Preload data on async Svelte breaks reactivity

[hyunbinseo]

Will be fixed by sveltejs/svelte#17362 and sveltejs/svelte#17335

Describe the bug

Hovering then clicking on the anchor

<!-- src\routes\+page.svelte -->

<script>
  import { resolve } from '$app/paths';
</script>

<a href={resolve('/blog/[slug]', { slug: 'lorem' })}>Preload Enabled</a>

breaks reactivity. {now} is not updated.

<!-- src\routes\blog\[slug]\+page.svelte -->

<script>
  import Component from './Component.svelte';
</script>

<Component>
  {#snippet snippet({ now })}
    {now}
  {/snippet}
</Component>

Reproduction

https://github.com/hyunbinseo/svelte-kit-15064

System Info

System:
  OS: Windows 11 10.0.26200
  CPU: (8) x64 Intel(R) Core(TM) Ultra 7 258V
  Memory: 5.20 GB / 31.48 GB
Binaries:
  Node: 24.12.0 - C:\Users\hyunb\AppData\Local\fnm_multishells\14132_1765800295302\node.EXE
  Yarn: 1.22.22 - C:\Users\hyunb\AppData\Local\fnm_multishells\14132_1765800295302\yarn.CMD
  npm: 11.6.2 - C:\Users\hyunb\AppData\Local\fnm_multishells\14132_1765800295302\npm.CMD
  pnpm: 10.25.0 - C:\Users\hyunb\AppData\Local\fnm_multishells\14132_1765800295302\pnpm.CMD
  Deno: 2.4.0 - C:\Users\hyunb\.deno\bin\deno.EXE
Browsers:
  Chrome: 143.0.7499.109
  Edge: Chromium (140.0.3485.54)
  Firefox: 146.0 - C:\Program Files\Mozilla Firefox\firefox.exe
npmPackages:
  @sveltejs/adapter-auto: ^7.0.0 => 7.0.0
  @sveltejs/kit: ^2.49.2 => 2.49.2
  @sveltejs/vite-plugin-svelte: ^6.2.1 => 6.2.1
  svelte: ^5.46.0 => 5.46.0
  vite: ^7.3.0 => 7.3.0

Severity

serious, but I can work around it

Additional Information

I stumbled on an issue that was fixed by removing preloadData similar to this:

• Async Svelte breaks reactivity when using preloadData #14875

but I've failed to create a reproduction for it.

I am using the same Viewer.svelte component with preloadData inside it.

The only difference being whether the server load function uses:

• import.meta.glob for getting the markdown modules
• params to select the correct module from above

[hyunbinseo]

Doing vite build then vite preview in the reproduction works fine.

Meanwhile in our production codebase, neither of these are helping.

• data-sveltekit-reload
• data-sveltekit-preload-data={false} - fixed in dev, broken in preview

[erikbg7]

There are several issues reporting the same bug and a pending fix sveltejs/svelte#17335.

I was able to work around it using data-sveltekit-preload-data="off" in links

[hyunbinseo]

I've stumbled on that PR after I've created this issue. See #14875 (comment)

Hopefully this gets fixed as well. The reproduction seems different from ones in the PR.

Also, false and "off" both seem to disable preloading data on hover, but I did update my reproduction.

- data-sveltekit-preload-data={false}
+ data-sveltekit-preload-data="off"

[hmnd]

Hopefully this gets fixed as well. The reproduction seems different from ones in the PR.

Could you elaborate on what you think is different in your reproduction compared to those listed in sveltejs/svelte#17335? This looks the same as all the others to me.

Have you tried installing the pr and seeing if it's fixed for you? pnpm add https://pkg.pr.new/svelte@17335

[hyunbinseo]

@hmnd The reproductions in that PR looked different.

"svelte": "https://pkg.pr.new/svelte@17335",

No, this version did not fix the issue in this reproduction.

[hmnd]

@hyunbinseo ahh it's because you're actually facing a combination of two of my fixes! If I run it with both sveltejs/svelte#17362 and sveltejs/svelte#17335 applied, the now updating works as expected :)

Hopefully those can get merged soon 🤞