Brian Jenkins writes:

 > It's not clear to me that X itself even *has* a concept of "active
 > application" distinct from "frame that has input focus".

No, it doesn't, and it can't.

Multiple applications can select events in the same X window at the same
time.  For example, the classic debugging tool xev(1) selects events on
a window you select (say, an Emacs frame), and outputs a trace of those
events.  Which is active: xev, or Emacs (which also receives those input
events and acts on them as if xev didn't exist)?

The only sane answer is "both of them".