Comment 17 for bug 1625846

OK, I think that "Freestyle" is the appropriate solution here. And that X11 overrideRedirect windows should equate to Freestyle surfaces which never receive input focus.

Mir 0.27/1.0 appears to have a new function for creating freestyle surfaces but I can't find any way to flag that they should not receive keyboard focus (note: I mean keyboard focus only as these still need to receive mouse events).

So a correct solution requires more API work in Mir 0.27/1.0 and possibly related WM logic in MirAL to enforce the "never receives keyboard focus" flag if set (see comment #16).