gdk:window-at-pointer ()

window -- a gdk:window object under the mouse pointer
xwin -- an integer with the origin of the window under the pointer
ywin -- an integer with the origin of the window under the pointer

Obtains the window underneath the mouse pointer, returning the location of that window in xwin, ywin. Returns nil if the window under the mouse pointer is not known to GDK. The gdk:window-at-pointer function has been deprecated since version 3.0 and should not be used in newly written code. Use the gdk:device-window-at-position function instead.