vercel / vercel/next.js

CSS order incorrect with the combination of server and client components

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CSS linear: next
Dominant language
JavaScript
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Description

Link to the code that reproduces this issue

https://github.com/Netail/repro-app-dir-css-order

To Reproduce
  1. yarn dev & open localhost:3000
  2. See 1 yellow and 1 blue component
  3. Uncomment the 'use client' in the navigation-wrapper.tsx
  4. Instead of 1 yellow & 1 blue component it becomes 2 blue ones (Even after a reload & restart)
Current vs. Expected behavior

CSS streaming of client components mess with the order of the css bundle, resulting in being appended later and thus overriding certain server side styling.

In the example it should stay a yellow and blue component instead of 2 blue components when uncommenting 'use client'.

It seem like the page.css overwrites css from the layout.css due to importing order issues. Some classes which get streamed to page.css are already in layout.css, maybe a check could be built in if they were already present in the layout.css bundle?

Provide environment information
Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 23.4.0: Fri Mar 15 00:10:42 PDT 2024; root:xnu-10063.101.17~1/RELEASE_ARM64_T6000
  Available memory (MB): 32768
  Available CPU cores: 10
Binaries:
  Node: 20.8.1
  npm: 10.5.0
  Yarn: 1.22.22
  pnpm: N/A
Relevant Packages:
  next: 14.3.0-canary.45 // Latest available version is detected (14.3.0-canary.45).
  eslint-config-next: N/A
  react: 18.2.0
  react-dom: 18.2.0
  typescript: 5.3.3
Next.js Config:
  output: N/A
Which area(s) are affected? (Select all that apply)

Webpack

Which stage(s) are affected? (Select all that apply)

next dev (local)

Additional context

No response

Contributor guide

Open the contributing guide

First steps

  1. Read the whole issue, then the project's contributing guide.
  2. Comment on the issue to say you are picking it up — it saves two people doing the same work.
  3. Fork the repository and make your change on a branch.
  4. Open a pull request that references the issue number.

Research direction

Start by running the linked repro app with yarn dev, then compare the output before and after uncommenting use client in navigation-wrapper.tsx. Inspect how layout.css and page.css are assembled during Next.js development with Webpack. Done means the example remains one yellow and one blue component in both configurations.

Written by the indexing model from the issue text.

Assessment

Tech stack
css, javascript, nextjs, react, typescript, webpack
Domain
build-system, frontend
Issue type
Bug
Difficulty
4/5
Estimated time
3-5 days
Activity status
Stale
Clarity
Mostly clear
Newbie friendliness
35/100

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