The ordering of the DestroyNotify events is such that for any given window, DestroyNotify is generated on all inferiors of the window before being generated on the window itself. The X protocol does not constrain the ordering among siblings and across subhierarchies.
To receive DestroyNotify events, set the StructureNotifyMask bit in the event-mask attribute of the window or the SubstructureNotifyMask bit in the event-mask attribute of the parent window (in which case, destroying any child generates an event).
The structure for this event type contains:
typedef struct {
int type; /* DestroyNotify */
unsigned long serial; /* # of last request processed by server */
Bool send_event; /* true if this came from a SendEvent request */
Display *display; /* Display the event was read from */
Window event;
Window window;
} XDestroyWindowEvent;
The event member is set either to the destroyed window or to its parent, depending on whether StructureNotify or SubstructureNotify was selected. The window member is set to the window that is destroyed.