AvalonEdit VS ScintillaNET

For the most part it's best to just use something like AvalonEdit that already implements it for you. There's some documentation and if you dig a long time ago there was a book about how it was made (along with SharpDevelop) that may still be findable.

I lied in the OP. The text box I'm using is actually the AvalonEdit text editor and it apparently doesn't support binding and they think it would cause performance issues: https://github.com/icsharpcode/AvalonEdit/issues/84

Not OP, but at work I recently implemented an HTML/CSS/JS code editor and used AvalonEdit to great effect for syntax highlighting in a large multi-line textbox. I was also able to add custom linting via regexs and it's got custom theming capabilities. Super happy with it.

I'm using the ScintillaNET NuGet package. It had its last update 6 years ago. Its last repo commit 5 years ago. The documentation... is lacking - for example, copying example code from there immediately gave me obsolete warnings. Still, this was simply the only library I could find for this.

If you're using Windows Forms, you can also take a look at ScintillaNET (or for WPF inside of a WindowsFormsHost).

It’s actually WinForms 😄. I’m using dockpanelsuite, DarkUI, and ScintillaNET. I forked the dockpanelsuite and DarkUI repos. My modifications to dockpanelsuite adds a docking manager and ability to use extra (non-default) theme files from a vstheme.gz format that the documentation specifies. I then extended DarkUI to accept ThemeBase objects and apply the necessary drawing/coloring styles to the controls.