[Turbopack | CSS]: CSS prioritization differs between next dev --turbopack and next build --turbopack

[GlebKodrik]

Link to the code that reproduces this issue

https://github.com/GlebKodrik/turbopack-style/

To Reproduce

Use route my project with problem : http://localhost:3000/intensive/31BoMCUUsroffEMtAmd6ALUmZ0y or https://turbopack-style.vercel.app/intensive/31BoMCUUsroffEMtAmd6ALUmZ0y

To Reproduce

1. Run the app in development with next dev --turbopack → Styles load correctly, no issues observed.

2. Build the app with next build --turbopack and then start with next start → Styles are applied in a different priority/order compared to dev mode.

Notes:

• The issue reproduces 100% on our current setup.
• Sometimes deleting the pages folder or a random component in entities makes the bug disappear, which makes the behavior unpredictable.

Current vs. Expected behavior

Current vs. Expected behavior

• Current: In production (next build --turbopack + next start), CSS order/priority differs from development, breaking the layout.
• Expected: Style prioritization should be consistent between dev and build modes when using Turbopack.

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 23.6.0
Binaries:
  Node: 20.9.0
  npm: 10.8.2
Relevant Packages:
  next: 15.6.0-canary.12
Next.js Config:
  output: N/A

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

Turbopack

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

next build (local)

Additional context

Additional context Happens locally with next build --turbopack → next start. Cannot reproduce when running next dev --turbopack. Issue sometimes disappears after removing a random folder/component, which suggests non-deterministic style bundling.

1 image - next dev --turbopack 2 image next build --turbopack → next start

[mischnic]

I can reproduce it, but we need a more minimal reproduction. I tried to trim it down by removing imports and reexports while still maintaining the broken prod behehavior, but there's still to much...

[GlebKodrik]

@mischnic

I can reproduce it, but we need a more minimal reproduction. I tried to trim it down by removing imports and reexports while still maintaining the broken prod behehavior, but there's still to much...

I’ve minimized the code a lot, so now it’s much easier to understand what’s going on. Please pay special attention to the page.

Reproduction link: http://localhost:3000/intensive

We are working with (pages).

• If I delete the course page, the bug occurs.
• If I move the <CourseProgramStudy /> block into the parent (page), the bug occurs.
• Same thing with the intensive (page).
• If I delete <WhoItSuitableFor />, the bug occurs.
• If I move the content of <WhoItSuitableFor /> directly into page, the bug also occurs.

Additional

• If I delete a component that is not used anywhere at all (<CourseProgramLessonCard />), the bug still occurs

Something very strange is happening here.

[GlebKodrik]

@mischnic Any information?

[GlebKodrik]

@mischnic In the new version, the next tank remains

[mischnic]

We will look into it further soon.

[GlebKodrik]

@mischnic I'm really looking forward to it. There's just a really important bug that's preventing us from moving. We're just waiting for it.

[GlebKodrik]

We will look into it further soon.

@mischnic Hey, any progress? I really need it.

[MattisAbrahamsson]

We're also having this issue and its blocking us from using turbopack in production. I expect that when I use the same bundler in dev and build the app should look identical in dev and build

[GlebKodrik]

@mischnic How are things going there?

[GlebKodrik]

@mischnic @icyJoseph You have a problem on the board.?

[GlebKodrik]

@mischnic How are things going there?

[MattisAbrahamsson]

@GlebKodrik I don't think they will respond 🫠

[GlebKodrik]

@GlebKodrik I don't think they will respond 🫠

I absolutely adore the Nextjs team. They should focus on stabilization instead of new features. It's always one problem after another. And the guys ignore it.

[camiblanch]

I am also seeing this. The more time goes on the more places I'm noticing it. I'm using turbo pack on next 16.0.10 if that's helpful

[GlebKodrik]

I suggest creating new ones issues so they can see the problem and start fixing it.

@camiblanch @MattisAbrahamsson