bug#4112: Follow mode breaks mouse wheel on Emacs 23

[Chong Yidong]

Dave Benjamin <dave@ramenlabs.com> writes:

> It looks like Emacs 23 introduced a new bug when using the mouse wheel
> within follow-mode. It's pretty simple to reproduce:
>
> 1. Start emacs under X (emacs -Q)
> 2. Load a file that's more than a few screenfuls
> 3. M-x follow-delete-other-windows-and-split
> 4. Move the mouse pointer so it's over the right column
> 5. Scroll the mouse wheel
>
> It looks like emacs is sort of fighting with itself, as it scrolls the
> column for a split second and then scrolls it back.

I've committed a fix to trunk.  Thanks for the bug report.