Opera lost titlebar. It is not a bug, is is a feature.
https://forums.opera.com/topic/28207/ti ... -ubu-16-04

Is it somehow possible to force Opera to use Kwin titlebar?

The Opera should have same kind of setting as the Chrome:


but as it doesn't have this option you could add a window rule for the Opera: KDE System Settings > Window Management > Window Rules:



The setting will force the Kwin titlebar and frame. Note - there will be double frames & buttons: