Menu "scrambled" in Opera

Issue Summary

I am on Mac OSX and using Opera.

I activated some primary menues at the top and whenever I hover over the menu terms to make the sub-terms display, the shown list appears scrambled in Opera. The first two entries are visible, but below that I can only see the right half (or less) of the menu.
When moving the mouse over the scrambled space, the highlighted sub-menu becomes readable, as long as the mouse curser is over it.

Safari and Firefox work fine.

Is this a known issue already?

Cheers