The reason why I've stopped using Opera was the adoption of the Windows proxy as the only option. It sucks! If you are not going to offer a decent many independent (not the Windows one) proxies management capability than Vivaldi is not for me. Also you must add a button/combo box to quickly choose the proxies. Also you must add a button/combo box to quickly change the user agent string.
Every decent browser should offer its own proxy settings!
It's not rocket science to change this deplorable custom introduced by Google. Some other Chromium-forks have their own proxy settings
As it is for now, Vivaldi is useless to me since none of my browsers are using the proxy settings of Internet Explorer.