autospa points out a post (with video) showing off the multi-tasking abilities
of the Blit terminal
, developed in 1982 by Rob Pike and Bart Locanthi. Before Windows, before X, and before the Mac (but somewhat later than the Xerox Alto
), the Blit terminal provided a multitasking, mouse-driven graphical interface; it took a Unix server on the other side to do the heavy lifting, though.