bug#15744: 23.3; ansi-term: prompt moves to bottom of screen

[Olsen, Stuart J]

In ansi-term, once the screen has filled with output -- that is, when
the prompt for the shell is at the bottom of the window -- if one scrolls the
buffer so that the prompt is in the middle of the screen, typing
anything into the terminal buffer causes the prompt to snap back down to
the bottom of the window. This is helpful if the prompt is not in view,
but is rather obnoxious if it is, especially when using ZSH, which
displays completions and other information below the prompt, meaning
that that information is hidden once the buffer fills to the size of the
window.

To reproduce the bug, issue `emacs -Q', run the `ansi-term' command,
select an appropriate shell (I have observed this behaviour with both
GNU bash and ZSH), and issue enough commands to the shell to fill the
window. Once the window has been filled, scroll the buffer so the prompt
appears above the bottom of the window. Send any input to the shell, and
the prompt will snap to the bottom of the window.

It may be helpful to note that term.el references at line 1005 a variable
named `term-scroll-to-bottom-on-input', which does not appear to be
referenced anywhere within the code, and which is undefined in my version of
Emacs.


In GNU Emacs 23.3.1 (x86_64-pc-linux-gnu, GTK+ Version 2.24.10)
 of 2013-05-16 on panlong, modified by Debian
Windowing system distributor `The X.Org Foundation', version 11.0.11103000
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/emacs23:/etc/emacs:/usr/local/share/emacs/23.3/site-lisp:/usr/local/share/emacs/site-lisp:/usr/share/emacs/23.3/site-lisp:/usr/share/emacs/site-lisp:/usr/share/emacs/23.3/leim'
 '--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=-DDEBIAN -g -O2' 'LDFLAGS=-g' 
'CPPFLAGS=-D_FORTIFY_SOURCE=2''

Important settings:
  value of $LC_ALL: en_US.UTF-8
  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: Term

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:
M-x a n s i - t e r m <return> <return> <return> <return> 
<return> <return> <return> <return> <return> <return> 
<return> <return> <return> <return> <return> <return> 
<return> <return> <return> <return> <return> <return> 
<return> <return> <return> <return> <return> <return> 
<return> <return> <return> <return> <return> <return> 
<return> <return> <return> <return> <return> <return> 
<return> <return> <return> <return> <return> <return> 
<return> <return> <return> <return> l s <return> <down-mouse-5> 
<mouse-5> <double-down-mouse-5> <double-mouse-5> <triple-down-mouse-5> 
<triple-mouse-5> s <down-mouse-5> <mouse-5> <double-down-mouse-5> 
<double-mouse-5> s <down-mouse-5> <mouse-5> <double-down-mouse-5> 
<double-mouse-5> <triple-down-mouse-5> <triple-mouse-5> 
s s s s <backspace> <backspace> <backspace> <backspace> 
<backspace> <backspace> <backspace> <backspace> <backspace> 
<backspace> <backspace> M-x s u b <tab> C-g <help-echo> 
<help-echo> <help-echo> <help-echo> <help-echo> <help-echo> 
<menu-bar> <help-menu> <send-emacs-bug-report>

Recent messages:
Loading /var/cache/dictionaries-common/emacsen-ispell-default.el (source)...done
Loading debian-ispell...done
Loading /var/cache/dictionaries-common/emacsen-ispell-dicts.el (source)...done
Loading /etc/emacs/site-start.d/50dictionaries-common.el (source)...done
Loading /etc/emacs/site-start.d/50emacs-goodies-el.el (source)...done
Loading /etc/emacs/site-start.d/50psvn.el (source)...done
Loading /etc/emacs/site-start.d/50slime.el (source)...
Loading /usr/share/emacs23/site-lisp/slime/slime-autoloads.elc...done
Loading /etc/emacs/site-start.d/50slime.el (source)...done
For information about GNU Emacs and the GNU system, type C-h C-a.

Load-path shadows:
/usr/share/emacs/23.3/site-lisp/debian-startup hides 
/usr/share/emacs/site-lisp/debian-startup
/usr/share/emacs23/site-lisp/dictionaries-common/ispell hides 
/usr/share/emacs/23.3/lisp/textmodes/ispell
/usr/share/emacs23/site-lisp/dictionaries-common/flyspell hides 
/usr/share/emacs/23.3/lisp/textmodes/flyspell

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 term
disp-table ehelp electric ring slime-autoloads emacs-goodies-el
emacs-goodies-custom emacs-goodies-loaddefs easy-mmode 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)