Please Allow Tabs To Be Dragged Outside of Window
As a developer I often need to move tabs outside of the browser to create a new tab window. I also open two browser windows and drag tabs between the browsers. Currently in Vivaldi the tabs are stuck in the same browser window, even if I have another Vivaldi window open.
This is a functionality that is still in the works at the moment. The Vivaldi team has acknowledged that they are going to implement this eventually, it's just taking a while because they have to write the code from scratch.
Meanwhile, you can right click on a tab and go to Move To > New Window