Re: prettify-symbols-mode, derived modes, and compose-region

[D]

> In your original use case you didn't have images, you had characters.
> Or did I misunderstand?

That's correct, my point was whether there was any common underlying
logic one could make use of.

> Yes, I think so.  And on text terminals this issue is moot anyway,
> because every character takes a single column (if you forget about
> those rare ones that take 2).

Exactly, it's the straightforward case.

> shr-fill-line, I guess.
> 
> You can also try using font-get-glyphs, it might be more suitable for
> your needs.  I think shr.el doesn't use it because that turned out to
> be slower than the current approach, but your use case is different.

I'll look into both, thank you!