Fix identification of CSD windows when checking whether to force fullscreen

We try to exempt CSD windows from being forced fullscreen if they are
undecorated and the size of the screen; however, we also catch almost
all windows that *do* need to be forced fullscreen in this check, since
they also have decorations turned off.

Identify actual CSD windows by checking whether _GTK_FRAME_EXTENTS is set -
GTK+ will always set this on CSD windows even if they have no invisible
borders or shadows at the current time.

https://bugzilla.gnome.org/show_bug.cgi?id=723029

src/core/constraints.c

@@ -426,7 +426,7 @@ setup_constraint_info (ConstraintInfo      *info,
    */
   if (meta_prefs_get_force_fullscreen() &&
       !window->hide_titlebar_when_maximized &&
-      window->decorated &&
+      (window->decorated || !meta_window_is_client_decorated (window)) &&
       meta_rectangle_equal (new, &monitor_info->rect) &&
       window->has_fullscreen_func &&
       !window->fullscreen)

src/core/window-private.h

@@ -694,4 +694,6 @@ Window meta_window_get_toplevel_xwindow (MetaWindow *window);
 void meta_window_get_client_area_rect (const MetaWindow      *window,
                                        cairo_rectangle_int_t *rect);
 
+gboolean meta_window_is_client_decorated (MetaWindow *window);
+
 #endif

src/core/window.c

@@ -10127,6 +10127,24 @@ meta_window_same_client (MetaWindow *window,
           (other_window->xwindow & ~resource_mask));
 }
 
+/**
+ * meta_window_is_client_decorated:
+ *
+ * Check if if the window has decorations drawn by the client.
+ * (window->decorated refers only to whether we should add decorations)
+ */
+gboolean
+meta_window_is_client_decorated (MetaWindow *window)
+{
+  /* Currently the implementation here is hackish -
+   * has_custom_frame_extents() is set if _GTK_FRAME_EXTENTS is set
+   * to any value even 0. GTK+ always sets _GTK_FRAME_EXTENTS for
+   * client-side-decorated window, even if the value is 0 because
+   * the window is maxized and has no invisible borders or shadows.
+   */
+  return window->has_custom_frame_extents;
+}
+
 void
 meta_window_refresh_resize_popup (MetaWindow *window)
 {