bug#2996: 23.0.92; Scrolling narrows region

[Chong Yidong]

> I have `transient-mark-mode t'
>
> When I right click into a buffer that creates an active region.
> If I now scroll down and right click again, the active region is
> extended as expected.
>
> If I keep on scrolling and right-clicking, so that the start of the
> regions scrolls off the window, the region is truncated and now starts
> on the first visible line.

I can't reproduce this.  Did you scroll using the mouse-wheel, the
scroll-bar, or some other method?  Please provide a *precise* set of
instructions for reproducing this bug, starting from emacs -Q.