A MapRequest event occurs when the functions XMapRaised() and XMapWindow() are called.
This event differs from MapNotify in that it delivers the parameters of the request before it is carried out. This gives the client that selects this event (usually the window manager) the opportunity to revise the size or position of the window before executing the map request itself or to deny the request. (MapNotify indicates the final outcome of the request.)
This event is selected by specifying the window ID of the parent of the receiving window with SubstructureRedirectMask. (In addition, the override_redirect member of the XSetWindowAttributes structure for the specified window must be False.)