I did a quick check of a CM6 project (customizing the JS lexer) with the base options and a JS-derived lexer and it came out to 405k / 124k gzip. I wouldn't consider it especially lightweight but I think the size is reasonable for the feature set.
Here it is around 300kb inflated https://codemirror-basic-setup.replitfaris.repl.co/ using the basic-setup package, and I don't think this build tree-shakes.
Code for this build is here https://codemirror-basic-setup.replitfaris.repl.co/__repl
> Unfortunately copy and paste doesn't work well on Firefox on Linux
Code editing in the browser has plenty of valid criticisms, but that criticism is as pointless as saying it won’t work in Lynx. Use any one of the many many browsers that do support copy and paste.
> which clocks in at 2 kB.
You aren’t comparing like-for-like in terms of functionality. You might as well say just use a <textarea> tag which clocks in at 0.02kB.
> but the minified JS is still over 1 MB
From the article: Modularity, bundling, and footprint: “CodeMirror is built with modern technologies. You can even use ES6 modules to import it with no bundler involved. Lazy-loading features is a breeze; dynamic ES6 imports are all you need. The project is very modular and has a very slim core.”.
CodeMirror is relatively light, compared against Monaco: “Monaco is huge. monaco-editor and related libraries contributed a whopping 51.17 MB to our bundle size (5.01 MB when parsed + gzipped)”, in Monaco “lazy-loading features are not possible as far as I'm aware”.
Unfortunately copy and paste doesn't work well on Firefox on Linux in any of these fancy editors because support for the primary selection is broken and Mozilla doesn't seem interested in fixing it: https://bugzilla.mozilla.org/show_bug.cgi?id=1593761
The article calls CodeMirror "lightweight" but the minified JS is still over 1 MB. If you want lightweight then you probably want something like CodeJar (https://github.com/antonmedv/codejar) which clocks in at 2 kB.