I'm showing the Fonto trial to someone, on their laptop with a resolution of 720p. After playing around a bit, the toolbar has become invisible.
After taking a look at the HTML, it seems that the content of the iframe decides to fill the screen.
I was working in Google Chrome, on a recent version.
After a refresh, the problem fixed itself, I cannot reproduce it, sorry.
Also, I've got more images, but since I'm a new user, I cannot post them all.