bug#2618: 23.0.90; Point shifts leftward during vertical motion in buffaced buffer

Open a text file and shrink the text size (using bufface) to -2.  Move
the cursor right several glyphs from the beginning of the line, then
move the cursor down one line.  On my machine, the cursor moves down,
but also moves left one glyph.  After any left or right cursor motion,
the next upward or downward cursor motion shifts the cursor one glyph
towards the left margin; further vertical motion is not affected.  The
problem stops when you return to normal fact size.

This is with fonts

  -jmk-Neep Alt-normal-normal-normal-*-20-*-*-*-c-*-iso10646-1 (bufface  0)
  -jmk-Neep Alt-normal-normal-normal-*-15-*-*-*-c-*-iso10646-1 (bufface -2)
  -jmk-Neep Alt-normal-normal-normal-*-11-*-*-*-c-*-iso10646-1 (bufface -4)

from the Debian distribution.


In GNU Emacs 23.0.90.1 (i486-pc-linux-gnu, GTK+ Version 2.12.11)
 of 2009-02-07 on elegiac, modified by Debian
 (emacs-snapshot package, version 1:20090207-1)
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' '--infodir=/usr/share/info' '--mandir=/usr/share/man' '--with-pop=yes' '--enable-locallisppath=/etc/emacs-snapshot:/etc/emacs:/usr/local/share/emacs/23.0.90/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/23.0.90/site-lisp:/usr/share/emacs/site-lisp' '--with-x=yes' '--with-x-toolkit=gtk' 'build_alias=i486-linux-gnu' 'host_alias=i486-linux-gnu' 'CFLAGS=-DDEBIAN -DSITELOAD_PURESIZE_EXTRA=5000 -g -O2' 'LDFLAGS=-g -Wl,--as-needed' '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.UTF-8
  value of $XMODIFIERS: nil
  locale-coding-system: utf-8-unix
  default-enable-multibyte-characters: t

Major mode: VM Presentation

Minor modes in effect:
  text-scale-mode: t
  shell-dirtrack-mode: t
  auto-image-file-mode: t
  show-paren-mode: t
  icomplete-mode: t
  mouse-wheel-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  blink-cursor-mode: t
  global-auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t