Bug 1586555 - Disable double buffering with D3D compositor when Glass is used r=bas

Differential Revision: https://phabricator.services.mozilla.com/D48469

--HG--
extra : moz-landing-system : lando

widget/windows/nsWindow.cpp

@@ -132,6 +132,7 @@
 #include "mozilla/dom/MouseEventBinding.h"
 #include "mozilla/dom/Touch.h"
 #include "mozilla/gfx/2D.h"
+#include "mozilla/gfx/GPUProcessManager.h"
 #include "nsIAppStartup.h"
 #include "mozilla/WindowsVersion.h"
 #include "mozilla/TextEvents.h"  // For WidgetKeyboardEvent
@@ -7470,6 +7471,14 @@ void nsWindow::SetWindowTranslucencyInner(nsTransparencyMode aMode) {
                reinterpret_cast<HBRUSH>(GetStockObject(BLACK_BRUSH)));
     ReleaseDC(mWnd, hdc);
   }
+
+  // Disable double buffering with D3D compositor for disabling compositor
+  // window usage.
+  if (HasGlass() && !gfxVars::UseWebRender() &&
+      gfxVars::UseDoubleBufferingWithCompositor()) {
+    gfxVars::SetUseDoubleBufferingWithCompositor(false);
+    GPUProcessManager::Get()->ResetCompositors();
+  }
 }
 
 #endif  // MOZ_XUL