Yes and no, 5% of the script, corresponding to its unique UI is proprietary, but 100% auditable and even moddeable by the user (in the Forum they show you even how to do it, at own risk, logical). And its better so this way, making it OpenSource too, Chrome and Edge are the first in forking it, which will be the end of Vivaldi and any other Chromium. Anyway, with more than 100 browsers curretntly in the market, OpenSource or not isn’t the most important poiny, more important the ethics and transparency respect the user of the company. Respect privacy it is irrelevant, it depends only of the manufactor of the product, not if it OpenSource or not, all spying APIs of Google, Facebook, MS & cia are all OpenSource and included in a lot of the FOSS in the market (also in Firefox, eg the “save” browsing API is from Google, not really needed if you use an adblocker (uBO), which contains a similar function, this API send your browsing data to Google who host the list of phising sites but also logs your activity, If you can, desactivate it, in Vivaldi you can do it in the privacy settings)