
         }
 

src/Wpf.Ui/Markup/Design.cs

@@ -28,10 +28,12 @@ public static class Design
     /// </summary>
     private static bool InDesignMode =>
         _inDesignMode ??=
-            (bool)DependencyPropertyDescriptor
+            (bool)
+                DependencyPropertyDescriptor
                     .FromProperty(DesignerProperties.IsInDesignModeProperty, typeof(FrameworkElement))
                     .Metadata.DefaultValue
-            || System.Diagnostics.Process.GetCurrentProcess()
+            || System
+                .Diagnostics.Process.GetCurrentProcess()
                 .ProcessName.StartsWith(DesignProcessName, StringComparison.Ordinal);
 
     public static readonly DependencyProperty BackgroundProperty = DependencyProperty.RegisterAttached(

src/Wpf.Ui/UiApplication.cs

@@ -36,8 +36,8 @@ public UiApplication(Application application)
         _application = application;
 
         System.Diagnostics.Debug.WriteLine(
-                $"INFO | {typeof(UiApplication)} application is {_application}",
-                "Wpf.Ui"
+            $"INFO | {typeof(UiApplication)} application is {_application}",
+            "Wpf.Ui"
         );
     }
 
@@ -95,14 +95,11 @@ public ResourceDictionary Resources
                     _resources.MergedDictionaries.Add(themesDictionary);
                     _resources.MergedDictionaries.Add(controlsDictionary);
                 }
-                catch
-                {
-                }
+                catch { }
             }
 
             return _application?.Resources ?? _resources;
         }
-
         set
         {
             if (_application is not null)
@@ -132,8 +129,10 @@ public void Shutdown()
 
     private static bool ApplicationHasResources(Application application)
     {
-        return application.Resources.MergedDictionaries
-            .Where(e => e.Source is not null)
-            .Any(e => e.Source.ToString().ToLower().Contains(Appearance.ApplicationThemeManager.LibraryNamespace));
+        return application
+            .Resources.MergedDictionaries.Where(e => e.Source is not null)
+            .Any(e =>
+                e.Source.ToString().ToLower().Contains(Appearance.ApplicationThemeManager.LibraryNamespace)
+            );
     }
 }