Rooms (Xerox PARC)

Henderson and Card had studied how people actually used their windows on PARC's Lisp machines and saw the same pattern as on a wooden desk: you hop between a handful of tasks, and each task has its own pile. Rooms, implemented in Interlisp-D and described in ACM Transactions on Graphics in 1986, let the user gather windows into rooms, move between them through "doors", and see every room at once in an overview. A window could live in several rooms simultaneously at different positions, and anything that should follow you everywhere went in a pocket.

The paper is the one everyone cites afterwards. vtwm, olvwm and fvwm built their virtual desktops on the idea in the early 1990s, CDE made four workspaces the default, and GNOME, KDE, macOS Spaces and Windows 10 reached the same conclusion in turn. Xerox also sold a commercial version, Rooms for Windows, around 1990, but the market was not yet convinced that one screen could hold more than one desktop.