ansi-term scroll bug

[Chong Yidong]

When I use ansi-term in a small (Emacs) window, and run a program that
scrolls the terminal (for example, compiling a program that generates
a lot of error messages), the prompt sometimes moves to the top line
of the terminal window:

-------
| ~ $ 
| ~ $ 
| ~ $ 
| ~ $ make
|
|___

(many error messages later)

-------
| ~ $
|
|
|
|_____

The previous output ends up above the first line of the terminal
window, so I have to scroll up to look at it. This is very annoying,
especially since any keystroke I enter moves the prompt back to the
top of the window. I think this only occurs when the width of the
output surpasses the width of the window.

Here is a test case:

Make a file ~/test.c with the following nonsense contents:

int main () {
  foo(a);
  foo(b);
  foo(c);
  foo(d);
  foo(e);
  foo(f);
}

Run "emacs -q" and enter these keystrokes:

C-x 3
C-x 2
M-x ansi-term<RET><RET>
gcc test.c -o foo

Repeat the last command a few more times, and the above behavior will
appear.


GNU Emacs 21.1 (i686-pc-linux-gnu, X toolkit)
value of $LC_ALL: C
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: C
locale-coding-system: nil
default-enable-multibyte-characters: t