Toggling a breakpoint will shift the line contents

[David Kastrup]

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Please describe exactly what actions triggered the bug
and the precise symptoms of the bug:

If I am in C mode and have gud-mode active, and am in a line that
starts with a single tab as white space, for example, like 6480 in
dispnew.c that says

        goto changed;

and then set a breakpoint with C-x SPC, then the text will shift to
the left when the breakpoint is indicated by a red circle in the
margin, and will shift to the right again if it is removed.  It would
appear that only TABs are afflicted: lines starting with spaces are ok.

In GNU Emacs 21.3.50.16 (i686-pc-linux-gnu, GTK+ Version 2.2.1)
 of 2004-02-29 on lola.goethe.zz
configured using `configure '--prefix=/usr/local/emacs-21' '--with-gtk''

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
  locale-coding-system: iso-latin-1
  default-enable-multibyte-characters: t

Major mode: C

Minor modes in effect:
  auto-compression-mode: t
  desktop-save-mode: t
  display-time-mode: t
  iswitchb-mode: t
  mouse-wheel-mode: t
  menu-bar-mode: t
  unify-8859-on-encoding-mode: t
  line-number-mode: t
  abbrev-mode: t

Recent input:
C-x o <escape> < C-s g u s <backspace> d C-s C-s C-s 
C-x o C-x SPC C-x SPC C-x SPC C-h k C-x SPC C-x C-a 
C-d C-h k C-x C-a C-d C-x C-a C-h C-M-v <down> <down> 
<down> <down> <down> <up> <up> <up> <up> <up> <up> 
<next> <next> <down> <down> <down> <down> <down> <down> 
<down> C-x C-a C-t C-x C-a C-d C-x C-a C-t <right> 
<left> <right> <right> <left> <left> C-x C-a C-d C-x 
C-a C-t C-x C-a C-d <up> <up> <up> C-x C-a C-t C-x 
C-a C-d M-x r e p o <tab> r t <tab> <return>

Recent messages:
Command: tbreak dispnew.c:6480
/home/tmp/emacs/src/dispnew.c and /usr/local/tmp/emacs/src/dispnew.c are the 
same file [2 times]
Command: clear dispnew.c:6480
/home/tmp/emacs/src/dispnew.c and /usr/local/tmp/emacs/src/dispnew.c are the 
same file
Command: tbreak dispnew.c:6477
/home/tmp/emacs/src/dispnew.c and /usr/local/tmp/emacs/src/dispnew.c are the 
same file [2 times]
Command: clear dispnew.c:6477
/home/tmp/emacs/src/dispnew.c and /usr/local/tmp/emacs/src/dispnew.c are the 
same file
Making completion list...
Loading emacsbug...done

-- 
David Kastrup, Kriemhildstr. 15, 44793 Bochum