I've noticed some behaviour which could explain why sometimes users find
that URLs are not clickable. The scenario to reproduce with CVS HEAD is this:

1. find an article that has a URL, and is big enough to have a scrollbar
2. display the top of the article
3. mouse over the URL and see the cursor change to a hand (clickable URL)
4. scroll the article one line
5. mouse over the URL and see that it's not clickable
6. move the mouse down to where the URL was before scrolling. The cursor
should change back to a hand

So, it looks like the tags we use to track URLs are calculated based on the
original position, but don't take scrolling into account.