That's the FVWM window manager. His FVWM config (or at least some version of it) is available if you're curious: https://www-cs-faculty.stanford.edu/~knuth/programs/.fvwm2rc .

Thanks so much for this link! It also explains what I am seeing in that picture:

"Now comes the fun part: Buttons to push that will take me from one desktop to another.

Being a control freak, I am not trusting FvwmButtons to find the correct button layout; I'm building it myself. The goal is to have a 64x64 ASClock at the upper right, preceded by four 32x32 buttons that will aim my display at another desktop, all above a 16x128 CPU load display. Geometry-wise, I consider it to be an 8x5 grid of 16x16 squares (although I could have regarded it as a 4x5 grid of 32x16s)."

Man, he's really bought in to that whole literate programming thing..

That's the most-pleasant .rc file I've ever read.

Thank you for this..

When I see the picture in the article, I was wondering what kind of editor he is using

Damn, even his .rc files are a joy to read.

Attended one of his lectures many years ago and he was indeed using Emacs back then. Don't remember which OS but I think the whole session he was just in Emacs most of the time. Also one time he stopped and was puzzled what was going on, quickly realizing that he'd forgotten to save the file. Still to this day when I forget to save I remember this and think : "If it can happen to Knuth it's ok if it happens to you"