Proximity events are generated when using GDK's wrapper for the XInput extension. The XInput extension is an add-on for standard X that allows you to use nonstandard devices such as graphics tablets. A proximity event indicates that the stylus has moved out of contact with the tablet, or perhaps that the user's finger has moved out of contact with a touch screen.
Event triggered when the control receives proximity_out_event signal(please check GTK documentation for more information).
Warning: this event is undocumented(or poorly documented) in GTK/GTKmm.
This event can close the application by returning MSG_APPLICATION_QUIT. Any other return value will be ignored.