bug#4996: problem found

[Mark Lillibridge <mark.lillibridge@hp.com>]

    Ah!  I have figured out the problem.  report-emacs-bug adds the
intangible property to the instructions.  Normally, forward-line skips
past text with the intangible property and this is what happens when
linum-update is called normally, hence those lines do not receive line
numbers normally.  [UPDATE: this is a different bug in linum as it
normally reports incorrect line numbers in the presence of intangible
text; ironically, the case I was reporting is one of the few when it
reports the *correct* line numbers.  Consider this a bug report for the
behavior of the window-scroll-functions callers.]

    However, for some reason inhibit-point-motion-hooks is set to t when
the window-scroll-functions hooks are called.  This causes linum-update,
which uses forward-line, to number the intangible lines only when called
from the scrolling hook.

    The following code change, which fixes the bug, demonstrates this:
[UPDATE: this actually breaks linum further]

(defun linum-after-scroll (win start)
  (let ((old inhibit-point-motion-hooks))
    (setq inhibit-point-motion-hooks nil)
    (linum-update (window-buffer win))
    (setq inhibit-point-motion-hooks old)))


    I am not sure if the actual bug here is with the callers of
window-scroll-functions incorrectly setting inhibit-point-motion-hooks
or with linum-after-scroll (in which case, the documentation for
window-scroll-functions should mention this behavior).  If the later, a
better patch should be used that uses unwind-protect or the like.

- Mark