Apparently lowering a focused window via a middle click on the frame so that it
drops below another window and such that the mouse enters the new window results
in some funny behavior.  It causes a LeaveNotify event to be generated with
xcrossing.mode equal to neither NotifyGrab nor NotifyUngrab AND an EnterNotify
event to be generated with xcrossing.mode equal to NotifyUngrab.  This asymmetry
is _really_ odd (I would expect both events to come from grabs, or neither event
to come from a grab; I don't understand why there's a mixture).

Anyway, since the EnterNotify is considered to have come from a grab, we need to
manually focus the correct window in all focus modes (not just click to focus).

Manually focusing the correct window, though, isn't quite enough for mouse focus
because of another issue:  LeaveNotify events in display.c use window->has_focus
in order to determine whether to defocus the window in mouse focus mode.  The
patch I will attach changes this to window==display->expected_focus_window, but
that depends on the patch in bug 154598 first being applied in order for this
change to be correct.  You'll note that there's a comment for EnterNotify events
that says that window->has_focus has to be ignored due to possible race
conditions, so making this change is a good idea for probably more than just
this bug.