bug#24091: 24.5; High CPU usage at startup while hidden

[martin rudalics]

> Only if x_top_window_to_frame returns non-nil, which it does not.
>
>      case MapNotify:
>        /* We use x_top_window_to_frame because map events can
>           come for sub-windows and they don't mean that the
>           frame is visible.  */
>        f = x_top_window_to_frame (dpyinfo, event->xmap.window);

Where in x_window_to_frame does it fail?

martin