Cool stuff.
What's funny is that when learning Smalltalk, I got used to proportional fonts for programming, but only in a Smalltalk environment. I still find it odd to see that in another editor, and I have no idea why.
I actually replaced my programming fonts with serif fonts in all languages after a few years in Smalltalk. It’s not quite as good in a brace language like Java, but for Python it works really well, I find.
Because Smalltalk already shows up the main structure in boxes.
I noticed the same thing: loved Smalltalk-80, but when editing documents or code in Emacs, I need monospaced fonts.