This is a JS/CSS combo that adds a floating header and sidebar. (Note that for all the features to work correctly it requires the Vector skin to be used.)
When the sidebar is larger than the window, it remains at the top, otherwise it follows the page.
If you expand the sidebar so that it is larger than the window, it will go to the top - in a pretty, animated manner.
You can also hide the sidebar by clicking the "«" and show it again by clicking "»". The header can also be hidden/shown by clicking on either one of the "˄ ˅" located at the top left-hand corner of the article.