Error when refreshing page: "HierarchyRequestError: Failed to execute 'appendChild' on 'Node': This node type does not support this method."

[NinjaInShade]

Describe the bug

I noticed this when updating my app's dependencies (notably svelte/sveltekit).

If you look at the reproduction stackblitz, click the "go to page" link, the page loads, and renders, without error, all good. As soon as you then refresh that page, you get a "Uncaught (in promise) HierarchyRequestError: Failed to execute 'appendChild' on 'Node': This node type does not support this method." error.

I'm not entirely sure what is causing this. Some observations:

• In PageComponent.svelte, the error goes away when you remove either:
  • The $props() call in the <script></script>
  • The --input-width style variable that is passed into the Fieldset component

I am able to provide more info and help in any way I can - this bug is currently blocking me deploying the next version of the app.

Reproduction

https://stackblitz.com/edit/sveltejs-kit-template-default-scix6sg7?file=src%2Froutes%2F%2Bpage.svelte

Logs

client.js?v=ea032cef:318 Uncaught (in promise) HierarchyRequestError: Failed to execute 'appendChild' on 'Node': This node type does not support this method.

	in PageComponent.svelte
	in +page.svelte
	in layout.svelte
	in root.svelte

System Info

System:
    OS: Linux 6.8 Linux Mint 21.3 (Virginia)
    CPU: (12) x64 AMD Ryzen 5 7600 6-Core Processor
    Memory: 18.18 GB / 30.32 GB
    Container: Yes
    Shell: 5.1.16 - /bin/bash
Binaries:
    Node: 23.6.0 - ~/.nvm/versions/node/v23.6.0/bin/node
    npm: 10.9.2 - ~/.nvm/versions/node/v23.6.0/bin/npm
Browsers:
    Chrome: 133.0.6943.98
npmPackages:
    @sveltejs/adapter-node: ^5.2.11 => 5.2.11 
    @sveltejs/kit: ^2.15.1 => 2.15.1 
    @sveltejs/vite-plugin-svelte: ^5.0.3 => 5.0.3 
    svelte: ^5.16.0 => 5.16.0 
    vite: ^6.0.6 => 6.0.6

Severity

blocking an upgrade

Additional Information

This happens both on dev and prod build.