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 +62,15 @@ public static unsafe bool DWMSetBackdropForWindow(Window window, BackdropTypes b
                 _ => DWM_SYSTEMBACKDROP_TYPE.DWMSBT_AUTO
             };
 
+            // The backdrop renders in the non-client frame area. WindowStyle=None
+            // removes that area, so we extend it across the entire client area.
+            // https://learn.microsoft.com/en-us/windows/win32/api/dwmapi/nf-dwmapi-dwmextendframeintoclientarea
+            if (backdrop is not BackdropTypes.None)
+            {
+                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,