bug#3688: 23.1.50; TAB in C style comment causes cursor jump

Hi, Kees,

On Fri, Jun 26, 2009 at 11:59:59AM +0200, Kees Bakker wrote:
> The following C example, with just 2 lines of comments (first is
> incomplete), shows the problem. The second line has a comment which ends
> with 3 TABs and then the closing C comment.

Actually, the two lines constitute a single comment, the second "/*"
being part of the comment, not it's delimiter.  This might not be
relevant, though.

> When you go back to the first line and type a space after the => you'll
> see that the cursor jumps back 21 positions. (The 21 appears to be
> equal to NumOfTabs * (8-1)).

> /*type a space here=>
> /* 3 tabs following after this=>			*/

I can't reproduce this.  I've tried with a newly build 23.1.50, yet don't
see the problem, whether running under a console or X-Windows, though I
think I'm using a different window manager.

Do you have any feel at all for what might be causing this?  Would you
please start emacs without your startup files (i.e. emacs -Q) and see if
you still get the bug.

Thanks for taking the trouble to report it.

> In GNU Emacs 23.1.50.1 (i486-pc-linux-gnu, GTK+ Version 2.12.12)
>  of 2009-06-26 on koli, modified by Debian
> Windowing system distributor `The X.Org Foundation', version 11.0.10402000
> configured using `configure  '--build=i486-linux-gnu' '--host=i486-linux-gnu' '--prefix=/usr' '--sharedstatedir=/var/lib' '--libexecdir=/usr/lib' '--localstatedir=/var/lib' '--infodir=/usr/share/info' '--mandir=/usr/share/man' '--with-pop=yes' '--enable-locallisppath=/etc/emacs23:/etc/emacs:/usr/local/share/emacs/23.1.50/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/23.1.50/site-lisp:/usr/share/emacs/site-lisp:/usr/share/emacs/23.1.50/leim' '--with-x=yes' '--with-x-toolkit=gtk' '--with-toolkit-scroll-bars' 'build_alias=i486-linux-gnu' 'host_alias=i486-linux-gnu' 'CFLAGS=-DDEBIAN -g -O2' 'LDFLAGS=-g' 'CPPFLAGS=''

> Important settings:
>   value of $LC_ALL: nil
>   value of $LC_COLLATE: POSIX
>   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.utf8
>   value of $XMODIFIERS: nil
>   locale-coding-system: utf-8-unix
>   default-enable-multibyte-characters: t

> Major mode: C/lah

> Minor modes in effect:
>   tooltip-mode: t
>   tool-bar-mode: t
>   mouse-wheel-mode: t
>   menu-bar-mode: t
>   file-name-shadow-mode: t
>   global-font-lock-mode: t
>   font-lock-mode: t
>   blink-cursor-mode: t
>   global-auto-composition-mode: t
>   auto-composition-mode: t
>   auto-encryption-mode: t
>   auto-compression-mode: t
>   column-number-mode: t
>   line-number-mode: t
>   transient-mark-mode: t
>   abbrev-mode: t

> Recent input:
> <help-echo> <f10> / t m p / t <backspace> e m <tab> 
> <help-echo> <help-echo> 2 <tab> <return> <down> <left> 
> <down> SPC <f4> <help-echo> <down-mouse-1> <mouse-1> 
> C-h a b u g <return> <help-echo> <help-echo> <down-mouse-5> 
> <mouse-5> <down-mouse-4> <mouse-4> <double-down-mouse-4> 
> <double-mouse-4> <triple-down-mouse-4> <triple-mouse-4> 
> <help-echo> <help-echo> <down-mouse-1> <mouse-1> <down-mouse-5> 
> <mouse-5> <double-down-mouse-5> <double-mouse-5> <triple-down-mouse-5> 
> <triple-mouse-5> <triple-down-mouse-5> <triple-mouse-5> 
> <triple-down-mouse-5> <triple-mouse-5> <down-mouse-5> 
> <mouse-5> <double-down-mouse-5> <double-mouse-5> <triple-down-mouse-5> 
> <triple-mouse-5> <triple-down-mouse-5> <triple-mouse-5> 
> <triple-down-mouse-5> <triple-mouse-5> <down-mouse-5> 
> <mouse-5> <double-down-mouse-5> <double-mouse-5> <triple-down-mouse-5> 
> <triple-mouse-5> <triple-down-mouse-5> <triple-mouse-5> 
> <triple-down-mouse-5> <triple-mouse-5> <help-echo> 
> <down-mouse-5> <mouse-5> <double-down-mouse-5> <double-mouse-5> 
> <triple-down-mouse-5> <triple-mouse-5> <down-mouse-5> 
> <mouse-5> <double-down-mouse-5> <double-mouse-5> <triple-down-mouse-5> 
> <triple-mouse-5> <help-echo> <down-mouse-1> <mouse-1> 
> <help-echo> <help-echo> <help-echo> <help-echo> <help-echo> 
> <help-echo> <help-echo> <help-echo> <help-echo> <menu-bar> 
> <help-menu> <send-emacs-bug-report>

> Recent messages:
> Loading /etc/emacs/site-start.d/50vc-svn.el (source)...done
> Loading /etc/emacs/site-start.d/70x-face-el.el (source)...done
> For information about GNU Emacs and the GNU system, type C-h C-a.
> Invalid or undefined bitmap `default' [4 times]
> kb/c-mode-customize
> Invalid or undefined bitmap `default' [4 times]
> Undo!
> Type C-x 1 to remove help window.  
> Invalid or undefined bitmap `default'
> Beginning of buffer
> Beginning of buffer

-- 
Alan Mackenzie (Nuremberg, Germany).