4.2 Translating Screen Coordinates
need to perform a coordinate transformation from the coordinate
space of one window to another window or need to determine which
window the pointing device is in.
fulfill these needs (and avoid any race conditions) by
asking the X server to perform these operations.
To obtain the screen coordinates of the pointer
or to determine the pointer coordinates relative to a specified window, use
Christophe Tronche, email@example.com