gistfile1.el

(defun display-buffer-above-selected (buffer alist)
  "Try displaying BUFFER in a window below the selected window.
This either splits the selected window or reuses the window below
the selected one."
  (let (window)
    (or (and (setq window (window-in-direction 'above))
             (eq buffer (window-buffer window))
             (window--display-buffer buffer window 'reuse alist))
        (and (not (frame-parameter nil 'unsplittable))
             (let ((split-height-threshold 0)
                   split-width-threshold)
               (setq window
                     (prog1 (selected-window)
                      (window--try-to-split-window (selected-window) alist))))
             (window--display-buffer
              buffer window 'window alist display-buffer-mark-dedicated))
        (and (setq window (window-in-direction 'below))
             (not (window-dedicated-p window))
             (window--display-buffer
              buffer (prog1 (selected-window) (select-window window))
              'reuse alist display-buffer-mark-dedicated)))))