One of my favorite features of Vivaldi is the ability to stack tabs. Have you ever been working in your browser and wished to group your tabs by project? With tab stacks, you can. I usually have a stack with Gmail and Google Calendar, and another stack with Google Drive and open files from Google Drive.
In this picture, three websites are stacked on the one tab. To stack tabs, drag one tab on top of another. The background tab will go dim. Once that happens, let up and the tabs will be stacked. To unstack the tab, right click on the tab and select Ungroup Tab Stack.
Tab stacks do more than just group a set of tabs under one. You can also tile the tab stack. Right click on the tab stack and select Tile Tab Stack. The open sites in the tab stack will then be tiles in the current window.
Finally, you can hibernate a tab stack. This shuts down any activity in those tabs, saving memory and cpu.