Can't assign result of `preloadData` to local state after using `pushState`

[danieldiekmeier]

Describe the bug

In my app, I'm using shallow routing together with preloadData. However, ever since the fork API was added in svelte@5.42.0, preloading does not work anymore.

The data gets loaded (I can see the fetch request, and I can log the result), but when I put the new data into local $state, my component does not rerender. I'm basically only doing this:

let data = $state("");

async function preload() {
  const result = await preloadData(`/other`);
  data = result.data.foo;
}

I added a reproduction that uses almost the same code. If I can do anything else, let me know.

Thank you for all the work on async/fork. I know you already fixed a bunch of bugs in the last few weeks, and I repeatedly read the changelog and thought that my bug would now be fixed, but sadly not yet. I hope the reproduction helps.

Reproduction

https://github.com/danieldiekmeier/sveltekit-repro-preload-data

If you start the reproduction with pnpm run dev, it will hopefully explain itself.

This is what it looks like when you go to /other with shallow routing and click the "Preload" button a few times:

As you can see, the Result: is not updated, and the $inspect call runs twice after ever click.

---

If you go to /other directly, clicking the button successfully replaces the result, and there's only one $inspect line per click.

Logs

System Info

System:
    OS: macOS 15.6.1
    CPU: (10) arm64 Apple M1 Max
    Memory: 1.26 GB / 64.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 22.21.0 - /Users/daniel/.local/share/mise/installs/node/22.21.0/bin/node
    Yarn: 1.22.19 - /Users/daniel/.local/share/mise/installs/node/22.21.0/bin/yarn
    npm: 10.9.4 - /Users/daniel/.local/share/mise/installs/node/22.21.0/bin/npm
    pnpm: 10.23.0 - /Users/daniel/.local/share/mise/installs/node/22.21.0/bin/pnpm
    Deno: 2.5.6 - /Users/daniel/.local/share/mise/installs/deno/2.5.6/bin/deno
  Browsers:
    Chrome: 142.0.7444.176
    Edge: 142.0.3595.90
    Firefox: 144.0.2
    Firefox Developer Edition: 143.0
    Safari: 18.6
    Safari Technology Preview: 18.4
  npmPackages:
    @sveltejs/adapter-auto: ^7.0.0 => 7.0.0
    @sveltejs/kit: ^2.47.1 => 2.49.0
    @sveltejs/vite-plugin-svelte: ^6.2.1 => 6.2.1
    svelte: ^5.41.0 => 5.43.14
    vite: ^7.1.10 => 7.2.4

Severity

blocking an upgrade

Additional Information

I can keep my project on 5.41.4 as a workaround, but of course I'd like to update one day.