I didn't even notice there was a hidden message triggered by the mouseover..... I remember that once this practice was discouraged by many usability recommendations!

I thought it was pretty clear with the hand mouse cursor to the bottom right.

it is the kind of spammy-like detail you are likely to find in popup windows that probably is blind to me

Check out the jitter if you leave your mouse just to the left of the hand.

This doesn't happen in Firefox so I'd assume it's a bug of the browser.

Worse in Firefox then in Chrome for me. I can not get a continuous jitter in chrome at all but easily get the button and hand to jitter continuously in Firefox if I leave the mouse at the right spot.

I agree !