I just went to the site again to copy-paste you the URL when all of a sudden… it works! Guess what. I tried it this time in my default browser (Firefox 25.0) and it works fine. The dropdown menu is over the youtube video, as it should be. I did try it with Opera 17.0 before (my secondary browser, where I am logged in with another user into my WP site) and it doesn’t work there. So it seems to be an Opera issue.