other-window-for-scrolling

other-window-for-scrolling is a built-in function in `C source code'.

(other-window-for-scrolling)

Return the other window for "other window scroll" commands.
If `other-window-scroll-buffer' is non-nil, a window
showing that buffer is used.
If in the minibuffer, `minibuffer-scroll-window' if non-nil
specifies the window. This takes precedence over
`other-window-scroll-buffer'.