Is S-TAB referring to <backtab>?  That is unbound (default) in this context
for me.

I'm not sure where/if the behaviour was documented.  I think it has behaved
like that for
as long as I've used emacs, so it was quite noticeable that something was
different without knowing exactly what at first.

I just tried out v24.5 and C-M-v / C-M-S-v do scroll the completions buffer
with any number of frames up.

The recent change that sticks out to me is from 898cdc67f1

On Thu, Dec 5, 2019 at 7:19 PM Juri Linkov <juri@linkov.net> wrote:

> > I think this is new within the last week or so.
> > When a completion frame pops up while using the minibuffer,
> > normally the minibuffer-scroll-other-window(-down) functions
> > scroll the completion window.  However, now when emacs is split
> > into two horizontal frames, these functions ignore the
> > completion buffer and scroll the others.
> >
> > To reproduce from emacs -Q:
> >
> >     C-x 3
> >     M-: (mak
> >     TAB for completions
> >     C-M-v
>
> TAB and S-TAB should scroll the the completion buffer.  But what about
> C-M-v?
> Is it documented somewhere what buffer it's intended to scroll?
> I can find only this text in (info "(emacs) Minibuffer Edit"):
>
>      The ‘C-M-v’ command in the minibuffer scrolls the help text from
>   commands that display help text of any sort in another window.  You can
>   also scroll the help text with ‘M-<PageUp>’ and ‘M-<PageDown>’ (or,
>   equivalently, ‘M-<prior>’ and ‘M-<next>’).  This is especially useful
>   with long lists of possible completions.
>
> Does this mean the help text is the same as the completion buffer?
>
> If yes, then this patch should fix it:
>
> diff --git a/lisp/simple.el b/lisp/simple.el
> index 47ce0364d1..7d91678ff7 100644
> --- a/lisp/simple.el
> +++ b/lisp/simple.el
> @@ -1517,6 +1517,7 @@ read-expression-map
>      ;; Might as well bind TAB to completion, since inserting a TAB char is
>      ;; much too rarely useful.
>      (define-key m "\t" 'completion-at-point)
> +    (define-key m [remap minibuffer-scroll-other-window]
> 'scroll-other-window)
>      (set-keymap-parent m minibuffer-local-map)
>      m))
>
>
>