bug#28978: 26.0; Regression: separate, dedicated `*Completions*' frame no longer has parameter `minibuffer'

[Drew Adams]

In my code I check (essentially) this:

(let ((swin  (selected-window)))
  (when (window-minibuffer-p swin) (setq swin  (minibuffer-selected-window)))
  (and (get-buffer-window "*Completions*")
       (window-live-p swin)
       (not (eq (window-buffer swin) (get-buffer "*Completions*")))
       (interactive-p)))

It does what I expect in all Emacs releases, but not in this
prerelease.  My `*Completions*' window is in its own one-window frame,
with the window dedicated.  Input for that frame is redirected to my
standalone minibuffer frame.

It should be the case in my context that this returns the minibuffer
window:

(cdr (assoc 'minibuffer (frame-parameters
                          (#<frame *Completions* 000000000cacf6c8>))))

Instead, it now returns nil.  Parameter `minibuffer' for the
`*Completions*' frame is no longer the minibuffer window; it is nil.

This is the code that redirects the frame focus, which I guess is
relevant:

(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)))

That code is in a function, `1on1-display-*Completions*-frame',
that I use with `special-display-buffer-names':

(add-to-list 'special-display-buffer-names
             `("*Completions*" 1on1-display-*Completions*-frame
               ((background-color ,@1on1-completions-frame-background)
                (mouse-color      ,@1on1-completions-frame-mouse+cursor-color)
                (cursor-color     ,@1on1-completions-frame-mouse+cursor-color)
                (menu-bar-lines . 0) (tool-bar-lines . 0) ; No menu bar or tool 
bar.
                ,@(and 1on1-completions-frame-width
                       `((width   ,@1on1-completions-frame-width))))))

In GNU Emacs 26.0.90 (build 3, x86_64-w64-mingw32)
 of 2017-10-13
Repository revision: 906224eba147bdfc0514090064e8e8f53160f1d4
Windowing system distributor `Microsoft Corp.', version 6.1.7601
Configured using:
 `configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static -g3''