Hey, I've got a development installation of Storefront on Joomla 3.0 with HikaShop, but the checkout module in the header bugs out any time you have something in your cart. Whenever there's a product in your cart, the overlay won't go away - it just stays on top of the page (see screenshot).
Take a look: http://projects.discoverrg.com/coffee
Is this a javascript issue? or CSS?