I made this repo to support my question posted here: https://discuss.codemirror.net/t/local-language-package-not-working-with-rollup-dedupe/4252

in lang-example/

npm i rollup-plugin-ts
rollup -c

in editor/

npm install --save ../lang-example
npm i @rollup/plugin-node-resolve rollup-plugin-ts
rollup -c

open editor/index.html in a browser.

The language package works, there is syntax highlighting and folding.

In editor/, if you

• in editor.js
  • uncomment line 3
  • comment line 4
• rollup -c
• reload index.html

there no longer is syntax highlighting and folding.

It seems that although {dedupe:["@codemirror/state"]} in editor/rollup.config.js is necessary (try to rollup -c and reload index.html without it), including the language package this way (as a dependency) instead of a .js file in the root of the editor project makes it not have any behavior, nor cause errors.