Vivaldi has no mouse locking functionality
Vivaldi lacks the ability to prompt me and allow a website to lock my mouse. In Chromium, the functionality is just like asking me if it can use my microphone, et cetera. It asks if the site is allowed to lock my mouse and I can press escape at any time to remove the mouse lock. This functionality is necessary most notably for WebGL video games such as UberStrike or QuakeJS, but some other webapps as well.