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

[martin rudalics <rudalics@gmx.at>]

 > I was checking the `minibuffer' parameter of THIS-FRAME,
 > to see if it was the `active-minibuffer-window'.  But it
 > should be just as good to check that the frame of the
 > `active-minibuffer-window' is THIS-FRAME.
 >
 > I don't think the code you showed earlier corresponds to
 > the same thing.  IIUC, the test you suggested earlier
 > checks whether the window that was selected immediately
 > before the current minibuffer window was selected is the
 > same as the selected window of THIS-FRAME.  That's not
 > the same thing as what I need to test, AFAICT.  But your
 > latest suggestion seems to check what I've been checking,
 > and it should work OK in all Emacs versions.

If you told me with plain words what you really wanted to check then I
might come up with another suggestion.  But "checking the `minibuffer'
parameter of THIS-FRAME, to see if it was the
`active-minibuffer-window'" could not have possibly done anything
reasonable even before my changes.  That test simply failed/succeeded
accidentally with Emacs 25 and just produces the opposite result with
Emacs 26.

 >   Return the window that was selected immediately
 >   before the current minibuffer window was selected.

Looks good but I need a first line that fits into 67 characters.  Pretty
please suggest a suitable first line and whatever you consider important
for the rest.  Also, since we nowhere specify "the current minibuffer
window" wouldn't "the currently active minibuffer window" be better?

martin