[j_koreth]

I'm not aware of any spyware in the browser.

I believe the source is online but not licensed under a free license (Someone is free to correct me on that).

[chucky]

The browser engine, being a Chromium variant, is open-sourced. The source code for the UI is not available under an open source license it seems: https://help.vivaldi.com/desktop/privacy/is-vivaldi-open-sou...

[dagurp]

More on the subject https://vivaldi.com/blog/vivaldi-browser-open-source/