bug#8390: follow-mode does not work when text is scaled

[Bosko Markovic]

When text size is bigger than initial size, cursor does not leave
first window no matter how far it is moved.

When text size is smaller than initial size cursor jumps to other
window before it reaches bottom of the window and part of the text
from the first window is repeated at the top of second window.

Steps to reproduce problem with increased text size:

0. emacs -Q
1. M-x text-scale-increase
2. visit sufficiently long file
3. C-x 3 [split-window-horizontally]
4. M-x follow-mode
5. C-n until cursor moves past bottom of window

Expected results:

   Cursor moves to top of next window.

Actual results:

   First window scrolls down

Steps to reproduce problem with decreased text size:

0. emacs -Q
1. M-x text-scale-decrease
2. visit sufficiently long file
3. C-x 3 [split-window-horizontally]
4. M-x follow-mode
5. C-n until cursor moves past bottom of window

Expected results:

   Cursor moves to top of next window.

Actual results:

   Before it reached bottom of first window cursor moves to second
window. Displayed text in two windows is partially overlapped, part of
the text at the bottom of first window is repeated at the top of
second window.

GNU Emacs 23.3.1 (i386-mingw-nt5.1.2600) of 2011-03-10 on 3249CTO