#ifdef USE_LUCID
      /* Bloodcurdling hack alert: The Lucid menu bar widget's
      redisplay procedure is not called when a tip frame over menu
      items is unmapped.  Redisplay the menu manually...  */
        Widget w;
     struct frame *f = SELECTED_FRAME ();
     w = f->output_data.x->menubar_widget;

     if (!DoesSaveUnders (FRAME_DISPLAY_INFO (f)->screen)
         && w != NULL)
         block_input ();
         xlwmenu_redisplay (w);
         unblock_input ();
#endif /* USE_LUCID */

This is part of the code responsible for tooltips. In case you were wondering why certain kinds of tooltips persist upon switching workspaces, it’s because they’re proper windows that are subject to the whims of your window manager. I’ve still got to investigate whether that’s just the case for non-system popups or all kinds of them.

So, Emacs creates a new, very bare-bones frame and controls it which works very well for the default tooltips displayed upon hovering on menu bar buttons, but less well for tooltips created by libraries such as pos-tip. The previous code snippet demonstrates another case when this design becomes obvious for the Lucid toolkit which apparently did not redraw the menu bar on hiding the tooltip.