@jamesisin
Sorry, I believe that I misunderstood your original note. I thought that you simply did not like the menu being in the "top of screen" position rather than the application's title bar (I'm with you on that) so i suggested that the double-click on the Opera side panel was at least the easy way to switch back and forth and get the menu when needed. The "O" button was there at one point but that seems to have been abandoned.

At any rate, on my system settings/appearance/behavior the "in the menu bar" & "always displayed" does at least work for Opera, but the fact that it then puts every program's menu there (that does not have a menu "button") at the screen top means that I stick with "windows title bar" & "always displayed" that and just do the double-click for Opera, for me the easiest way to toggle full screen.

On my boxes (combinations of 16.04, 18.04 Unity and Gnome) Chrome on most boxes seems to be operating like Opera with no window menu, Firebird and Chromium work just fine, Vivaldi uses a menu button. I have a few other browsers and Windows 10 but this note is far too long already..

At least I can get it title bars somewhere on them all. Bummer for you.

@jamesisin
Sorry, I believe that I misunderstood your original note. I thought that you simply did not like the menu being in the "top of screen" position rather than the application's title bar (I'm with you on that) so i suggested that the double-click on the Opera side panel was at least the easy way to switch back and forth and get the menu when needed. The "O" button was there at one point but that seems to have been abandoned.

At any rate, on my system settings/appearance/behavior the "in the menu bar" & "always displayed" does at least work for Opera, but the fact that it then puts every program's menu there (that does not have a menu "button") at the screen top means that I stick with "windows title bar" & "always displayed" that and just do the double-click for Opera, for me the easiest way to toggle full screen.

On my boxes (combinations of 16.04, 18.04 Unity and Gnome) Chrome on most boxes seems to be operating like Opera with no window menu, Firebird and Chromium work just fine, Vivaldi uses a menu button. I have a few other browsers and Windows 10 but this note is far too long already..

At least I can get it title bars somewhere on them all. Bummer for you.

@jamesisin

I am no expert on this subject but I do use Opera on three Ubuntu boxes on different multi-boot combinations of a few different Ubuntu and Opera releases and desktops. The title bar/menu options seem to differ, some have the Opera Button at top left on, some do not and so on.

What I can say is that so far double-clicking the sidebar (even totally empty) toggles between full screen and back and the tile/menu bar then appears on all of mine. A bit of a nuisance but if it works for you and you love Opera it gets by.

However, I do hang with LTS Ubuntu releases, and it does seem that the Opera Button itself MIGHT agree with Gnome more than Unity- but of that I am not sure.

Since the double-click works for me in all cases SO FAR , I just use that on my Unity installs.

Good luck.