There were three window systems that I know of that ran on the various incarnations of that machine. The first, mpx, was very basic. The second, mux, allowed editing of text on the screen and had some interesting properties, such as "hold mode", which stopped all reads to the host until hold mode was cancelled. This allowed one to use the terminal window as a general text editor for any application, and was obviously an influence on various followons. It also didn't satisfy any read to the host until you hit return, so all hold mode did was make its basic one-line operation multiline, but it felt liberating and was used constantly for writing mail messages and so on.