Bug report

Describe the bug

In development mode, when a page imports styles from a .module.css file ( import styles from "./home.module.css" ) and then imports another component after that ( import { Container } from "../components/Container" ), and passes the imported styles to the component like <Container className={styles.blue}>, these styles get overriden by the imported component. This component has it’s own styles and passes the className prop to a DOM element:

import styles from "./Container.module.css";

function classes(...classNames) {
  const joined = classNames.join(' ');
  return joined;
}

export const Container = ({
  children,
  className= "",
}) => {
  return (
    <div className={classes(styles.container, className)}>
	 {children}
    </div>
  );
};

The Container has the following style:

.container {
	background-color: yellow;
}

But the page provides the following style:

.blue {
	background-color: blue;
}

In development, the styles are injected into the head as follows:

<style>
.index_blue__3Iu-s {
	background-color: blue;
}
</style>
<style>
.Container_container__1UmhU {
	background-color: yellow;
}
</style>

In production, these styles are combined into one stylesheet:

.Container_container__1UmhU {
    background-color: #ff0
}

.index_blue__3Iu-s {
    background-color: #00f
}

As you can see, there is a difference in order. This causes the inconsistency across environments with CSS ( yellow in development, blue in production ).

I believe that the production result is the correct one, as you should be able to override the component styles.

I’d be happy to try to contribute to a fix. However, I am a beginner developer so I might need some guidance.

To Reproduce

Clone my reproduction repository and run npm run dev to see the styles applied incorrectly, and run npm run build; npm run start to see that the styles work as intended.

Expected behavior

I would expect that in both development and production mode the classes get defined in the same order so that the styles are applied correctly.

System information

• OS: Windows 10
• Browser: Chrome 81.0.4044.138
• Version of Next.js: 9.4.1
• Version of Node.js: 12.16.1

More information

The style tags are injected by the code in this file. It uses style-loader in development and the MiniCssExtractPlugin in production. What is the reason as to why it isn’t used in development mode?

_NEXT-1386