windows: fix window_background_opacity

I'm not sure what exactly changed (perhaps it was a Windows updated?)
but window_background_opacity was only taking effect for windows
with no title bar.

I found that explicitly configuring a region makes transparency
work again.

refs: #553

docs/changelog.md

@@ -15,6 +15,7 @@ brief notes about them may accumulate here.
 * Fixed: detection and handling of fonts such as terminus-bold.otb that contain only bitmap strikes. [#560](https://github.com/wez/wezterm/issues/560)
 * Fixed: the pixel size reported by the pty to the kernel wasn't adjusted for font metrics/dpi until the config was reloaded or window resized. [#563](https://github.com/wez/wezterm/issues/563)
 * Fixed: greatly reduce memory consumption when system fallback fonts are loaded [#559](https://github.com/wez/wezterm/issues/559)
+* Fixed: Windows: `window_background_opacity` was only taking effect when `window_decorations="NONE"` [#553](https://github.com/wez/wezterm/issues/553)
 
 ### 20210314-114017-04b7cedd
 

window/src/os/windows/window.rs

@@ -729,16 +729,21 @@ unsafe fn wm_ncdestroy(
 fn enable_blur_behind(hwnd: HWND) {
     use winapi::shared::minwindef::*;
     use winapi::um::dwmapi::*;
-
-    let bb = DWM_BLURBEHIND {
-        dwFlags: DWM_BB_ENABLE,
-        fEnable: TRUE,
-        hRgnBlur: null_mut(),
-        fTransitionOnMaximized: FALSE,
-    };
+    use winapi::um::wingdi::*;
 
     unsafe {
+        let region = CreateRectRgn(0, 0, -1, -1);
+
+        let bb = DWM_BLURBEHIND {
+            dwFlags: DWM_BB_ENABLE | DWM_BB_BLURREGION,
+            fEnable: TRUE,
+            hRgnBlur: region,
+            fTransitionOnMaximized: FALSE,
+        };
+
         DwmEnableBlurBehindWindow(hwnd, &bb);
+
+        DeleteObject(region as _);
     }
 }