We also have http://www.cs.dartmouth.edu/~doug/IX/, in particular "Multilevel Windows on a Single-Level Terminal" (http://www.cs.dartmouth.edu/~doug/IX/163f.ps):

"The workings of mux, a windowed-terminal handler, that can run differently classified sessions in different windows."

[google search tip for historians/computer archeologists who don't want to find SE-Linux when searching for Unix history: enter "before 1/1/{years ago} as a custom time range]