The hover region is limited to max-height: 2.2em. Certain browsers take this literally. That's why the dropdown collapses in Opera 12 when you move the mouse down below the 2.2em limit. I'm not even sure if there is a rule in the specs about this. I don't think Opera is wrong. It's just different. The CSS is asking for this limit, right?
Removing the limit solves the issue.
Possible patch to discuss and improve: