Extend DWM frame into client area for backdrop rendering

Author: DavidGBrett

Flow.Launcher.Infrastructure/NativeMethods.txt

@@ -14,6 +14,7 @@ VIRTUAL_KEY
 EnumWindows
 
 DwmSetWindowAttribute
+DwmExtendFrameIntoClientArea
 DWM_SYSTEMBACKDROP_TYPE
 DWM_WINDOW_CORNER_PREFERENCE
 

Flow.Launcher.Infrastructure/Win32Helper.cs

@@ -21,6 +21,7 @@
 using Windows.Win32.Graphics.Dwm;
 using Windows.Win32.System.Power;
 using Windows.Win32.System.Threading;
+using Windows.Win32.UI.Controls;
 using Windows.Win32.UI.Input.KeyboardAndMouse;
 using Windows.Win32.UI.Shell.Common;
 using Windows.Win32.UI.WindowsAndMessaging;
@@ -61,6 +62,13 @@ public static unsafe bool DWMSetBackdropForWindow(Window window, BackdropTypes b
                 _ => DWM_SYSTEMBACKDROP_TYPE.DWMSBT_AUTO
             };
 
+            // Extend the non-client frame into the entire client area so the
+            // backdrop (Mica/Acrylic) has a surface to render on. Without this,
+            // WindowStyle="None" removes the non-client area and the backdrop
+            // has nothing to paint.
+            var margins = new MARGINS { cxLeftWidth = -1, cxRightWidth = -1, cyTopHeight = -1, cyBottomHeight = -1 };
+            PInvoke.DwmExtendFrameIntoClientArea(GetWindowHandle(window), in margins);
+
             return PInvoke.DwmSetWindowAttribute(
                 GetWindowHandle(window),
                 DWMWINDOWATTRIBUTE.DWMWA_SYSTEMBACKDROP_TYPE,