(follow-calculate-first-window-start-from-below): Avoid looping

forever if vertical-motion returns an unexpected value.

diff --git a/lisp/follow.el b/lisp/follow.el
index 59b8a8a38fdf242a2cb1452f7fdaab75b87a5b36..7af3a049299a0dbf299c710ea60b8087aa015885 100644 (file)
--- a/lisp/follow.el
+++ b/lisp/follow.el
@@ -1208,22 +1208,25 @@ should be a member of WINDOWS, starts at position START."
   (setq win (or win (selected-window)))
   (setq start (or start (window-start win)))
   (save-excursion
-    (let ((done nil)
-         win-start
-         res)
+    (let (done win-start res opoint)
       ;; Always calculate what happens when no line is displayed in the first
       ;; window. (The `previous' res is needed below!)
       (goto-char guess)
       (vertical-motion 0 (car windows))
       (setq res (point))
       (while (not done)
+       (setq opoint (point))
        (if (not (= (vertical-motion -1 (car windows)) -1))
            ;; Hit roof!
            (setq done t res (point-min))
          (setq win-start (follow-calc-win-start windows (point) win))
-         (cond ((= win-start start)    ; Perfect match, use this value
-                (setq done t)
-                (setq res (point)))
+         (cond ((>= (point) opoint)
+                ;; In some pathological cases, vertical-motion may
+                ;; return -1 even though point has not decreased.  In
+                ;; that case, avoid looping forever.
+                (setq done t res (point)))
+               ((= win-start start)    ; Perfect match, use this value
+                (setq done t res (point)))
                ((< win-start start)    ; Walked to far, use preious result
                 (setq done t))
                (t                      ; Store result for next iteration