[Aoyagi]

FWIW there are bits of the code here, but I don't know what is the extent of it

https://vivaldi.com/source/

[Etheryte]

The browser is proprietary, but includes open source components, without checking, what you've linked to above is probably just the open source bits.

[hobarrera]

I'm honestly curious as to who'd move to a proprietary browser in 2016 given all the history of the last couple of decades!

[chias]

Google, for one. Chromium is an open-source project, but Chrome is and has always been proprietary.

[welly]

It's still using an open source rendering engine, which I think is the important bit, it's just the container that is proprietary.