[Turbopack] [Pages] Hydration error (or double render) when using dynamic imports in _app.tsx

Why this is a good first issue

Hydration issue with dynamic imports in _app.tsx that may require Turbopack-specific fixes.

AI Summary

The issue involves a hydration error when using dynamic imports in _app.tsx with Turbopack, with reports of double rendering. The problem appears related to specific Next.js versions and may require changes in how Turbopack handles dynamic imports during hydration. The scope is somewhat clear but involves complex interactions between Next.js, React hydration, and Turbopack.

Issue Description

Link to the code that reproduces this issue

https://github.com/rtrembecky/next-turbopack-hydration-repro

To Reproduce

Install the deps - pnpm i
Start the app - pnpm dev
Navigate to http://localhost:3000

Current vs. Expected behavior

Current behavior:

Expected: No errors.

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): 32768
  Available CPU cores: 10
Binaries:
  Node: 24.2.0
  npm: 11.4.0
  Yarn: 1.22.19
  pnpm: 9.15.9
Relevant Packages:
  next: 16.0.1-canary.2 // Latest available version is detected (16.0.1-canary.2).
  eslint-config-next: N/A
  react: 19.2.0
  react-dom: 19.2.0
  typescript: 5.9.3
Next.js Config:
  output: N/A

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

Pages Router, Module Resolution, Turbopack

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

next dev (local), next build (local), Other (Deployed)

Additional context

Pinpointed broken versions:

canary: >= 15.1.1-canary.14, up to 16.0.1-canary.2 (latest at the time of writing)
stable: >= 15.2.0

Working versions:

15.1.1-canary.13
15.1.6

Background:

_app.tsx imports a component via next/dynamic - this causes a hydration issue
when the same component is imported from the page level as well, it works - http://localhost:3000/works page (in repro repo) works
when the <main> wrapper in _app.tsx is removed or replaced by a <> fragment, the dynamic element is rendered TWICE instead
- so this issue looks related: https://github.com/vercel/next.js/issues/77712
- and also this one where @gurkerl83 reported it's fixed in 15.1.1-canary.14, though it apparently broke something: https://github.c

GitHub Labels

TurbopackPages Routerlinear: turbopackModule Resolution