ADDENDUM
I tried re-installing Opera on top of existing version, but still no change.
Then. I suddenly jerked the Menu tab downwards with the left mouse key. The window became horizontally half-screen and when I restored it to maximum, the problem was solved with the Menu tab back at its normal place.
http://i.imgur.com/EhsMj7e.jpg
I suspect, was caused the problem was an unintended similar but opposite move.
I suggest Opera developers investigate how this can happen and eliminate it, as it could occur unintentionally.