In this particular case, the characters that get copied are clearly rendered far outside of the area selected by the mouse (in fact, outside the visible area),

In this particular case, the characters that get copied are clearly rendered far outside of the area selected by the mouse (in fact, outside the visible area), so I don't see a reason why the browser should think I meant to copy them. Yes, it's probably not easy to fix, and once fixed there are probably a dozen different ways of cheating. I'm just pointing out that it's certainly possible to fix this particular attack vector.