It's just a plain textarea with no syntax highlighting, and for some reason it feels the need to scroll the texteditor to make it so where you're typing is always the last line, it's nearly unusable and so much worse then the already pretty bad editor that's used for themes. The same goes for the editor for additional scripts on checkout and everywhere you can write liquid outside of the theme editor, it makes for a terrible user experience and has me relying on copy and pasting all the liquid into VS Code using a third party extension for syntax highlighting and then copy and pasting it back into the text area, not ideal at all.
Yep, it sucks alright.
I would recommend using something like the Atomic Chrome extension, I would guess there's probably something similar for VS Code. That way you can edit code in those fields in VS Code without having to copy/paste.