I'm running two displays and whenever the Opera window is on my RIGHT display, the dropdown menus for folders in the bookmarks bar show up on the rightmost edge of the LEFT display. This seems to only happen when running dual displays with the LEFT display set as the primary (the one with the menubar in System Preferences > Displays > Arrangement). If I make the right display the primary, it works as it should on BOTH displays. Also if I set the left display to a lower resolution, the menus then appear on the correct display, but in the wrong position -- but again, only on the RIGHT display. Not a major issue, but a pretty ugly-looking one!
Edit: Here's a screenshot of the problem: