Several of the lean GUI text editors are built on Scintilla (https://scintilla.org/), which provides a cross-platform editing component that can be integrated in GTK, Windows or Mac GUI apps. Maybe that has too much bells and whistles for you, since it's both about editing and presentation.
I guess I might be misunderstanding what Scintilla is? Everything I've seen with it has it coupled with native controls, like a winform control or a Qt control. Are you saying that the library can be used, on its own, without a graphical component? If so, that might fit the bill!
Yes, Scintilla is a text editor engine library. It's not tied to any particular UI or technology. Out of the box it's not a text editor yet; you provide the frontend. You get all the "table stakes" right away if you build your editor on this library.
Same engine, different frontends. The engine has a series of hooks that you implement in whichever way you please for your particular interface. It's definitely the presumptive choice here.
Ah, I see! Very cool! Yeah, this is the kind of thing I was looking for, so this should give me what I need to test some proof of concepts. Thanks for the links! I do wish there were something a little more ergonomic, but I'm way too far into the begging to be choosing, here, so I'm quite happy to take what I can get.
In any case, I really do appreciate the dual links. It's so much harder to suss out the boundaries of a library with only one implementation. This was really helpful.