> on-screen text that can serve as an entry point or continuation to perform all sorts of computations, things you'd normally write hacky scripts for.

The Acme editor famously works this way. So, in a rudimentary way, does the Emacs scratch buffer. It's a weird amalgam of text editor and REPL: you evaluate snippets of text and get the output in-place, and then edit the text to get what you want next. This live-text-as-code way of working is not how I'm used to interacting with a programming system and always slightly rewires my brain. But even in that simple form, you can grasp in it the beginnings of a whole computational paradigm, all the way up to UI. (Computational models don't usually imply a UI—this is an exception, as are spreadsheets.) Oberon must be the most systematic realization of this. I'd like to try it.