I am using Kubuntu 16.04, after Opera update to the 55.x, I cannot normally use developer tools anymore. I am working with two monitors and I am always detaching developer tools so developer tools could be on one screen and webpage on another.
After detaching developer tools (to be in a seperate window) everything is fine, but when you are trying to move seperated window somewhere it loses title bar (see attached image). It's super annoying and it's hard to work with a bug like this.
P.S. After update to the 55.0.2994.44 the bug is still there.