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

[Juri Linkov]

>>> And shouldn't a window, if specified, override a specified buffer?
>>
>> 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);
    }