`$inspect` can cause an error in `$derived` based on `data`

Describe the bug

$inspect can somehow cause a $derived to operate on data belonging to the next page when navigating. When the $inspect is removed, the problem disappears.

Reproduction

StackBlitz

// /some-route/+page.server.js
export function load() {
  return {
    items: [
      { id: 1, name: 'Item 1' },
      { id: 2, name: 'Item 2' },
      { id: 3, name: 'Item 3' },
      { id: 4, name: 'Item 4' },
      { id: 5, name: 'Item 5' },
    ],
  };
}

<!-- /some-route/+page.svelte -->
<script lang="ts">
  import List from './list.svelte';

  let { data } = $props();

  const { items } = $derived(data);

  const even = $derived(items.filter((item) => item.id % 2 === 0));
</script>

<List items={even} />

<!-- /some-route/list.svelte -->
<script lang="ts">
  const { items } = $props();

  $inspect(items); // <- remove this
</script>

{#each items as item}
  <div>
    <a href="/">{item.id}</a>
  </div>
{/each}

Logs

+page.svelte:13 Uncaught (in promise) TypeError: Cannot read properties of undefined (reading 'filter')

	in <unknown>
	in layout.svelte
	in root.svelte

    at +page.svelte:13:49
    at update_reaction (chunk-DJJYHU3C.js?v=ad8341b0:2754:23)
    at execute_derived (chunk-DJJYHU3C.js?v=ad8341b0:1240:15)
    at update_derived (chunk-DJJYHU3C.js?v=ad8341b0:1257:15)
    at is_dirty (chunk-DJJYHU3C.js?v=ad8341b0:2681:11)
    at flush_inspect_effects (chunk-DJJYHU3C.js?v=ad8341b0:1398:9)
    at internal_set (chunk-DJJYHU3C.js?v=ad8341b0:1386:7)
    at set (chunk-DJJYHU3C.js?v=ad8341b0:1337:10)
    at Object.set (chunk-SRUB5KV5.js?v=ad8341b0:829:11)
    at Object.assign (<anonymous>)

System Info

(StackBlitz)

  System:
    OS: Linux 5.0 undefined
    CPU: (8) x64 Intel(R) Core(TM) i9-9880H CPU @ 2.30GHz
    Memory: 0 Bytes / 0 Bytes
    Shell: 1.0 - /bin/jsh
  Binaries:
    Node: 20.19.1 - /usr/local/bin/node
    Yarn: 1.22.19 - /usr/local/bin/yarn
    npm: 10.8.2 - /usr/local/bin/npm
    pnpm: 8.15.6 - /usr/local/bin/pnpm
  npmPackages:
    @sveltejs/adapter-auto: ^6.0.0 => 6.0.1 
    @sveltejs/kit: ^2.22.0 => 2.25.1 
    @sveltejs/vite-plugin-svelte: ^6.0.0 => 6.1.0 
    svelte: ^5.25.0 => 5.36.13 
    vite: ^7.0.4 => 7.0.5

Severity

annoyance

Additional Information

This seems like more of a Svelte issue, but I was unable to create a clean reproduction showing the difference between having the $inspect and not having it without SvelteKit.

Start with the linked StackBlitz reproduction and inspect +page.server.js, +page.svelte, and list.svelte, comparing navigation with and without $inspect(items). Trace why the $derived value receives missing data during navigation. Done means the reproduction no longer throws and the rendered list uses the current page's data.