> From: noah <noah.v.peart@gmail.com>
> Date: Thu, 05 Dec 2019 14:03:18 -0500
>
> 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

Probably related to recent message/minibuffer-message changes.