bug#12403: 24.2; scroll-margin breaks emacs24

[Yuri D'Elia <wavexx@thregr.org>]

emacs24 as shipped by debian breaks "scroll-margin".

   http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=681991

It seems that setting scroll-margin on *any* value breaks the redisplay
while scrolling.

Try with the following (under a graphical session):

emacs -q

open the tutorial, scroll with the arrow keys until you reach the bottom.
Everything looks ok.

kill emacs. Start again, this time:

C-x set-variable scroll-margin 5

open the tutorial, scroll down with the arrow keys until you reach the
bottom.  Text on the page looks *badly* garbled (this is not just
character left-overs from the previous screenfull, a lot of lines look
repeated at some point in the same line).


In GNU Emacs 24.2.1 (x86_64-pc-linux-gnu, GTK+ Version 2.24.10)
  of 2012-09-09 on trouble, modified by Debian
Windowing system distributor `The X.Org Foundation', version 11.0.11203902
Configured using:
  `configure '--build' 'x86_64-linux-gnu' '--build' '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/emacs24:/etc/emacs:/usr/local/share/emacs/24.2/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/24.2/site-lisp:/usr/share/emacs/site-lisp'
  '--with-crt-dir=/usr/lib/x86_64-linux-gnu' '--with-x=yes'
  '--with-x-toolkit=gtk' '--with-toolkit-scroll-bars'
  'build_alias=x86_64-linux-gnu' 'CFLAGS=-g -O2 -fstack-protector
  --param=ssp-buffer-size=4 -Wformat -Werror=format-security -Wall'
  'CPPFLAGS=-D_FORTIFY_SOURCE=2''

Important settings:
   value of $LC_ALL: nil
   value of $LC_COLLATE: C
   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: Fundamental

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
   blink-cursor-mode: t
   auto-composition-mode: t
   auto-encryption-mode: t
   auto-compression-mode: t
   line-number-mode: t
   transient-mark-mode: t

Recent messages:
Loading /etc/emacs/site-start.d/50php-elisp.el (source)...done
Loading /etc/emacs/site-start.d/50psvn.el (source)...done
Loading /etc/emacs/site-start.d/50pymacs.el (source)...done
Loading /etc/emacs/site-start.d/50python-docutils.el (source)...done
Loading /etc/emacs/site-start.d/50python-mode.el (source)...done
Loading /etc/emacs/site-start.d/50w3m-el.el (source)...done
Loading /etc/emacs/site-start.d/50yaml-mode.el (source)...done
Loading /etc/emacs/site-start.d/51debian-el.el (source)...done
For information about GNU Emacs and the GNU system, type C-h C-a.
Making completion list...

Load-path shadows:
/usr/share/emacs/24.2/site-lisp/cmake-data/cmake-mode hides 
/usr/share/emacs/site-lisp/cmake-mode
/usr/share/emacs24/site-lisp/dictionaries-common/ispell hides 
/usr/share/emacs/24.2/lisp/textmodes/ispell
/usr/share/emacs24/site-lisp/dictionaries-common/flyspell hides 
/usr/share/emacs/24.2/lisp/textmodes/flyspell
/usr/share/emacs/site-lisp/rst hides 
/usr/share/emacs/24.2/lisp/textmodes/rst

Features:
(shadow sort gnus-util mail-extr emacsbug message format-spec rfc822 mml
mml-sec mm-decode mm-bodies mm-encode mail-parse rfc2231 mailabbrev
gmm-utils mailheader sendmail regexp-opt rfc2047 rfc2045 ietf-drums
mm-util mail-prsvr mail-utils help-mode easymenu view debian-el
debian-el-loaddefs w3m-load mmm-auto mmm-vars mmm-compat cl
emacs-goodies-el emacs-goodies-custom emacs-goodies-loaddefs easy-mmode
time-date tooltip ediff-hook vc-hooks lisp-float-type mwheel x-win x-dnd
tool-bar dnd fontset image fringe lisp-mode register page menu-bar
rfn-eshadow timer select scroll-bar 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 minibuffer loaddefs button faces
cus-face files text-properties overlay sha1 md5 base64 format env
code-pages mule custom widget hashtable-print-readable backquote
make-network-process dbusbind dynamic-setting system-font-setting
font-render-setting move-toolbar gtk x-toolkit x multi-tty emacs)