POC of Svelte async compiler option bug

Summary

The async experimental compiler option causes a client-side navigation failure on a weird edge case: When a +layout.svelte exists on a route, client-side navigation to another route causes futher client-side navigation to fail.

Setup:

• repo generated with sv create
• svelte.config.js experimental async flag is set to true
• +layout.svelte is present in a route (/start is used in this example)

To reproduce:

• start the dev server
• open browser
• Follow the links:
  • /start -> /dashboard -> /dashboard/test
    • URL correctly updates to /dashboard/test, but page does not load

I've also included a working example: /start-no-layout -> /dashboard -> /dashboard/test

• In this case, the navigation works because there is no +layout.svelte in the /start-no-layout route.

Additional observations

If data-sveltekit-preload-data="hover" is removed from app.html, the navigation works again.