Reproduce

Sources
Demo

Reproduced in Chrome, Opera and Edge (perhaps any Chromium based browser).
It works fine in Firefox and Safari.

Notes

It seems the issue is related to using iframe for block-level element bindings in Svelte 4, see sveltejs/svelte#7583.
Perhaps no longer an issue with block-level element bindings in Svelte 5, see sveltejs/svelte#7583 (comment).