fallback page accesses __data.json with a relative URL and fails

[hybridherbst]

Describe the bug

According to #10730 fallback URLs need to access everything by absolute path which makes sense.
According to #9343 (comment) data is passed to +error.svelte from all previous loads.

However, that data is currently loaded from a relative URL, so passing the data fails in a build. I think it would need to be an absolute URL.

Reproduction

https://stackblitz.com/edit/sveltejs-kit-template-default-fb5xvp?file=svelte.config.js

npm run build
npm run preview (uses serve package on the build folder)

Logs

sveltejskittemplatedefaultfb5x-vqmp--3000--5291dfc1.local-credentialless.webcontainer.io/my/test/page/__data.json?x-sveltekit-invalidated=1:1     Failed to load resource: the server responded with a status of 404 (Not Found)

System Info

System:
    OS: Linux 5.0 undefined
    CPU: (8) x64 Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
    Memory: 0 Bytes / 0 Bytes
    Shell: 1.0 - /bin/jsh
  Binaries:
    Node: 16.20.0 - /usr/local/bin/node
    Yarn: 1.22.19 - /usr/local/bin/yarn
    npm: 9.4.2 - /usr/local/bin/npm
    pnpm: 8.6.10 - /usr/local/bin/pnpm
  npmPackages:
    @sveltejs/adapter-static: ^2.0.3 => 2.0.3 
    @sveltejs/kit: ^1.20.4 => 1.25.0 
    svelte: ^4.0.5 => 4.2.0 
    vite: ^4.4.2 => 4.4.9

Severity

blocking usage of SvelteKit error pages