bug#13723: 24.2.93; Fringe icons corrupt buffer separator line

[David Engster <deng@randomsample.de>]

Maybe a small thing, but nonetheless irritating: I noticed that when
scrolling buffers on a split frame with disabled scrollbars, the small
line separating two buffers can get corrupted by icons in the fringe. It
seems that this line doesn't get redrawn correctly, or maybe not at all.

You should be able to reproduce this issue with the following recipe:

* emacs -Q

* In the *scratch* buffer, insert

(progn
  (scroll-bar-mode -1)
  (setq truncate-partial-width-windows nil)
  (toggle-truncate-lines -1)
  (with-current-buffer (get-buffer-create "test")
    (erase-buffer)
    (dotimes (i 100)
      (insert (make-string 500 65) "\n")))
  (pop-to-buffer-same-window "test")
  (goto-char (point-max))
  (split-window-right)
  (other-window 1)
  (split-window-right)
  (pop-to-buffer-same-window "test")
  (goto-char (point-min))
  (while (not (eobp))
    (scroll-up-command)
    (redisplay t)))

and evaluate.

* The separator line between the first and the second buffer should now
  have gaps. Here's a screenshot:

  http://www.randomsample.de/emacs-bug.jpg

As soon as the Emacs window gets redrawn, everything's back to
normal.

-David