It is a stuck dropdown. If you make your browser smaller, it will turn into an actual dropdown. It looks like someone heard about responsive design, but didn't really understand why or how.
It's actually not bad - it's likely too complex to make it responsive all the way down to mobile, and they have a mobile site for that function, but it fits perfectly for those browsing on different screens.
I just wish they'd make it fixed width. The site's barren on my wide monitor.
I like the idea of the menu being responsive, but I dislike the drop shadow when it's open by default. The drop shadow makes it look too out of place.
I don't have a problem with the menu being responsive. It's just poorly designed and doesn't contribute very much.
I've been seeing it for at least a couple of months as well. I only see it every now and then.