Firefox extension: Positioning panels

I'm making a Firefox extension and I want to position a XUL panel element exactly above a search box when a person types text in the search box-- almost like autocomplete. How do I do that?

Thanks!

Edit: I've been looking at using auto complete and using my own nxIAutoComplete implementation but I'开发者_如何学Pythonm a bit too new to extension development to do that and know what I am doing. Can anyone provide any help? I feel the solution is simple, but I just can't figure it out. Thanks.

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Maybe this helps: https://developer.mozilla.org/en/XUL/PopupGuide/Positioning