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

[Juri Linkov]

>> Often scroll-other-window uses a wrong window for scrolling
>> when there are more than 2 windows on the frame.
>
> Which one is the "right" window?

The right window would be most-recently-used window like can be
customized in e.g. compare-windows-get-window-function.

> And can you show a recipe for the issue?

A recipe is to create 3 windows and to try scrolling
a window that is not next-window.