>  > I don't call `make-frame' to create frame `*Completions*'.
>  > It is created when ` 1on1-display-*Completions*-frame' is
>  > called, and that is done by `special-display-function'.
>  > I showed the code for that in my previous message.
> 
> The 'minibuffer' parameter must be set up specially by whoever calls
> ‘make-frame’.  If this is not done, you can't change it afterwards.  The
> default value of ‘special-display-function’ is
> ‘special-display-popup-frame’ and I don't see the latter setting up the
> 'minibuffer' parameter anywhere.

I use the default value of `special-display-function',
`special-display-popup-frame'.

I don't call `make-frame' for `*Completions*'.  I just
provide an alist to `special-display-buffer-names'.
That alist includes, as its second element, function
`1on1-display-*Completions*-frame', which displays the
frame.

This is `special-display-buffer-names':

(("*Completions*" 1on1-display-*Completions*-frame
  ((background-color . "LavenderBlush2")
   (mouse-color . "VioletRed")
   (cursor-color . "VioletRed")
   (menu-bar-lines . 0)
   (tool-bar-lines . 0)
   (width . 100)))
 ("*Help*" 1on1-display-*Help*-frame
  ((background-color . "Thistle")
   (mouse-color . "Blue Violet")
   (cursor-color . "Blue Violet")
   (height . 40))))

The only thing that matters there is
`1on1-display-*Completions*-frame'.

>  > I don't ever set parameter `minibuffer' explicitly for
>  > *Completions*.  I'm guessing that it has always gotten
>  > set automatically when frame input was redirected from
>  > frame *Completions* to the standalone minibuffer.
>  >
>  > (It is redirected to `completion-reference-buffer' if
>  > the minibuffer is not active (and if `c-r-b' is not
>  > frame *Completions*)).
> 
> Maybe you mean "focus redirection" here which is something
> different from setting up the 'minibuffer' parameter.

Yes, I mean input/focus redirection, with
`redirect-frame-focus'.  I'm guessing that redirecting
focus is somehow behind the changed value of parameter
`minibuffer' for frame `*Completions*'.  The redirection
of frame focus is the only connection I'm aware of
between those two frames.

I don't change parameter `minibuffer' for *Completions*,
that I'm aware of.  Frame *Completions* is created by
the special-display code, just by my using
`special-display-buffer-names'.  I don't create it
directly - I don't call `make-frame' for it.  If it
is now getting the wrong value of parameter `minibuffer'
I don't see how it could be my code that is doing
that.
 
> I understand that you want that parameter to have a non-nil value there.
> So make sure that it is.  For this you will have to debug your earlier
> version to see how they set up the 'miniuffer' parameter and compare
> them with the current version to see how it fails to do that.

I have no idea what part of the special-display code might
have managed parameter `minibuffer' before vs now, or
whether that is done in Emacs C code or Emacs Lisp code.
I don't know what has changed in the Emacs code.

However, I realize now that I reported the exact opposite
of the real problem.  Apologies for that.

The problem is NOT that (1) PREVIOUSLY, frame *Completions*
had a `minibuffer' parameter whose value was the active
minibuffer window (on the minibuffer frame) and (2) NOW,
frame *Completions* has a nil `minibuffer' parameter.

The problem is that (1) PREVIOUSLY, frame *Completions*
had a nil `minibuffer' and (2) NOW, frame *Completions*
has a `minibuffer' parameter whose value is the active
minibuffer window (on the minibuffer frame).

IOW, the problem is that the separate, dedicated frame
*Completions* somehow gets a `minibuffer' parameter
(whose value is the minibuffer window on the minibuffer
frame).

`*Completions*' is not a minibuffer frame.  The only
minibuffer frame has a `minibuffer' parameter whose
value is `only' - the value is not a minibuffer
_window_.

Does the problem description make more sense now?
Sorry for the confusion.

I am not setting parameter `minibuffer'.  The only
connection I know of between the *Completions* frame
and the standalone minibuffer frame is the focus
redirection.  Is it possible that that redirection
code now mistakenly gives a non-nil `minibuffer'
parameter to the frame whose input focus is
redirected to the minibuffer frame?

Attached is the debug output that I see, in case it
helps.  The cause of my problem seems to be that
somehow frame `*Completions*' is getting a non-nil
parameter `minibuffer' - the value being the
minibuffer window (on the minibuffer frame).
Or do you interpret the attached debug info
differently?

Hoping you can set me straight about this.
I don't think I should need to do anything
different in my code, but if I need to make
some minor adjustment for some change in
Emacs, please let me know what I'll need to do.

So far, this seems like an Emacs bug, to me.