I've never done this before: consider this entirely speculation. However, I'd probably use relative positioning. This is speculation - I haven't opened Coda to try this - but perhaps make each top button a div that reveals its child content when it's being hovered over. That way, there's no confusion over where the drop-down menu's going to appear. It might be a little tricky fitting the things that appear under the drop-down menu, but since relative positioning allows for overlaps, I'd say that this would be a fairly simple issue. Drop-downs in CSS aren't particularly difficult: they just get more complex the more children you add.
