Prerendering with `ssr: false` does not work correctly when a `basename` is set

[wonu]

I'm using React Router as a...

framework

Reproduction

1. npx create-react-router@latest my-react-router-app
2. modify react-router.config.ts
   • ssr: false
   • prerender: true
3. npm run build
   • build/client/__spa-fallback.html: 2.19kB
   • build/client/index.html: 6.71kB
4. set basename and rebuild
   • build/client/__spa-fallback.html: 2.2kB
   • build/client/fake-base/index.html: 2.2kB (the two files are identical)

System Info

System:
  OS: Windows 11 10.0.22631
Binaries:
  Node: 22.10.0
  npm: 10.9.0
npmPackages:
  @react-router/dev: ^7.5.3 => 7.6.0
  @react-router/node: ^7.5.3 => 7.6.0
  @react-router/serve: ^7.5.3 => 7.6.0
  react-router: ^7.5.3 => 7.6.0
  vite: ^6.3.3 => 6.3.5

I tested it on WSL(Ubuntu 22) as well, and it was the same.

Used Package Manager

npm

Expected Behavior

Shouldn't prerendering work even if a basename is set?

Actual Behavior

If a basename is set, prerendering becomes ineffective.
This seems to have occurred since v7.2.0, which is when 'Prerendering with a SPA Fallback' was added.

[brophdawg11]

Is this a duplicate of #12083?

[wonu]

@brophdawg11 No, I am also experiencing the same problem as that issue and my settings are identical, so I was going to comment on that issue, but since the problem itself is different, I created a separate issue.

The repro for that issue is v7.0.0-pre, and my issue is not reproducible on that version.
My issue is not a path-related problem; it's that all prerendered routes become __spa-fallback.html with only the name being different.