Bug 1633343. Also use the style WS_EX_TRANSPARENT on the compositor window. r=sotaro

The original workaround given by Microsoft was both WS_EX_TRANSPARENT and WS_EX_LAYERED. In bug 1627505 we tried to just add WS_EX_LAYERED because it was all that was needed to fix that bug and we hoped it wouldn't re-introduce the blank window problem. But it did. So we may as well go with both flags as recommended by Microsoft.

Differential Revision: https://phabricator.services.mozilla.com/D72598

widget/windows/WinCompositorWindowThread.cpp

@@ -162,7 +162,7 @@ WinCompositorWnds WinCompositorWindowThread::CreateCompositorWindow() {
         DWORD extendedStyle = WS_EX_NOPARENTNOTIFY | WS_EX_NOREDIRECTIONBITMAP;
 
         if (!StaticPrefs::apz_windows_force_disable_direct_manipulation()) {
-          extendedStyle |= WS_EX_LAYERED;
+          extendedStyle |= WS_EX_LAYERED | WS_EX_TRANSPARENT;
         }
 
         compositorWnd = ::CreateWindowEx(