preloadCode does not handle relative URLs

Describe the bug

I noticed that the preloadCode function behaves differently than preloadData: preloadCode requires an absolute URL while preloadData works well both with relative and absolute URLs.

The documentation does not mention the requirement specific to preloadCode and lets us think that preloadCode and preloadData are very similar.

Could preloadCode be amended to accept relative URLs in addition to absolute ones?
Could the documentation could be amended to state the kind of URL the methods accept?

If for some reason, preloadCode cannot accept relative URLs, maybe it should throw or at least log a warning? Currently, we have no way to know nothing is done with a relative URL.

Reproduction

Here is repro with links and buttons to preload a page in a subfolder named "x".

1. When we hover over the preloadCode link (with data-sveltekit-preload-code="hover"), there is a request for x/+page.svelte
2. When we hover over the preloadData link (with data-sveltekit-preload-data="hover"), there is a request for x/+page.svelte
3. When we click the preloadCode("x") button, there is no request for x/+page.svelte
4. When we click the preloadData("x") button, there is a request for x/+page.svelte
5. When we click the preloadCode("/x") button, there is a request for x/+page.svelte
6. When we click the preloadCode("/x") button, there is a request for x/+page.svelte

⚠️ Refresh the preview before each step above and look at the network tab of the dev tools of your browser

Note that the href for both links is "x", so preloading works well with a relative URL on a link.

Logs

No response

System Info

Tested with Firefox and Chrome

  Binaries:
    Node: 18.17.1 - C:\Program Files\nodejs\node.EXE
    npm: 9.6.7 - C:\Program Files\nodejs\npm.CMD
    pnpm: 8.9.0 - ~\AppData\Local\pnpm\pnpm.EXE
  Browsers:
    Edge: Chromium (118.0.2088.46)
    Internet Explorer: 11.0.22621.1
  npmPackages:
    svelte: ^4.2.0 => 4.2.0

Severity

annoyance

Start by reproducing the relative-versus-absolute URL behavior described for preloadCode and preloadData in the linked SvelteLab example, while watching the browser network tab. Trace the preloadCode and preloadData entry points and determine whether relative URLs should trigger the x/+page.svelte request or produce an explicit warning; update the documented URL behavior accordingly.