hydration_mismatch when pending snippet is passed as prop to <svelte:boundary> #16886
Description
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_mismatchSystem 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 => 7.1.9Severity
annoyance
Additional Information
No response