Slots in Svelte customElements not working

[vekunz]

Describe the bug

In our company, we want to build a UI component library for use with several frontend frameworks. Therefore we have to build them as Web Components (aka custom elements). We are trying to build them with Svelte because Svelte is just cool.
We are using Rollup for this because we couldn't get it to work with Vite (maybe someone has a guide for Vite for custom elements as build output from Svelte files).

Our problem now is that everything seems to work, except Slots. We have a <slot /> in our Svelte component, but after compiling the Svelte component to a Web component, the slot does not work at all.

Reproduction

I created a reproduction repository with our Rollup setup https://github.com/vekunz/svelte-rollup-webcomponents-slot.

1. npm install
2. npm run package
3. Open demo.html in browser.

-> Slot content from <my-button> in demo.html is not shown in the browser

Logs

---

System Info

System:
    OS: macOS 14.1.2
    CPU: (10) arm64 Apple M2 Pro
    Memory: 537.33 MB / 32.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 18.18.0 - ~/.nvm/versions/node/v18.18.0/bin/node
    Yarn: 1.22.21 - ~/.nvm/versions/node/v18.18.0/bin/yarn
    npm: 9.8.1 - ~/.nvm/versions/node/v18.18.0/bin/npm
    pnpm: 8.11.0 - ~/.nvm/versions/node/v18.18.0/bin/pnpm
  Browsers:
    Chrome: 120.0.6099.109
    Safari: 17.1.2
  npmPackages:
    rollup: 4.9.0 => 4.9.0 
    svelte: 4.2.8 => 4.2.8

Severity

blocking all usage of svelte

[dummdidumm]

I tried the reproduction repository. It fails the install for me, something in the package.lock.json seems to be broken. If I remove it or use pnpm, it installs correctly and the slot feature works as expected.
Please provide a reproduction that does reproduce this.

[vekunz]

Oh, sorry. I forgot this again. We have a company internal NPM registry, which mirrors the public one. I recreated the package-lock.json with the correct entries.

When I try this out, the first button just says "Hello", but it should say "Hello world!". The "Hello" comes from an attribute, the "world!" from the slot.

[dummdidumm]

Thanks, can reproduce it now - closing as duplicate of #8997, the workaround mentioned there should work here, too.