It is indeed a shame Vivaldi lacks so much in this sort of UI integration. Accessibility should be one of Vivaldi's assets. You can enable the Chromium setting, yet the UI integration isn't there. It would be better if Vivaldi had integrated it into its own user interface including custom shortcuts (currently F7 is focus panel and Shift+Arrows is for spatial navigation).
The Scroll Lock suggestion is a great one, I also mentioned that to the devs some time ago. Integration with spatial navigation is still something to think about.