diff --git a/src/gtkutil.c b/src/gtkutil.c
index 0203a5d5c1a..0da70399193 100644
--- a/src/gtkutil.c
+++ b/src/gtkutil.c
@@ -1217,7 +1217,10 @@ xg_create_frame_widgets (struct frame *f)
      with regular X drawing primitives, so from a GTK/GDK point of
      view, the widget is totally blank.  When an expose comes, this
      will make the widget blank, and then Emacs redraws it.  This flickers
-     a lot, so we turn off double buffering.  */
+     a lot, so we turn off double buffering.
+     FIXME: gtk_widget_set_double_buffered is deprecated and might stop
+     working in the future.  We need to migrate away from combining
+     X and GTK+ drawing to a pure GTK+ build.  */
   gtk_widget_set_double_buffered (wfixed, FALSE);
 
   gtk_window_set_wmclass (GTK_WINDOW (wtop),