Precondition:
The Mac Dock is ordered on the left side of screen.
The Opera Icon in the Dock is at the same height, as Opera labels.
Opera has activated the side menu on the left side. !
Opera is closed.

How to reproduce it:
Click the Opera Icon. Opera opens, and because the Opera Icon is on the same height than for example the icon personalized news (in German: personalisierte Nachrichten), label shows up "personalisierte Nachrichten" automatically, even the mouse is still on the dock layer. The label which showed up from the opera side menu, remains until you are closing opera.