Comment 11 for bug 401823

I can't see a reasonable way for either Xlib or the Xserver to guarantee that
XIDs in client's event queues are unique.

The X server has handed off the DestroyNotify event, so it thinks it has
finished with the event.

Xlib could ensure not to allocate an XID referenced in its own event queue
(for known event types), but it wouldn't know what other clients might have a
reference to a candidate XID sitting in their event queues.

If the server were to keep XIDs of destroyed windows allocated until clients
have processed events on that window, it would need to know when the events in
Xlib's queue have been processed. I can't see how the Xserver can know this
(without some change in protocol).

The other way of looking at this is that the events are a history of what has
happened and need to be interpreted in the context of when they happened.