To clarify, "treego" is refering to an installation of "Opera for Linux" in the Crostini Linux container of a Chromebook.
I am refering to the more usual way to install Opera on a Chromebook - from the Google Play Store where a slightly different version of "Opera for Android" is offered. However, "Opera for ChromeOS" seems very poorly documented.

BTW I have found the main settings, only available as a gear in the "profile" dialogue, which is only available in part-screen mode because scrolling of the task-bar is inhibited in full-screen mode. There seems to be no setting to auto-hide the side bar so it constantly takes up screen real estate. So does the tab bar because it is not integrated into the widget bar.