bug#38502: 27.0.50; minibuffer-scroll-other-window with multiple frames

[Juri Linkov]

>> 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?
>
> It should scroll 'minibuffer-selected-window' whichever buffer is
> displayed there.

C-v and M-v should scroll 'minibuffer-selected-window', but
C-M-v should scroll the other window from 'minibuffer-selected-window'
like it does now.  But maybe this should be configurable?

Is there a variable/function with a name like 'minibuffer-selected-other-window'
that gives an other window to scroll with C-M-v from the minibuffer or
from the window defined by 'minibuffer-selected-window'?