bug#12170: save-excursion fails boundary case with recenter

[martin rudalics]

>                 (defun f (n) (save-excursion (forward-line (- n)) (recenter
> 0)))
>
>
>
> Let H be the window height in lines, and suppose that point is positioned at
> the beginning of line L > H.
>
>
>
> Despite 'save-excursion', (f K) moves point if K = H.  It leaves point
> unchanged for any other value of K, 0 <= K < L.

I'm not sure whether I clearly understand your scenario.  Suppose with
emacs -Q I evaluate

(progn
  (defun f (n)
    (save-excursion (forward-line (- n)) (recenter 0)))
  (let ((buffer (switch-to-buffer "foo"))
        (height (window-height (get-buffer-window "foo"))))
    (insert-char 10 (* height 2))
    (let ((pt (point)))
      (f height)
      (message "old %s new %s" pt (point)))))

Then the values printed by the message are equal.  Please elaborate.

Thanks, martin