@vitejs/plugin-react 'can't detect preamble' on module-shipped .tsx pages #66

Summary

@vitejs/plugin-react raises "can't detect preamble. Something is wrong." for any module-shipped .tsx page (i.e. files served from outside host/client_app/).

Module pages live either in:

• modules/<m>/lacowiki_<m>/pages/Login.tsx (workspace member), or
• .venv/lib/python3.12/site-packages/<m>/pages/Login.tsx (wheel-installed)

Vite's dev server is configured with server.fs.allow to read these (the bundled vite.config.ts does this correctly). But @vitejs/plugin-react's React Fast Refresh preamble is only injected into the host's index.html. When the .tsx is served via Vite's /@fs/ mechanism from outside client_app/, the per-component preamble check fires before the entry-point preamble has executed in the same JS realm, and the plugin throws.

Reproduction

1. Scaffold a host with sm new --preset full.
2. Install any framework module that ships pages (e.g. simple_module_users ships Login.tsx from its wheel).
3. npm run dev.
4. Navigate to /login. Console:

   Error: @vitejs/plugin-react can't detect preamble. Something is wrong.
       at http://localhost:5050/@fs/.../site-packages/users/pages/Login.tsx:277:11
   

   White screen.

Workarounds I've tried

• Clearing .vite cache → no change.
• Adding optimizeDeps.include for the user-facing entry → no change.
• Hard reload → no change. Issue is consistent.

Suggested investigation

The host's main.tsx already wraps Inertia setup with createRoot(). The plugin's preamble is sourced from __vite_plugin_react_preamble_installed__ on globalThis. When the cross-origin .tsx is loaded via /@fs/, the realm matches but the load order may race. Possibly the React plugin needs include configured for the absolute paths in server.fs.allow, or the host scaffold should enable fastRefresh: false for non-host paths.

A working test would be: configure react() plugin to skip Fast Refresh entirely for files outside __dirname, or ship the host's main.tsx with a manual preamble bootstrap.

Impact

This blocks the entire dev server for any host that consumes module-shipped pages — which is every real installation, since simple_module_users ships Login.tsx and all --preset full hosts use it.

Environment

• vite@6.4.2, @vitejs/plugin-react@5.x
• simple_module_users page served from .venv/lib/python3.12/site-packages/users/pages/Login.tsx
• Discovered while building laco_wiki_python rewrite