(A note on code style though: some of your lines are a bit too long. Have pity on those with small screens. Seriously, it's not that hard to break up line 94.)

Where can I learn enough about X to read this code?

It's still one file, but it has more features & is less obfuscated

Otherwise I mostly learnt by reading XCB header files

That said things are going the way of Wayland, so I've since switched: https://github.com/serprex/noway

How small are your screen(s)? What resolution do you run them at? What size font? Do you have eyesight issues?

I have decent vision at monitor distance. On a 13-inch Macbook Pro, using a font of reasonable size, I can easily diff two files with a line length of 108 - side-by-side.

I'm curious, then, why others can't do that. (Eyesight issues aside, of course.)

(Whether code lines should be 108 characters long is a separate issue.)

But in this case, all that is not relevant. The link was to Github, and Github enforces a width of less than a thousand pixels, regardless of screen size.

Excellent, thanks. That's something I hadn't considered.

But in this case, all that is not relevant. The link was to Github, and Github enforces...

Ah. I assumed you were speaking generally.