bug#2749: 23.0.91; Visual Line Mode: incorrect line wrapping in corner case

[pent <pent@aparamon.msk.ru>]

Hello!

I've discovered a minor problem with the new Visual Line Mode. To
reproduce:

0) emacs -Q

1) Switch to the *scratch* buffer.

2) Select menu item Options/Line Wrapping in this Buffer/Wrap at
   Window Edge.

3) Type '-' symbol. Keep typing '-' until the the line is
   wrapped. Delete the last symbol (the one which caused
   wrapping). The line should now be fully filled with '-'s, and there
   should be no wrapping.

4) Hit Enter and repeat 3). There are now 2 fully filled lines one
   after another, with no space between them (expected).

5) Select menu item Options/Line Wrapping in this Buffer/Word Wrap
   (Visual Line Mode). An empty line between 2 lines has appeared
   (unexpected).

I've found the inconsistency while working with horizontally split
Ediff windows, where the probability of wrapping exactly at the line
width is high. Hope the fix is trivial.

I'm ready to provide any additional info,
Andrey Paramonov

In GNU Emacs 23.0.91.1 (i486-pc-linux-gnu, GTK+ Version 2.14.7)
 of 2009-03-20 on elegiac, modified by Debian
 (emacs-snapshot package, version 1:20090320-1)
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/23.0.91/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/23.0.91/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: 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: ru_RU.UTF-8
  value of $XMODIFIERS: nil
  locale-coding-system: utf-8-unix
  default-enable-multibyte-characters: t

Major mode: svn-status

Minor modes in effect:
  TeX-PDF-mode: t
  diff-auto-refine-mode: t
  delete-selection-mode: t
  show-paren-mode: t
  pc-selection-mode: t
  pretty-control-l-mode: t
  shell-dirtrack-mode: t
  tooltip-mode: t
  mouse-wheel-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-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
  global-visual-line-mode: t
  visual-line-mode: t
  transient-mark-mode: t