`hydration_mismatch` when `pending` snippet is passed as prop to `<svelte:boundary>`

[Tinggaard]

Describe the bug

hydration_mismatch error shows up when passing the pending snippet as a prop to <svelte:boundary> instead of as a child.

I.e. this works

<svelte:boundary>
  {@const _ = await new Promise((r) => setTimeout(r, 2000))}
  <p>Content has now loaded</p>

  {#snippet pending()}
    loading...
  {/snippet}
</svelte:boundary>

While this does not

{#snippet pending()}
  loading...
{/snippet}

<svelte:boundary {pending}>
  {@const _ = await new Promise((r) => setTimeout(r, 2000))}
  <p>Content has now loaded</p>
</svelte:boundary>

Furthermore. The latter briefly shows undefined right after the pending snippet, which quickly disappears again.

Reproduction

https://stackblitz.com/github/tinggaard/boundary-pending-prop

Logs

[svelte] hydration_mismatch

System Info

System:
    OS: Linux 6.16 Arch Linux
    CPU: (16) x64 AMD Ryzen 7 8845HS w/ Radeon 780M Graphics
    Memory: 14.68 GB / 27.15 GB
    Container: Yes
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 24.8.0 - /usr/bin/node
    npm: 11.6.1 - /usr/bin/npm
    pnpm: 10.17.1 - /usr/bin/pnpm
  Browsers:
    Brave Browser: 139.1.81.137
    Chromium: 140.0.7339.207
  npmPackages:
    @sveltejs/adapter-auto: ^6.1.1 => 6.1.1
    @sveltejs/kit: 2.43.8 => 2.43.8
    @sveltejs/vite-plugin-svelte: ^6.2.1 => 6.2.1
    svelte: ^5.39.8 => 5.39.8
    vite: ^7.1.9 => 7.1.9

Severity

annoyance

Additional Information

No response