Sets whether input to the window is passed through to the window below.
The default value of this is FALSE, which means that pointer events that happen inside the window are send first to the window, but if the event is not selected by the event mask then the event is sent to the parent window, and so on up the hierarchy.
If pass_through is TRUE then such pointer events happen as if the window wasn't there at all, and thus will be sent first to any windows below window. This is useful if the window is used in a transparent fashion. In the terminology of the web this would be called "pointer-events: none".
Note that a window with pass_through TRUE can still have a subwindow without pass through, so you can get events on a subset of a window. And in that cases you would get the in-between related events such as the pointer enter/leave events on its way to the destination window.