I've often said OSes and DEs have stagnated, so web browsers started innovating in areas they shouldn't have needed to. Tabs is one such area. And now with Cosmic:
Stacks, snapping, and sticky windows
◦ Stack windows to combine them into tab groups like a browser
••• Right click on the header and choose Create Window Stack. Then drag another window to the stack.
••• When tiling windows, simply drag the window on top of another to create a stack.
Tabs are an interesting way to handle multiple instances of the same app (though this sounds like cosmic might mix them too). But in Windows for example, each app would have to do it's own implementation of muli-document handling. Browsers just brought us the tabs metaphor to manage them. I always thought that should be done at a higher level than the apps, and now it's here! I was thinking toolkit level, but go a level up to the DE and mix apps.