CORS errors during server rendering

Describe the bug

When fetching an endpoint using CORS from a +page.js file, SvelteKit throws a CORS error when trying to render the page on the server, such as during development mode or prerendering for production. CORS doesn't seem to make sense in this context, since the fetch request is being performed on the server, which has no origin.

This seems related to https://github.com/sveltejs/kit/issues/7441, but that issue involved +page.server.js files. In this issue, we want to perform the fetch on the client AND the server (during SSR/prerendering).

Reproduction

https://stackblitz.com/edit/cors-error-during-server-render-repro?file=src%2Froutes%2Ftest%2F%2Bpage.js

Using the client-side navigation from / to /test fetches data on the client perfectly fine, but doing a browser navigation/reload on the /test page throws a CORS error on the server. Errors are also thrown when trying to run a build.

Logs

No response

System Info

StackBlitz WebContainer

Binaries:
    Node: 16.14.2 - /usr/local/bin/node
    Yarn: 1.22.19 - /usr/local/bin/yarn
    npm: 7.17.0 - /usr/local/bin/npm
  npmPackages:
    @sveltejs/adapter-auto: ^1.0.0 => 1.0.0 
    @sveltejs/kit: ^1.0.0 => 1.0.1 
    svelte: ^3.54.0 => 3.55.0 
    vite: ^4.0.0 => 4.0.3

Severity

serious, but I can work around it

Additional Information

No response

Start with the linked StackBlitz reproduction and its src/routes/test/+page.js file, then compare the client navigation with a browser reload and a production build. Done means the endpoint fetch works during server rendering and prerendering without a CORS error while continuing to work during client navigation.