The usual history of graphic user interfaces goes like this: Xerox PARC invented them in the 1970s and Apple got inspired to build their own closely followed by X Windows and Microsoft... Well it didn't quite happen that way. Xerox were not the first to develop GUI technology and in the late 1970s and early 1980s there were a number of other GUIs in development.
Blit was a multi-tasking windowing environment developed by AT&T in the early 1980s. It ran on the AT&T 5620 which might be termed more a smart terminal than a computer but it had a M68K processor - like the first Macs - and so was quite powerful for it's day. The 5620 could be connected to a Unix computer via an RS-232 connection. The GUI is familiar but also had some interesting ideas, somewhat similar to Smalltalk systems in some ways and early X Windows. The video is well worth a watch.