jQuery fixed content that scrolls if larger than view

This is hard to explain but if you go here: https://developers.facebook.com/docs/reference/api/

You will see that BOTH the sidebar and header are fixed but because the height of the sidebar may be larger than the viewport it scrolls slightly until the user has been able to view all of it and then becomes fixed again.

My question is how could I do something like this with jQuery? I have built similar things using just CSS but would like to overcome the problem with the sidebar.

Thanks

Can't log into facebook but I think you mean something like this?

http://jsfiddle.net/b43hj/24/

If so, that's quick and dirty but should get you started. My have a few kinks to work out.