Vivaldi on desktop enables the “Browser windows in Portal”: an underlying change to how Vivaldi handles multiple windows to open faster & reduces memory usage.
Why? Maybe because of Google? Google has the same influence in Firefox, there even Google devs working in Mozilla on Firefox. Don't forget, Chromium is FOSS and every company is free to gut Chromium to they like, which Vivaldi devs are doing. EDGE is also Chromium, it is a privacy Nightmare, but all calls are of MS and participating companies, but zero calls to Google.
The original engine was KDE's KHTML, forked by Apple into WebKit which Google improved, creating Blink, which currently is the best engine, because of this also the most used.
That said, none of your reasons really. Well, in a sense because of Google. But not their influence on FF, but Manifest 3.
Plus, having Blink as the only engine on the web paves the way to non-standardized practices and technologies. And Google would control that. And one company having too much control has never been a good idea. Isn't that right, Microsoft (cough Internet Explorer)?
I mean, thats only because I use it as my main browser and have hundreds of tabs across seven different workspaces so I would be really surprised if it wasnt.
Since every major release od Vivaldi is unstable until it gets 2-3 minor updates, I feel like "massive code refactoring" means it'll need at least 5 updates this time. I'll give it a few weeks. But the changelog looks cool nonetheless.