bug#7910: 23.2; inconsistent scrolling

[Oleksandr Manzyuk]

This bug report will be sent to the Free Software Foundation,
not to your local site managers!
Please write in English if possible, because the Emacs maintainers
usually do not have translators to read other languages for them.

Your bug report will be posted to the bug-gnu-emacs@gnu.org mailing list,
and to the gnu.emacs.bug news group.

Please describe exactly what actions triggered the bug
and the precise symptoms of the bug.  If you can, give
a recipe starting from `emacs -Q':

1. M-x eval-expression RET (setq scroll-preserve-screen-position t) RET

2. Open a long file that takes up more than one screen.

3. Make sure that the beginning of the file is at the top of the screen
and go to a line near the bottom of the screen.

4. Press C-l to bring that line to the center of the screen.

5. Press M-v and then C-v.

The expected behavior is that point goes back to the line where it
started.  It is not so, however.  When M-v is pressed, the window is
scrolled so that the beginning of the file is at the top of the screen
and point is placed in the center-most line.  When C-v is subsequently
pressed, the window is scrolled up one screenful, and the position of
point relative to the screen is preserved, so point ends up in the
center-most line.

I would like to remark that if I perform dual actions near the end of
the file, then scrolling is consistent: if the end of the file is not
visible and scrolling the window one screenful up would bring the point
past the end of the file, the window is scrolled so that the point ends
up at the last line of the file (which is put in the center of the
window), and subsequently pressing M-v brings point back to the line
where it started.

If Emacs crashed, and you have the Emacs process in the gdb debugger,
please include the output from the following gdb commands:
    `bt full' and `xbacktrace'.
For information about debugging Emacs, please read the file
/usr/share/emacs/23.2/etc/DEBUG.


In GNU Emacs 23.2.1 (x86_64-pc-linux-gnu, GTK+ Version 2.20.1)
 of 2010-09-02 on pluot, modified by Debian
Windowing system distributor `The X.Org Foundation', version 11.0.10706000
configured using `configure  '--build=x86_64-linux-gnu'
'--host=x86_64-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/23.2/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/23.2/site-lisp:/usr/share/emacs/site-lisp:/usr/share/emacs/23.2/leim'
'--with-x=yes' '--with-x-toolkit=gtk' '--with-toolkit-scroll-bars'
'build_alias=x86_64-linux-gnu' 'host_alias=x86_64-linux-gnu'
'CFLAGS=-DDEBIAN -g -O2' 'LDFLAGS=-g' '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: en_US.UTF-8
  value of $XMODIFIERS: nil
  locale-coding-system: utf-8-unix
  default enable-multibyte-characters: t

Major mode: Emacs-Lisp

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

Recent input:
C-x b s c r C-g ( s e t q SPC s c r o l l - p r e s
<M-tab> SPC t ) C-x C-e C-x C-f . e m a <tab> <return>
C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n
C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n
C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-n C-f C-f
C-f C-f C-f C-f C-f C-f C-f C-f C-f C-f C-f C-f C-f
C-f C-f C-f C-f C-l <prior> M-x r e p o r t SPC e m
a c s SPC b u g <return>

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Quit
t
Loading vc-git...done

Load-path shadows:
/usr/share/emacs/23.2/site-lisp/debian-startup hides
/usr/share/emacs/site-lisp/debian-startup
/usr/share/emacs/23.2/site-lisp/dictionaries-common/ispell hides
/usr/share/emacs/23.2/lisp/textmodes/ispell
/usr/share/emacs/23.2/site-lisp/dictionaries-common/flyspell hides
/usr/share/emacs/23.2/lisp/textmodes/flyspell
/usr/share/emacs/23.2/site-lisp/ruby1.9.1-elisp/ruby-mode hides
/usr/share/emacs/23.2/lisp/progmodes/ruby-mode

Features:
(shadow sort mail-extr message sendmail regexp-opt ecomplete rfc822 mml
easymenu mml-sec password-cache mm-decode mm-bodies mm-encode mailcap
mail-parse rfc2231 rfc2047 rfc2045 qp ietf-drums mailabbrev nnheader
gnus-util netrc time-date mm-util mail-prsvr gmm-utils wid-edit
mailheader canlock sha1 hex-util hashcash mail-utils emacsbug vc-git
tooltip ediff-hook vc-hooks lisp-float-type mwheel x-win x-dnd
font-setting tool-bar dnd fontset image fringe lisp-mode register page
menu-bar rfn-eshadow timer select scroll-bar mldrag mouse jit-lock
font-lock syntax facemenu font-core frame cham georgian utf-8-lang
misc-lang vietnamese tibetan thai tai-viet lao korean japanese hebrew
greek romanian slovak czech european ethiopic indian cyrillic chinese
case-table epa-hook jka-cmpr-hook help simple abbrev loaddefs button
minibuffer faces cus-face files text-properties overlay md5 base64
format env code-pages mule custom widget hashtable-print-readable
backquote make-network-process dbusbind system-font-setting
font-render-setting gtk x-toolkit x multi-tty emacs)