gdk:device-last-event-window (device)

device -- a gdk:device object with a source other than :keyboard

The last gdk:window object the device is in.

Gets information about which window the given pointer device is in, based on events that have been received so far from the display server. If another application has a pointer grab, or this application has a grab with owner-events is false, nil may be returned even if the pointer is physically over one of this application's windows.