Loading.js fallback of dynamically rendered pages is not prefetched in the majority of cases

[dannytlake]

Link to the code that reproduces this issue

https://github.com/dannytlake/next-14-link-prefetch-bug

To Reproduce

1. Start the application in production mode, pnpm build && pnpm start
2. Navigate to http://localhost:3000 with Firefox Developer Edition.
3. Open dev tools and go to the Network tab
4. Reload the page, see the responses of each prefetch of each <Link>. The responses that are ~450 bytes in transfter size contain an invalid prefetch. The responses that are ~860 bytes contain the expected response with the loading.js fallback. See the screenshot in the README for reference.

Current vs. Expected behavior

The current behavior of prefetching (using the default setting prefetch of next/link) dynamically rendered pages is not correctly returning the loading.js fallback in the streamed response. This occurs for the majority of routing cases. There are some cases where it does function as expected, but only on top level route segments or a top level dynamic segment.

The expected behavior of prefetching (using the default setting prefetch of next/link) dynamically rendered pages is to include the loading.js fallback of the prefetched route.

Provide environment information

Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 23.6.0: Mon Jul 29 21:14:30 PDT 2024; root:xnu-10063.141.2~1/RELEASE_ARM64_T6000
  Available memory (MB): 32768
  Available CPU cores: 10
Binaries:
  Node: 20.16.0
  npm: 10.8.2
  Yarn: 1.22.22
  pnpm: 8.15.9
Relevant Packages:
  next: 14.2.13 // Latest available version is detected (14.2.13).
  eslint-config-next: 14.2.7
  react: 18.3.1
  react-dom: 18.3.1
  typescript: 5.5.4
Next.js Config:
  output: N/A

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

Navigation, Performance

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

next start (local), Vercel (Deployed)

Additional context

This bug is not present in next@canary. I am opening this issue to hopefully have the fix for this bug backported to v14, or at least be migrated to next@rc.

The possible fix for this prefetch bug was implemented via #67358

[ztanner]

Thanks for the report! This will be backported to v14 via:

• backport: fix prefetch bailout detection for nested loading segments #70618

[github-actions]

This closed issue has been automatically locked because it had no new activity for 2 weeks. If you are running into a similar issue, please create a new issue with the steps to reproduce. Thank you.