diff --git a/src/gtkutil.c b/src/gtkutil.c
index 0da70399193..9f05524738b 100644
--- a/src/gtkutil.c
+++ b/src/gtkutil.c
@@ -577,6 +577,10 @@ xg_check_special_colors (struct frame *f,
     if (get_fg)
       gtk_style_context_get_color (gsty, state, &col);
     else
+      /* FIXME: gtk_style_context_get_background_color is deprecated
+         in GTK+ 3.16.  New versions of GTK+ don’t use the concept of
+         a single background color any more, so we shouldn’t query for
+         it.  */
       gtk_style_context_get_background_color (gsty, state, &col);
 
     unsigned short