[kit 2.1.0] Runtime error when `onNavigate` callback returns a function

[f-elix]

Describe the bug

When an onNavigate callback returns a function, a runtime error occurs:

ReferenceError: callbacks is not defined

The problem comes from #11547, specifically this line: https://github.com/sveltejs/kit/pull/11547/files#diff-6bc786c5d82299b5eed36bb073cd7e3c8347af2951019da2b5dff1d5e3bad611R1289.

Its preceded by a @ts-ignore, thats probably why it was missed during the refactor.

Reproduction

Go to https://stackblitz.com/edit/sveltejs-kit-template-default-jsehy7?description=The%20default%20SvelteKit%20template,%20generated%20with%20create-svelte&file=src%2Froutes%2F%2Blayout.svelte&title=SvelteKit%20Default%20Template and navigate by clicking a link. The error will appear in the console.

Logs

No response

System Info

System:
    OS: macOS 12.7.1
    CPU: (12) x64 Intel(R) Core(TM) i7-9750H CPU @ 2.60GHz
    Memory: 390.63 MB / 32.00 GB
    Shell: 5.8.1 - /bin/zsh
  Binaries:
    Node: 20.6.0 - ~/.nvm/versions/node/v20.6.0/bin/node
    npm: 9.8.1 - ~/.nvm/versions/node/v20.6.0/bin/npm
    pnpm: 8.7.4 - ~/.nvm/versions/node/v20.6.0/bin/pnpm
    bun: 1.0.0 - ~/.nvm/versions/node/v20.6.0/bin/bun
  Browsers:
    Brave Browser: 116.1.57.53
    Chrome: 120.0.6099.199
    Safari: 17.1.2
  npmPackages:
    @sveltejs/adapter-vercel: 4.0.4 => 4.0.4 
    @sveltejs/kit: 2.3.0 => 2.3.0 
    @sveltejs/vite-plugin-svelte: 3.0.1 => 3.0.1 
    svelte: 4.2.8 => 4.2.8

Severity

blocking all usage of SvelteKit

Additional Information

No response