@atomicthumbs I fully agree! In fact one of the main reasons why I switched from Firefox to Opera after well over a decade with Firefox was precisely because of Opera's vertical tab cycler. I thought it was a near-perfect implementation with the previews. It just needed a tweak to stop accidental interference from the mouse.
I used to use Tab Mix Plus for this in Firefox Classic but when Quantum was released we had to settle for the horizontal tab cycler that is even worse than the current Opera implementation.
The flag to disable the horizontal cycler did work OK in 67 but is, as you say, broken in 68.
68 also broke my instant search customisation, where I had set it up to do tab search only, by default. I'd set this up as F2. Now I get both tabs and sites but I can live with that or possibly tweak it back again. I haven't tried. Plus I know you can get tab search through the new magnifying glass or Ctrl+Space.
I do wish devs would stop fixing what ain't broken. I don't mind if they add new features or even change defaults. But don't break previous behaviour unless you're sure that a replacement is superior.
It's not just Opera though. The whole industry is guilty - and I'm a dev myself!
Looks like I may have to implement your fix - although I also live with instant tab search. Yet another major reason why I switched from Firefox!
(Edit: I now have back my F2 which just does tab search, which is fine by me - and the old Ctrl+Tab. Thanks @atomicthumbs But I bet it gets broken again in 69!)
At the moment I have Opera as default and Vivaldi as No. 2 but I can easily see this being reversed given a few more "improvements."