Your Favourite Undocumented Feature
Full-screen a video, hit F11 to have the video fill the entirety of the window. May be a bug, but if it is, it's a bug I hope the Vivaldi team never squash.
Currently this I guess. Tabs on side, icon only
I believe new things like "tab audio auto mute" or "plugins settings" won't be included. They're so new, that no official document/help yet, except blogs or forum knowledge.