vertical-motion primitive fails specification

[Josh Buhl]

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In GNU Emacs 21.2.1 (i386-debian-linux-gnu, X toolkit, Xaw3d scroll 
bars) of 2002-03-22 on raven, modified by Debian
configured using `configure  i386-debian-linux-gnu --prefix=/usr 
--sharedstatedir=/var/lib --libexecdir=/usr/lib --localstatedir=/var/lib 
--infodir=/usr/share/info --mandir=/usr/share/man --with-pop=yes 
--with-x=yes --with-x-toolkit=athena --without-gif'
Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE:
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: C
  locale-coding-system: nil
  default-enable-multibyte-characters: t

Please describe exactly what actions triggered the bug
and the precise symptoms of the bug:


The vertical-motion primitive is supposed to move point to the *start*
of a screen line, i.e. a column which is equal to 0 modulo
frame-width.

In the documentation it says:


vertical-motion is a built-in function.
(vertical-motion LINES &optional WINDOW)

Move point to start of the screen line LINES lines down.
----------------^
.
.
.

The new position may be the start of a line,
------------------------------^
or just the start of a continuation line.
--------------^


This is the behavior of emacs 20 on my system. In Emacs 21 running in
X on my system, vertical-motion is moving point down by screen lines,
but it is preserving the starting column modulo frame-width.

e.g. if I start in column 40 on a line which is wrapped onto the next
screen line, in a frame whose width is 80 and I evaluate the
expression (vertical-motion 1), then point lands in column 120 instead
of column 80, which would be the start of the next screen line.

Emacs 21 is not doing this when running in a terminal on my system and
it does occur when I run "emacs -q --no-site-file".



Here's how to set up a concrete example:

Start emacs 21 in X. Set the frame width to 80 by using the mouse to
adjust the width of the window. Check the frame-width by evaluating
(frame-width). Make sure toggle-truncate-lines is turned off
(i.e. lines are wrapped and not truncated.) In an empty buffer press
the 'a' key and insert 140 a's without spaces inbetween. There should
now be one text line consisting of 140 letter a's which starts in the
first screen line and is wrapped onto the second screen line. Move
point to column 40 in the first screen line. Type

M-x eval-expression

and evaluate the expression

(vertical-motion 1)

Point lands in column 120 in the second screen line. If you do this in
a terminal or with emacs 20, point will land at the start of the
second screen line (which will be column 80 in emacs 21 and column 79
in emacs 20, since in emacs 20 there is a '\' displayed at the end of
the first screen line to indicate the wrapped line.)

Recent input:
<help-echo> <help-echo> <help-echo> <help-echo> <menu-bar>
<help-menu> <report-emacs-bug>

Recent messages:
Loading 50tdtd (source)...done
Loading easy-mmode...done
Loading /usr/lib/emacs/21.2/i386-debian-linux-gnu/fns-21.2.1.el 
(source)...done
Bootstrapping objects...done
Loading cl-extra...done
Loading cl-macs...done
Loading mule-util...done
Loading paren...done
Loading server...done
Loading emacsbug...done