Vivaldi is not open source. Not quite an option.

https://vivaldi.com/source/

I think the UI code is not open source (so you can't build the browser yourself).

https://vivaldi.com/blog/technology/why-isnt-vivaldi-browser...

Wait, what? Vivaldi is open source? Now I am confused and really not sure what was the reason I ignored it for so long. Was there something iffy with Linux desktop integration?

It is not open source. Some of the backend is.