gdk:device-surface-at-position (device)

device -- a gdk:device object
surface -- a gdk:surface object under the device position
xwin -- a double float for the x coordinate of the device location
ywin -- a double float for the y coordinate of the device location

Obtains the surface underneath device, returning the location of the device in xwin and ywin in double precision. Returns nil if the surface tree under device is not known to GDK, for example, belongs to another application.