Link to the code that reproduces this issue

https://github.com/mators/nextjs-error-overlay-reproduction

To Reproduce

1. Run next dev

Current vs. Expected behavior

Current:

Expected (next 15.4.7):

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 24.6.0: Mon Jul 14 11:30:29 PDT 2025; root:xnu-11417.140.69~1/RELEASE_ARM64_T6000
  Available memory (MB): 16384
  Available CPU cores: 10
Binaries:
  Node: 22.12.0
  npm: 10.9.0
  Yarn: 1.22.22
  pnpm: N/A
Relevant Packages:
  next: 15.6.0-canary.2 // Latest available version is detected (15.6.0-canary.2).
  eslint-config-next: 15.5.3
  react: 19.1.0
  react-dom: 19.1.0
  typescript: 5.9.2
Next.js Config:
  output: N/A

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

CSS, Error Overlay, Pages Router

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

next dev (local)

Additional context

The reproduction is using webpack since I've experienced it in that setup, but it's the same with turbopack. Works fine with the app router.
Works if the error happens on the server.

Worked as expected before 15.5.0