Also, not everything is a file. Tabs should apply to all edges of all windows, including top level windows, not just one edge of only windows with files in them. And you should be able to drag any window out to top level and it still has its tab attached, then move it around to any position along any edge, or hide it, and of course snap windows together along their tabbed edges, either tiling or overlapping.

How do you control all of that? That's where the pie menus on the tabs come in, of course. Thanks to the tabs, you can even pop up pie menus on windows that are completely covered up, and perform commands on them even though they're not visible, like bringing them to the top (stroke up) or down (stroke down), or closing them (diagonal stroke for confirmation submenu, then stroke up to confirm), or whatever (paste into terminal emulator, evaluate code in editor, etc).