[Async] Spread imported `pushState` handler completely breaks `page.state`

[colecrouter]

Describe the bug

This is one wacky. It took me all afternoon and evening to figure it out, and I'm still not sure I know what's going on.

I had something like this:

/** Returns anchor attributes to open a dialog at the given href */
export const openDialog = (href: Pathname) => {
  const onclick = async (event: MouseEvent) => {
    event.preventDefault();
    event.stopImmediatePropagation();

    // Imagine this is copied and pasted from the "shallow nav" docs
    await shallowNavigate(href);
  };

  return {
    href,
    onclick,
  } satisfies Partial<HTMLAnchorAttributes>;
};

—used like this:

<a
	{...openDialog("/test")}
	class="btn btn-test"
>
	openDialog (spread)
</a>

Doing so results in page.state completely desychronizing from everything. Future updates to page.state fail to trigger reactivity. If you log it manually, it's updated.

Other Things of Note

• This can be seemingly partially subverted by moving the affected code further up in the component tree (yikes), but runs into issues like this:

  {#if page.state.x !== undefined}
    <MyComponent data={page.state.x.whatever} />
    //                              ^ cannot read property "whatever" of undefined
  {/if}

• This sometimes resolves itself once by reloading on a page, then soft-navigating back
• This only occurs with experimental async enabled
• Tried this with .svelte.ts file

Reproduction

https://stackblitz.com/edit/sveltejs-kit-template-default-ckxtrshn

Logs

No logs directly related to the bug

System Info

System:
    OS: Windows 11 10.0.26200
    CPU: (20) x64 12th Gen Intel(R) Core(TM) i7-12700K
    Memory: 3.39 GB / 15.72 GB
  Binaries:
    Node: 24.11.0 - C:\nvm4w\nodejs\node.EXE
    npm: 11.6.1 - C:\nvm4w\nodejs\npm.CMD
  Browsers:
    Chrome: 143.0.7499.169
    Edge: Chromium (140.0.3485.54)
    Internet Explorer: 11.0.26100.7309
  npmPackages:
    @sveltejs/vite-plugin-svelte: ^6.2.1 => 6.2.1
    rolldown-vite:  7.2.5
    svelte: ^5.43.8 => 5.45.10

I tried on my work MacBook, phone, etc. Latest Svelte/kit, older versions, etc.

Severity

serious, but I can work around it

Additional Information

No response