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

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. 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=>			*/



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