The white box only stays up for a second or so, then goes blue, but I think it's still leaving a layer there so that what's behind it can't be accessed.
I've tried it in Safari, which didn't show the white box but I couldn't click on anything behind where it would be. I also tried it in IE9 which flashes up the box (which could be anything from fractions of seconds to around 2 seconds). Once again, couldn't click on the login fields, though the options that it would cover once logged in seem uneffected, so maybe they have a higher z-index or something like that?