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Originally from BeOS, I believe. (Thus the funky BeOS window shape. Still quite difficult to discover.)

You could drag the short title bar with a modifier key to move it from the leftmost position, and non-overlapping title bars would snap together to form tabs.

Several Linux WMs and DEs had this feature a long time ago, when screen space was kind of at a premium. Fluxbox had it for as long as I can remember it. KDE used to have it at some point in its 3.x or 4.x-series releases, I don't recall which one.

I would love to see this in xfwm4

Also available for KWin since a very long time (at least a decade).

I've been running PopOS, but don't see it - the Super+S stack shortcuts don't seem to be available unless I'm missing something.

I think it doesn't work unless you're in the tiling mode. You can also drag one one window into another window.