C-x o is moving between frames. [Was: Stop frames stealing eachothers' minibuffers!]

[Alan Mackenzie]

Hello, Eli.

I'll deal with the (few) points in your last mail separately, but for
now ....

On Thu, Oct 22, 2020 at 19:14:09 +0300, Eli Zaretskii wrote:
> > Date: Wed, 21 Oct 2020 20:04:38 +0000
> > Cc: emacs-devel@gnu.org
> > From: Alan Mackenzie <acm@muc.de>
> > 
> > Here's a corrected version of the patch, incorporating fixes and
> > suggestions from Stefan and Drew:

[ .... ]

> I may be missing something: where does this code handles the case of
> minibuffer-only frames?

They almost take care of themselves.  Almost, but not quite.  I've had a
closer look at minibuffer-only frames, and it feels like I'm opening
Pandora's box[*].  Even in the unchanged master branch, I get this:

(i) emacs -Q --exec "(push '(minibuffer . nil) initial-frame-alist)" ;
this creates two frames, one without a minibuffer, the other which _is_
a minibuffer.
(ii) C-x b
(iii) C-x o can now move point between the two frames.  This feels
wrong.  Surely C-x o should move between windows within a single frame.

[*] Pandora's box: From Greek mythology, the opening of this releases
previously confined troubles upon the world.

-- 
Alan Mackenzie (Nuremberg, Germany).