Side element fixes to top of page once scrolled to

I'm not even sure I'm describing this properly, but basically I want to display a leaderboard ad the way www.okcupid.com has setup.

I found this question helpful (http://stackoverflow.com/questions/3303173/position-element-fixed-vertically-absolute-horizontally) but it stays fixed. I would pr开发者_运维知识库efer it to scroll with the page until it reaches the top and then be fixed. Any ideas?

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You would use JavaScript.

The element will have normal flow on your page, and when the document.body.scrollTop is larger than the element's y position, give it position: fixed.