Custom elements cannot be used inside content scripts on extensions (v4 to v5 regression)

[ceifa]

Describe the bug

After upgrading to svelte 5, I cannot use anymore custom elements that was already mounted on the page inside content scripts on my chrome extension.

The bug apparently it's because svelte 5 now rely on window.customElements, which is not available inside content scripts.

I think it is just a matter of adding a check if typeof window.customElements is undefined or not, to fallback to the previous solution that worked on v4.

Reproduction

1. Create a page and add a custom element <foo-bar>
2. Create a chrome extension for that page and mount a svelte component on it that uses the <foo-bar>

Logs

Uncaught (in promise) TypeError: Cannot read properties of null (reading 'get')
    at set_custom_element_data
    at update_reaction
    at update_effect

System Info

System:
    OS: Windows 11 10.0.22635
    CPU: (6) x64 Intel(R) Core(TM) i5-9400F CPU @ 2.90GHz
    Memory: 2.23 GB / 15.92 GB
  Binaries:
    Node: 22.10.0 - C:\Program Files\nodejs\node.EXE
    Yarn: 1.22.22 - C:\Program Files\nodejs\yarn.CMD
    npm: 11.0.0 - C:\Program Files\nodejs\npm.CMD
    bun: 1.1.29 - ~\AppData\Roaming\npm\bun.CMD
  Browsers:
    Chrome: 131.0.6778.140
    Edge: Chromium (127.0.2651.8)
    Internet Explorer: 11.0.22621.3566

Severity

blocking an upgrade

[paoloricciuti]

If that's the problem you can just create a custom element passing true as the compile option or not passing a tag name instead of a tag name and it will not be registered for you

Here's an example

https://svelte.dev/playground/b508172a42f7499ca4d1b41693ae5bf9?version=5.14.2

[ceifa]

If that's the problem you can just create a custom element passing true as the compile option or not passing a tag name instead of a tag name and it will not be registered for you

I'm not registering any elements using svelte. I'm trying to use a custom element already registered by the site I'm hooking into using manifest's content script.

[Conduitry]

The dependency on the customElements global was introduced in #14508 as part of the improvements to the logic about when to set a property vs when to set an attribute. If we don't have that available to us, yeah, we could conceivably fall back to the value && typeof value === 'object' logic.