bug#51210: Customizable other-window-for-scrolling

[Juri Linkov]

> The advantage of having separate commands is that users can then have
> several behaviors at once.  By contrast, a single customizable
> variable can provide only one of the possible behaviors.  Not everyone
> wants only ever scroll the most-recently-used window or the single
> window on another frame, some of the users might want sometimes to
> scroll next-window, sometimes the most-recently-used one, and
> sometimes the one on the other frame.
>
> And it isn't like it would be hard to write those few commands, it
> should be almost trivial.  Not harder than writing those functions to
> return the window you want, anyway.

If you want to write such commands, still it would be much easier
to implement them using the new variable, e.g.

(defun scroll-mru-window ()
  (interactive)
  (let ((other-window-scroll-window 'get-mru-window)
    (scroll-other-window))))