And here only the сlose button, it's normal for Gnome, but not for KDE.

I have Opera developer 27.0.1670.0, i can use this "workaround" outlined above, but it would be good to add a native option in browser.

KDE unfortunately doesn't support window manager hints for removing the title bar on maximization

Okay, but Chromium have option "Use system title bar and borders", so we want have the same in Opera.