`default.js` does not trigger sibling `layout.js`

Why this is a good first issue

Layout behavior in parallel routes needs investigation to determine if it's a bug or documentation gap.

AI Summary

The issue reports that a default.tsx file in a parallel route slot is not being wrapped by its sibling layout.tsx as expected. This requires understanding Next.js's parallel routes and layout behavior to determine if it's a bug or needs documentation clarification. The reproduction case is clear but the expected behavior needs verification against the framework's design.

Issue Description

Link to the code that reproduces this issue

https://github.com/amannn/nextjs-bug-repro-default-layout/commit/0470cf213e7ede792d7b75b9167c748885f64df7

To Reproduce

Run app
Open /

Current vs. Expected behavior

Structure:

src/
└── app
    ├── @slot
    │   ├── default.tsx
    │   └── layout.tsx
    ├── layout.tsx
    └── page.tsx

On /, app/@slot/default.tsx renders, but it's not enclosed by app/@slot/layout.tsx.

Based on the available documentation on layouts, I'd expect that they always wrap their sibling entry points within a folder.

If this is not intended, I think it should be documented.

Provide environment information

Operating System:
  Platform: darwin
  Arch: x64
  Version: Darwin Kernel Version 25.2.0: Tue Nov 18 21:09:34 PST 2025; root:xnu-12377.61.12~1/RELEASE_X86_64
  Available memory (MB): 16384
  Available CPU cores: 12
Binaries:
  Node: 24.13.0
  npm: 11.6.2
  Yarn: 1.22.22
  pnpm: 10.28.0
Relevant Packages:
  next: 16.2.0-canary.37 // Latest available version is detected (16.2.0-canary.37).
  eslint-config-next: N/A
  react: 19.2.4
  react-dom: 19.2.4
  typescript: 5.9.3
Next.js Config:
  output: N/A

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

Parallel & Intercepting Routes

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

next dev (local)

Additional context

No response

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