Bug 1423869 - don't draw to mContainer when decorations are rendered by window manager, r=jhorak

Bug 1421974 introduced new mechanism to hide system titlebar by enabling client-side decorations for main Firefox window. It also causes a regression when the CSD window setup is enabled when system titlebar is hidden by window manager.

This patch fixes that and enables the CSD window setup only for case when it's actualy used.

MozReview-Commit-ID: 6OzoPyxlhCp

--HG--
extra : rebase_source : 9bcfc96cd27cb088f8d9671780304c2ac3772fdf

widget/gtk/nsWindow.cpp

@@ -3795,8 +3795,9 @@ nsWindow::Create(nsIWidget* aParent,
          *    and we listen to the Gtk+ events on mContainer.
          */
         GtkStyleContext* style = gtk_widget_get_style_context(mShell);
-        drawToContainer = mIsCSDAvailable ||
-                          gtk_style_context_has_class(style, "csd");
+        drawToContainer =
+            (mIsCSDAvailable && GetCSDSupportLevel() == CSD_SUPPORT_FLAT) ||
+            gtk_style_context_has_class(style, "csd");
 #endif
         eventWidget = (drawToContainer) ? container : mShell;