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

[amannn]

Link to the code that reproduces this issue

amannn/nextjs-bug-repro-default-layout@0470cf2

To Reproduce

1. Run app
2. 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