bug#998: after split-window-horizontally scroll works only on first window

[martin rudalics]

> Doesn't look too terrible, but I'd first want to understand why we're
> only seeing this now.  Shouldn't Emacs-22 already have suffered from
> that same problem?

I think so.  IIRC the stereotypical answer to this was something like
"get yourself another mouse-driver, another mouse, ...".

martin, who didn't try to reinvent the mouse-wheel.