bug#26513: 25.2; pop-up-frames and *Completions* buffer

[Drew Adams <drew.adams@oracle.com>]

FWIW, I reported such problems a couple of decades ago.
Unfortunately (for me at least), there is not enough
use or interest in using separate frames by default,
including for *Completions*, so this kind of thing has
not gotten the love it would really need for progress.

I've tried to do what I can in my own environment to
handle this, especially in the context of a standalone 
minibuffer frame.  And Martin has been helpful wrt
problems that resulted from changes in the Emacs code
over the years.

Here's what I do for *Completions*, FWIW:

I add an entry to `special-display-buffer-names* that
has a function, `1on1-display-*Completions*-frame',
which takes care of displaying the *Completions* frame.

The main thing that function does is redirect the
focus of the *Completions* frame to the standalone
minibuffer frame (if the minibuffer is active) or
(if not) to the buffer that was current before
*Completions* display was requested:

(let ((redirect
       (if (active-minibuffer-window)
           1on1-minibuffer-frame
         (and completion-reference-buffer
              (get-buffer-window
                completion-reference-buffer 'visible)
                (not (eq (get-buffer "*Completions*") 
                         completion-reference-buffer))
                (window-frame
                  (get-buffer-window
                    completion-reference-buffer t))))))
      (when redirect
        (redirect-frame-focus (selected-frame)
                              redirect)))

I've said it before, but I think it is relevant:
Back in the early 1990s the Emacs implementation
named `Epoch' worked very well with a standalone
minibuffer frame, out of the box.

All I've done is try to work around Emacs's poor
(non-existent) support for this kind of use case -
essentially trying to emulate Epoch behavior.

But frames remain the poor cousin to windows in
Emacs.  Part of that is likely due to the fact that
Emacs cannot completely control the behavior of
frames for all window managers.  Window mgrs are
different, and they have ultimate control.

The other reason is probably just because few Emacs
users try to use separate frames by default, or if
they try they soon give up due to the many hurdles.

Anyway, you might try, to start with, redirecting
the frame focus back to `completion-reference-buffer'.

For reference, in case it helps, my code for
`1on1-display-*Completions*-frame' is in `oneonone.el'
(https://www.emacswiki.org/emacs/download/oneonone.el).
My code for keybindings in `completion-list-mode-map'
is in `icicles-mode.el'
(https://www.emacswiki.org/emacs/download/icicles-mode.el).