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

[Juri Linkov]

close 51210 29.0.50
thanks

>>> Its purpose is to override only the part that hard-codes 'next-window'.
>>
>> How about a variable 'other-window-to-scroll-function' whose default
>> value is 'next-window'?
>
> The default behavior is more complex than just 'next-window':
>
>   else if (FUNCTIONP (Vother_window_scroll_default))
>     /* Nothing specified; try to get a window from the function.  */
>     window = call0 (Vother_window_scroll_default);
>   else
>     {
>       /* Otherwise, look for a neighboring window on the same frame.  */
>       window = Fnext_window (selected_window, Qlambda, Qnil);
>
>       if (EQ (window, selected_window))
>       /* That didn't get us anywhere; look for a window on another
>            visible frame on the current terminal.  */
>         window = Fnext_window (window, Qlambda, Qvisible);
>     }

So I pushed this.  If you have better suggestions, this could be reopened.