[custom-element] Crash in _renderSlots when Teleport target is ancestor of custom element host (shadowRoot: false)

[canciodev]

Vue version

3.5.22

Link to minimal reproduction

https://stackblitz.com/edit/vitejs-vite-ycis7iro?file=src%2FApp.vue

Steps to reproduce

Steps to reproduce

1. Use defineCustomElement to create a custom element with shadowRoot: false.

2. Inside its template, include a ….

3. Mount the custom element so that the body contains the custom element host.

4. Open the page and observe console.

What is expected?

Teleport should work reliably even if its to target is an ancestor of the custom element host. The custom element should correctly render its slotted content without errors.

What is actually happening?

vue.js:11802 Uncaught TypeError: Cannot read properties of null (reading 'insertBefore')
at VueCustomElement._renderSlots (vue.js:11802:18)
at callWithErrorHandling (vue.js:2296:19)
at callWithAsyncErrorHandling (vue.js:2303:17)
at invokeVNodeHook (vue.js:9916:3)
at vue.js:7537:19
at flushPostFlushCbs (vue.js:2481:28)
at render2 (vue.js:8196:7)
at mount (vue.js:6117:13)
at app.mount (vue.js:12387:19)
at VueCustomElement._mount (vue.js:11610:15)

System Info

Any additional comments?

#13900