Subject: 23.1.93; scroll bar redisplay bug locks up Emacs hard

From: Stephen Berman <[email-masked]>
Date: Wed, 03 Mar 2010 19:56:16 +0100

(I used M-x report-emacs-bug to report a bug, and it bounced, with this
feedback from the mailer-daemon: "A message that you sent could not be
delivered to one or more of its recipients. This is a permanent
error. The following address(es) failed: [email-masked]".  The
report included two long backtraces and perhaps the length is a
problem.  So I'm trying to repost now without the backtraces.  If this
works, I'll send the backtraces in separate followups.)

In GNU Emacs 23.1.93.1 (i686-pc-linux-gnu, GTK+ Version 2.18.6)
 of 2010-03-02 on escher
Windowing system distributor `The X.Org Foundation', version 11.0.10605000
Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: @im=local
  locale-coding-system: utf-8-unix
  default enable-multibyte-characters: t

I've encountered a redisplay bug which locks up Emacs completely, C-g
has no effect.  I've seen two manifestions resulting from essentially
the same recipe, both involving the scroll bar.  In one manifestation,
the scroll bar vanishes when Emacs locks up; in the other, the scroll
bar flickers very rapidly.  I haven't been able to pin down exactly when
which manifestation occurs; maybe it's a timing issue.  However, the
lock up is reproducible, but under rather complex conditions.  I've
managed to pare down my initializations to a minimum amount that induces
the lock up.  Here is the recipe:

1. Let ~/.emacs consist of the attached file.  This loads the third
party library tabbar.el, available at
<http://emhacks.cvs.sourceforge.net/viewvc/*checkout*/emhacks/emhacks/tabbar.el?revision=1.69>,
so make sure to adjust the path in the load sexp.

2. Let ~/.newsrc consist of the following line:
gmane.linux.suse.opensuse.user: 1-110249
(Also, maybe there shouldn't be a nonempty directory ~/News/, though it
might not matter).

3. Start emacs.

4. Type M-x gnus, and when the Group buffer comes up, enter the
newsgroup gmane.linux.suse.opensuse.user

5. Type `j' (gnus-summary-goto-article) and at the prompt paste in this
Message-ID: [email-masked]

=> The scroll bar disappears and Emacs locks up hard.

I've reproduced this under gdb and gotten two different backtraces,
which I will post in followups, if this report makes it into the
bugtracker.