Use case:

• package A has a css file, a/style.css and builds with this plugin, class names use modules.
• package B has its own css file, b/style.css and also imports a/style.css

Result:

• package B uses modules on both a/style.css and b/style.css, meaning that a/style.css goes through modules 2nd time, which makes class names different than what a JS files refer to.

Solution:
Having include/exclude patterns like many other rollup plugins, it'd be possible to apply postcss only on a portion of the files with some config, and apply it on others with a different config.

In webpack, each loader has this option, which allows me to apply css-loader?modules on src and css-loader without modules on node_modules files.

Workaround:

To prevent applying modules on css files in node_modules, in a local .css file in package-b, add an import for a css file in package-a using postcss :global. This ensures the class names inside the block don't get mangled.

Noice the ~ which tells the processor to include the file from a module in node_modules.
Also the imported file needs to be the already-built file, e.g. the one in dist. This file contains the mangled classes as they were built for package-a source.

/* package-b/src/style.css */

:global { @import "~package-a/dist/style.css"; }