According to the documentation of
other-window-for-scrolling we should be able to specify the "other window" by setting the variable
other-window-for-scrolling is a built-in function in ‘src/window.c’.
Return the other window for "other window scroll" commands. If in the minibuffer, ‘minibuffer-scroll-window’ if non-nil specifies the window. Otherwise, if ‘other-window-scroll-buffer’ is non-nil, a window showing that buffer is used, popping the buffer up if necessary. Finally, look for a neighboring window on the selected frame, followed by all visible frames on the current terminal.
However, in reality this seems to be ignored:
(setq other-window-scroll-buffer "*R*") (other-window-for-scrolling) --> #<window 558 on header.html>
Is this a bug or feature that was removed?