reproduction-webpack-css-order

This repository demonstrates a CSS order issue in Webpack 5 when using a pnpm monorepo with sideEffects: false in package.json files.

The Issue

When sideEffects: false is set in package.json files across a monorepo, CSS import order becomes unpredictable.

References

This bug happens only for webpack 5

The same code works for:

• rspack
• vite
• parcel

Expected Behavior

The css declaration background-color: orange; should be the last rule in the CSS file

• webpack 5 (broken)
• turbopack (broken)
• webpack 5 (sideEffects: true)
• webpack 5 (no barrel)
• rspack
• vite
• parcel

Steps to Reproduce

1. Clone the repository

git clone https://github.com/jantimonpn/reproduction-webpack-css-order.git
cd reproduction-webpack-css-order

2. Install dependencies

pnpm install

3. Build

pnpm build

4. See CSS output in @applications/base/dist: The background-color: orange; is not the last rule in the CSS file in the dist directory:

.hDE5PT5V3QGAPX9o9iZl {
  padding: 20px;
  border: 1px solid #ddd;
  border-radius: 8px;
  margin: 16px;
}

.yqrxTjAG22vkATE1VjR9 {
  background-color: orange;
}

.R_y25aX9lTSLQtlxA1c9 {
  padding: 8px 16px;
  background-color: #007bff;
  color: white;
  border: none;
  border-radius: 4px;
}

1. Change to one of the comparison branches:

   • side-effects-true branch: git checkout side-effects-true (all packages have sideEffects: true)
   • no-barrel branch: git checkout no-barrel (no barrel file in @segments/carousel)

2. Build again

pnpm build

1. See CSS output in @applications/base/dist: The background-color: orange; is now the last rule in the CSS file in the dist directory:

.R_y25aX9lTSLQtlxA1c9 {
  padding: 8px 16px;
  background-color: #007bff;
  color: white;
  border: none;
  border-radius: 4px;
}

.hDE5PT5V3QGAPX9o9iZl {
  padding: 20px;
  border: 1px solid #ddd;
  border-radius: 8px;
  margin: 16px;
}

.yqrxTjAG22vkATE1VjR9 {
  background-color: orange;
}

Project Structure

├── @applications/base
├── @libraries/teaser
└── @segments/carousel

Each package has sideEffects: false in its package.json, which triggers the issue.

Environment

• Webpack 5
• pnpm
• Node.js >= 20
• All packages have sideEffects: false

Module Graphs

graph TD
    subgraph "sideEffects: false"
        A1["@applications/base/src/index.ts preOrder: 0, postOrder: 6"]
        B1["@libraries/teaser/src/teaser.ts preOrder: 1, postOrder: 5"]
        C1["@libraries/teaser/src/teaser.module.css preOrder: 2, postOrder: 1"]
        D1["teaser.module.css|0|||}} preOrder: 3, postOrder: 0"]
        E1["@segments/carousel/src/buttons.ts preOrder: 4, postOrder: 4"]
        F1["@segments/carousel/src/button.module.css preOrder: 5, postOrder: 3"]
        G1["button.module.css|0|||}} preOrder: 6, postOrder: 2"]
        
        A1 --> B1
        B1 --> C1
        C1 --> D1
        B1 --> E1
        E1 --> F1
        F1 --> G1

        style A1 fill:#0a0a4a,stroke:#333
        style C1 fill:#294b51,stroke:#333
        style D1 fill:#294b51,stroke:#333
        style F1 fill:#294b51,stroke:#333
        style G1 fill:#294b51,stroke:#333
    end

    subgraph "sideEffects: true"
        A2["@applications/base/src/index.ts preOrder: 0, postOrder: 8"]
        B2["@libraries/teaser/src/index.ts preOrder: 1, postOrder: 7"]
        C2["@libraries/teaser/src/teaser.ts preOrder: 2, postOrder: 6"]
        D2["@segments/carousel/src/index.ts preOrder: 3, postOrder: 3"]
        E2["@segments/carousel/src/buttons.ts preOrder: 4, postOrder: 2"]
        F2["@segments/carousel/src/button.module.css preOrder: 5, postOrder: 1"]
        G2["button.module.css|0|||}} preOrder: 6, postOrder: 0"]
        H2["@libraries/teaser/src/teaser.module.css preOrder: 7, postOrder: 5"]
        I2["teaser.module.css|0|||}} preOrder: 8, postOrder: 4"]
        
        A2 --> B2
        B2 --> C2
        C2 --> D2
        D2 --> E2
        E2 --> F2
        F2 --> G2
        C2 --> H2
        H2 --> I2

        style A2 fill:#0a0a4a,stroke:#333
        style F2 fill:#294b51,stroke:#333
        style G2 fill:#294b51,stroke:#333
        style H2 fill:#294b51,stroke:#333
        style I2 fill:#294b51,stroke:#333
    end

    subgraph "No Barrel"
        A3["@applications/base/src/index.ts preOrder: 0, postOrder: 6"]
        B3["@libraries/teaser/src/teaser.ts preOrder: 1, postOrder: 5"]
        E3["@segments/carousel/src/buttons.ts preOrder: 2, postOrder: 2"]
        F3["@segments/carousel/src/button.module.css preOrder: 3, postOrder: 1"]
        G3["button.module.css|0|||}} preOrder: 4, postOrder: 0"]
        H3["@libraries/teaser/src/teaser.module.css preOrder: 5, postOrder: 4"]
        I3["teaser.module.css|0|||}} preOrder: 6, postOrder: 3"]
        
        A3 --> B3
        B3 --> E3
        E3 --> F3
        F3 --> G3
        B3 --> H3
        H3 --> I3
        style A3 fill:#0a0a4a,stroke:#333
        style F3 fill:#294b51,stroke:#333
        style G3 fill:#294b51,stroke:#333
        style H3 fill:#294b51,stroke:#333
        style I3 fill:#294b51,stroke:#333
    end

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