how can mouseover work and not mouseleave, it doesn't make sense, no way to miss an event... We can read that an event can be missed if we move the mouse very quickly in and out of the frame; this is unthinkable.

I'm coming to the conclusion that either the browser's JavaScript context is a load of crap, or, more likely, we're reading a lot of rubbish on the subject. This deserves to be clarified.