tl;dr: +1, and please make sure it's also easy to do operations with tabs in a tree
On desktop, this is the one remaining thing that's absolutely crucial to me. There's obviously many things that I would love to be added or improved, but I keep running into this one. Just now, I have a window with a couple dozen tabs, and I'd like to move them to a new window, and there's no way to do this without doing it one by one — there's not even any way that I can see to move a tab tree over, you have to do each one individually, and then re-nest them once they arrive at their destination!