State issues with preloading and async svelte

[nebwards]

Describe the bug

I understand there are some issues with recent svelte versions. I have a simple example for active links which highlights a bug with (what I assume to be) async svelte and/or preloading data.

Consider this +layout.svelte

<script lang="ts">
    import { page } from '$app/state';

    let { children } = $props();

    const links = ['/', '/page-1', '/page-2', '/page-3'];

    function isActivePath(path: string) {
        return page.url.pathname === path;
    }
</script>

<div>
    svelte page.url.pathname = {page.url.pathname}
</div>

<div>
    <nav>
        {#each links as link}
            <a href={link} data-active={isActivePath(link)}>{link}</a>
        {/each}
    </nav>
    {@render children()}
</div>

I would expect that when navigating to each of these pages, data-active becomes true on the respective anchor tag. However, instead I am seeing some odd behaviour:

• If hovering on a link, then clicking it, without hovering over other links first, this all works as expected.
• If hovering on a few different links, then clicking any one of them, data-active is no longer updated. Now navigating between the pages does not call isActivePath.

If async svelte is enabled, then removing data-sveltekit-preload-data="hover" 'fixes' these issues.
If aysnc svelte is not enabled, these issues do not appear.

All pages in this example simply contain an h1 tag - no async loading or remote functions.

In a more complex application, the actual page.url.pathname seems to be getting updated on hover preload aswell, and wrong pages are being loaded on clicking the links. I was not able to reproduce that in a simple example.

svelte_nav_bug.mp4

Reproduction

https://github.com/nebwards/svelte_nav_bug

Logs

System Info

System:
  OS: Windows 11 10.0.26100
  CPU: (12) x64 AMD Ryzen 5 3600 6-Core Processor
  Memory: 5.73 GB / 15.93 GB
Binaries:
  Node: 22.14.0
  npm: 10.9.2
  pnpm: 10.18.2
  bun: 1.3.0
Browsers:
  Edge: Chromium (140.0.3485.54)
  Internet Explorer: 11.0.26100.1882
npmPackages:
  @sveltejs/adapter-auto: ^7.0.0 => 7.0.0
  @sveltejs/kit: ^2.47.1 => 2.48.4
  @sveltejs/vite-plugin-svelte: ^6.2.1 => 6.2.1
  svelte: ^5.41.0 => 5.43.4
  vite: ^7.1.10 => 7.2.2

Severity

blocking an upgrade

Additional Information

No response

[gevera]

Can confirm and reproduce

Here is repo

"@sveltejs/kit": "^2.48.4"

---

Downgraded to 2.47.1 and navigation works