bug#31546: 27.0.50; macOS child frames with no mode-line mouse click problem

[martin rudalics <rudalics@gmx.at>]

 > Confirmed. I get the tooltip for whatever is under the mouse in the
 > parent frame, even though the child frame is obscuring it

Just for the record: Aaron, do you also see the tooltips adherent to
the parent frame appear over the child frame?

 > I think it’s because the parent is the selected frame.
 >
 > Here’s some fun for you to try:
 >
 >      (set-frame-parameter nil 'z-group 'below)
 >
 > put another application’s window partially over the Emacs frame, make
 > sure the Emacs frame is selected, then mouse‐over where the modeline
 > should be. The pointer changes and tooltips are displayed as though
 > the other application window isn’t there.

Do you really need the

 >      (set-frame-parameter nil 'z-group 'below)

for this experiment?  I wonder why (it doesn't happen here because I
always use a strict focus-follows-mouse + autoraise policy).

 > I guess we need to work out whether the frame is obscured and if so
 > don’t change the mouse pointer or create tooltips?

On my system I've occasionally seen Thunderbird tooltips appear on top
of a Firefox window (but not vice versa) so I suspect that this
problem may be quite common.  But with my mouse policy it's certainly
less likely to occur.

martin