I went shopping to buy myself some wings and couldn't switch from USD to CAD. Not a huge deal because I'm used to converting in my head, but it looks like the picklist tries to expand, but doesn't actually expand. The arrow switches direction, but that's it.
When this happened to me I saw a box at the bottom of the page and I could type letters, such as GBP, and they went in the box and it changed.
The tricky bit is that the currency changing thing is an 'app' and not an integral part of the actual Store, and the Store is already from a vendor and not really under our control, and the currency changer is one step further away from that... :-/