bug#24804: 25.1; posn-at-point erroneously signals an error

[Andreas Politz]

The documentation of the function posn-at-point states

"Return nil if position is not visible in window.",

but it may also signal an error in this case.  This happens, if
Fpos_visible_in_window_p returns a list of (X Y RTOP RBOT ROWH VPOS)
and at least Y is negative (which indicates, that pos is not visible
IIUC).  The error is then signaled by Fposn_at_x_y, which only accpets
non-negative numbers (neglecting the exceptional case of -1 for X).

I think this function should include a similar test for y, as is already
in place for x, returning nil if it is negative (see below).

diff --git a/src/keyboard.c b/src/keyboard.c
index bb411e7..215fcb9 100644
--- a/src/keyboard.c
+++ b/src/keyboard.c
@@ -10791,9 +10791,10 @@ The `posn-' functions access elements of such lists.  
*/)
       Lisp_Object x = XCAR (tem);
       Lisp_Object y = XCAR (XCDR (tem));
 
-      /* Point invisible due to hscrolling?  X can be -1 when a
-        newline in a R2L line overflows into the left fringe.  */
-      if (XINT (x) < -1)
+      /* Point invisible due to hscrolling or vscrolling?  X can be -1
+        when a newline in a R2L line overflows into the left
+        fringe.  */
+      if (XINT (x) < -1 || XINT (y) < 0)
        return Qnil;
       tem = Fposn_at_x_y (x, y, window, Qnil);
     }

--

Reproducing this state may be a little bit tricky, anyway evaluate the
following lines, starting with `emacs -Q'.

(defvar img "foo.png");; The image should be taller then the window
                      ;; it's displayed in.
(setq debug-on-error t)
(find-file img)
(redisplay t)         ;; If in batch mode.
(image-scroll-up 999)
;; The image should be scrolled to the bottom now, while point equals 1.
(posn-at-point (point-max) (selected-window))

--

Note, that this bug may make switching buffers difficult in certain
situations, as was reported here:
https://github.com/politza/pdf-tools/issues/200

-ap