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@burnout426 Thank you for these steps, worked nicely for me to revert back to the previous version. I hope Opera reconsiders and provides the more bubbly, rather irritating Opera One style optional, I like to keep it up to date for security and core functionality otherwise. 
@watersotr Perhaps the Opera team should be congratulated for achieving what I suspect is a milestone, if a dubious one, in software development. I think, perhaps, they allowed either a single "rock star" developer, or that person and a gaggle of devotees, to push for what they thought were slick, shiny design and usability changes that turned out to be not so slick and not as usable as they'd hoped.
Whether young or inexperienced or overconfident or a combination, it's entirely possible for people to hyperfocus on some singular goal in development and ultimately neglect the rest, in other words not seeing the forest for the trees, as they say. I think with Opera One they did just that, and however unclear the circumstances may be to mere users choices were made that transcended subjectivity and have proven objectively poor.
Speaking as a longtime IT guy with a background including web and database development, and one having worked with and shaken my head sadly in the aftermath of such people, well-intentioned at best and frightfully egotistical at worst, I can only hope Opera will step back, course correct, and work more closely with their user base to devise a solution that not only deploys the bleeding edge some of their aspirants crave, but also cater to their devoted longtime user base that's stuck with them until recently.
Challenge for the Opera team.
Cracked open Microsoft Edge this morning and noticed a border around my browser tab area, not as big as that of Opera One but a bit irritating to me, I prefer borderless.
Microsoft has this simple option in among its Appearance settings for Edge. Disabling it renders the screen borderless just like before:
I look forward to such a capability available in Opera One, if it becomes available I'll definitely consider switching back to it.