[Svelte 5] Components that are rendered as children remount when props change

[mattjennings]

Describe the bug

When a component is rendered as children in another component, if it references any props at the script top-level, it will remount when that prop changes.

<!-- App.svelte -->
<script>
    let count = $state(0)

    setInterval(() => count++, 100)
</script>

<Parent>
   <Child {count} />
<Parent />


<!-- Child.svelte -->
<script>
  const { count } = $props()
  
  console.log('mount', count)
</script>

Reproduction

https://svelte-5-preview.vercel.app/#H4sIAAAAAAAAE71STU_DMAz9K1ZBaqdV67hO3QTixAGJ-7pDaV2IlI8qcSdQ1P9O0lThY2PixCnxy_Nz_GybdIyjSTZ7m8haYLJJ7vo-yRN6731gjsgJXWzUoBuPlKbRrKddJStioleawCr5qAZJI3RaCUhDUlpJgJnxVGuUND-vihCuIi8q3b8y3kbaFH1hOT2OBI2vBVu4NlQTZuuFf_Ai8zeybAHbHVgPVdQoaQiY4xukB0mojzWfKZPScpnDzdqrhASNNGgJ30ScDMdax3S2CPjoj9EFZfFpi_tNGTrc-buLQlu2CSYVE1wWkePsFaplHcM22ZAecMzjNALpbwOJdluYrYAzIwmG2NB8Do3_m6vhqM7SXqveZD8bslesi8TRi9hbd2tRRzRbONwWrBsvdTMZ8f_N-CkVhSMJ4ddQIKiBoJYtMEoNdOwNW8_xaorjiquXLN1PYgd4xk5pBOErpHmoFMZ_aeVOZM7k_7Zq59IHGQQuLN6p8YfxAwxgravgAwAA

Check the console

Logs

No response

System Info

System:
    OS: macOS 14.0
    CPU: (10) arm64 Apple M1 Max
    Memory: 384.41 MB / 32.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 20.10.0 - ~/Library/pnpm/node
    npm: 10.1.0 - /opt/homebrew/bin/npm
    pnpm: 8.8.0 - ~/Library/pnpm/pnpm
    bun: 1.0.0 - ~/.bun/bin/bun
  Browsers:
    Chrome: 118.0.5993.70
    Safari: 17.0
  npmPackages:
    svelte: 5.0.0-next.12 => 5.0.0-next.12

Severity

annoyance

[mattjennings]

I've also been able to reproduce this without effects but instead console logging the prop at the top-level. So, this is perhaps unrelated to effects, but interacting with props at the top-level rather than inside of an onMount().

Updated the thread with a more accurate repro and explanation of the issue. Sorry for the the number of title changes - just been able to narrow it down a bit more since creating the issue.