(xdialog_show): Call Fredisplay before showing the dialog.

src/xmenu.c

@@ -3333,6 +3333,14 @@ xdialog_show (f, keymaps, title, header, error_name)
   /* No selection has been chosen yet.  */
   menu_item_selection = 0;
 
+  /* Force a redisplay before showing the dialog.  If a frame is created
+     just before showing the dialog, its contents may not have been fully
+     drawn, as this depends on timing of events from the X server.  Redisplay
+     is not done when a dialog is shown.  If redisplay could be done in the
+     X event loop (i.e. the X event loop does not run in a signal handler)
+     this would not be needed.  */
+  Fredisplay (Qt);
+
   /* Actually create and show the dialog.  */
   create_and_show_dialog (f, first_wv);