Bug 1583852 - [Linux] Disable round corners/alpha blending for PictureInPicture windows, r=jhorak

This bug depends on Bug 1584492.

Differential Revision: https://phabricator.services.mozilla.com/D47415

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

widget/gtk/nsWindow.cpp

@@ -3694,8 +3694,9 @@ nsresult nsWindow::Create(nsIWidget* aParent, nsNativeWidget aNativeParent,
 
         // There's no point to configure transparency
         // on non-composited screens.
+        // Also disable transparency for PictureInPicture windows.
         GdkScreen* screen = gdk_screen_get_default();
-        if (gdk_screen_is_composited(screen)) {
+        if (gdk_screen_is_composited(screen) && !mIsPIPWindow) {
           // Some Gtk+ themes use non-rectangular toplevel windows. To fully
           // support such themes we need to make toplevel window transparent
           // with ARGB visual.