Turbopack's CSS Modules Compose Chaining Issue: A Deep Dive

Hey guys! Let's talk about a pretty frustrating issue that's been bugging a lot of Next.js developers using Turbopack: the problem with chained composes in CSS Modules. If you're using Next.js and have encountered this, you know how crucial CSS Modules are for keeping your styles organized and maintainable. The composes feature is a powerful tool within CSS Modules that allows you to reuse and combine styles from different classes, creating a clean and efficient way to manage your CSS. However, when using Turbopack, the new bundler for Next.js, things can get a little wonky when it comes to chaining these composes rules. This can break the expected behavior of your stylesheets. Let's dig in and figure out what's going on.

Understanding the Problem: Chained composes in CSS Modules

So, what exactly is the issue? Imagine you have a set of CSS Modules where you're trying to build up styles by composing them together. You might have a base class, say .a, that provides a certain style, like underlining text. Then, you create another class, .b, that composes .a and adds some additional styles, such as italicizing the text. Finally, you have a class .c that composes .b and adds even more styles, maybe making the text bold. With Webpack, this works perfectly. Your element with className={styles.c} will inherit all the styles from .a, .b, and .c, effectively applying underline, italic, and bold styles. The expected behavior is a cascade of styles, but with Turbopack, the chaining breaks down. It seems to only recognize the direct composition, missing out on the styles from the initial composed classes. This is where the core issue arises.

In the provided code example, the expected behavior is that the text styled with className={styles.c} should be bold, italic, and underlined. The Webpack build correctly applies these styles by composing the classes: .c, .b, and .a. However, when you run this code using Turbopack, the styles are not correctly applied. The text may be bold and italic, but the underline style is missing, indicating that the composes chain isn't working as intended. This is a critical problem, as many developers rely on this chaining behavior to structure their CSS effectively. Without this, your application's design may not render correctly, resulting in broken layouts and a frustrating user experience. It's like the styles are partially applied, but not all of them, leading to an inconsistent appearance.

The core issue explained in SCSS code

Let's take a look at the SCSS code that demonstrates this problem:

.a {
    text-decoration: underline;
}

.b {
    composes: a;
    font-style: italic;
}

.c {
    composes: b;
    font-weight: bold;
}

In this setup, .c should inherit styles from both .b and .a. But in Turbopack, this chaining doesn't work correctly, which results in the underline style from .a not being applied. This discrepancy can significantly impact how your components look, especially when you have complex styling dependencies.

Reproduction and Expected Behavior

To reproduce this issue, you can use the provided CodeSandbox link in the original issue description. You'll start the Next.js app in Turbopack mode, observe that the chained styles are not applied as expected, and then restart in Webpack mode to confirm that the styles work correctly. The difference is stark and immediately highlights the problem. The core problem is that Turbopack does not fully support chained composes in CSS Modules. This is a big deal, as it affects how styles are applied and can lead to unexpected visual results. The correct functionality is crucial for maintaining the intended design and user experience of your web apps. Without the expected style inheritance, your components might look inconsistent, and it could be harder to maintain the style of your website or app. This can lead to extra debugging and time spent trying to find out why the styles aren't working as expected.

Environment Information and Affected Areas

Let's look at the environment information. The problem occurs on Linux systems and other platforms where the latest versions of Next.js are used with Turbopack. The relevant packages are next, react, and react-dom. The affected areas are the CSS and Turbopack itself, specifically when you're using next dev for local development and next build for production builds. This issue affects both the development and production stages, making it critical to find a resolution. This bug can throw a wrench into your development workflow and can potentially impact the final product. So, fixing it becomes an immediate concern for anyone using Turbopack with CSS Modules.

Why This Matters: Impact and Implications

So why is this a big deal? Well, guys, because it can break your web apps. Many web apps are coded with the expectation that composes chaining will work. The failure of Turbopack to correctly handle chained composes can lead to broken layouts, rendering inconsistencies, and a less-than-ideal user experience. The cascading nature of CSS is fundamental to its design. When one part of that cascade fails, it causes a lot of problems. For developers, this means more time spent debugging, more time trying to figure out why things don't look right, and generally a less efficient workflow. It also means that you might have to find workarounds, which adds complexity to your codebase. Essentially, this issue disrupts the predictable and reliable behavior that developers rely on. This can be especially problematic for large projects where the styling is built up through multiple layers of composition. The core principle of modularity in CSS, where you build reusable style components, is undermined when composition doesn't work as expected. Therefore, this issue necessitates a fix to ensure that applications render correctly and that the development experience remains smooth and efficient.

Potential Workarounds and Solutions

While there's no perfect workaround until Turbopack addresses this issue, here are a few things you can try:

• Flattening the Compositions: Instead of chaining composes, you could directly include all the styles in the final class. This is the most straightforward, but it can lead to more verbose CSS. For example, instead of .c { composes: b; font-weight: bold; }, you'd manually include the styles of .a and .b into .c. This can work, but it sacrifices some of the elegance and reusability that composes provides. It makes your CSS files larger and harder to manage.
• Using Webpack: If you're not ready to switch to Turbopack, you can stick with Webpack for now. This will ensure that your composes chains work correctly. You can configure Next.js to use Webpack instead of Turbopack. This will ensure that your composes chains work correctly, and it's a stable choice if you are in a production setting. This is a good option if you need to maintain the current styling behavior.
• Careful Code Review: Make sure you thoroughly review your CSS to catch any style issues related to composition early on. Manual inspection can help identify potential problems. Keep a close eye on components using composed styles. This is something that you should do anyway, but even more important when dealing with Turbopack.
• Waiting for a Fix: The best solution is for the Turbopack team to address this issue. Keep an eye on the Next.js and Turbopack updates. Check the official documentation and release notes for any fixes. This is often the most reliable way to resolve the problem. Hopefully, a fix will be implemented soon, and you can go back to using Turbopack without any issues.

Conclusion: The Path Forward

In conclusion, the issue with chained composes in CSS Modules within Turbopack is a real challenge for Next.js developers. It hinders the correct rendering of styles and disrupts established CSS workflows. The problem stems from Turbopack's incomplete support for chained compositions, resulting in style inheritance failures. While workarounds like flattening compositions or sticking with Webpack can provide temporary solutions, the optimal outcome depends on a Turbopack update that resolves the underlying issue. Developers must stay aware of this limitation and adapt their strategies to maintain efficient and consistent styling in their projects. Keep an eye out for updates and patches, and in the meantime, adopt these strategies to keep your apps looking good!