Switch to other desktop
I've been using Vivaldi since pretty early and really like it, however there's one weird point of behaviour:
I usually work with multiple virtual desktops on Windows, so I'll have multiple browser windows open. When I open a link from an external program, e.g. the mail client, the website opens in the browser window I last used, but instead of showing it to me directly, Vivaldi switches to the browser window in the virtual desktop I opened first, away from the stuff I want to see right now. So, I have to switch back. Sure, it only takes a second or two, but over time it gets a little annoying.