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

[Chong Yidong]

Bosko Markovic <boskoma@gmail.com> writes:

> 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

Fixed in trunk now.  Thanks for the bug report.