Now I'm definitely puzzled. I've just done exactly that in Opera and pressing back and then forward again brings me back to this message with all the text intact. If I deliberately try to lose it by pressing refresh then I get the "Restore Autosaved content" option in the bottom left of the editor which will bring everything back since the last auto save (every 60 seconds).
Pressing back in other browsers (e.g. Chrome) gives a pop-up warning that you're going to lose content by navigating away but even if you override that you can still restore the auto saved content.