events will go to the bottommost window which has registered interest.
client A owns window A.
client B owns window B, which is a subwindow of window A.
client A listens for keypress/keyrelease on window A.
keypress on window B -> event delivered to client A, window A.
client B listens for keypress/keyrelease on window A.
keypress on window B -> event delivered to clients A & B, window A.
client B listens for keypress/keyrelease on window B.
keypress on window B -> event delivered to client B, window B.
note client A never receives any event in the last one here, because window B now has interest registered on it. it doesn't matter that it's a different client.