XFilterEvent() - filter X events for an input method
Bool XFilterEvent (XEvent *event, Window w)
If the window argument is None, XFilterEvent applies the filter to the window specified in the XEvent structure. The window argument is provided so that layers above Xlib that do event redirection can indicate to which window an event has been redirected.
If XFilterEvent(3) returns True, then some input method has filtered the event, and the client should discard the event. If XFilterEvent(3) returns False, then the client should continue processing the event.
If a grab has occurred in the client, and XFilterEvent(3) returns True , the client should ungrab the keyboard.
Xlib