The original opera developers had moved to Vivaldi, it provides a check box in settings to use System's Frame/Theme that could bring back the Title Bar & Border.
Looks like the Opera Developers are Windoz heads to favor Windoz users' requests with no reason (https://autohotkey.com/board/topic/66060-opera-11-remove-titlebar-but-keep-taskbar/)
And it even removed the option to bring back System's setting like Vivaldi does. (So you have to stay with Windoz).
The intension (Windoz like UI, no response to Linux Users' requests) is so obvious. And it's your choice to stay with it.