Lifecycle events don't run for components inside `svelte:boundary` pending snippet with `experimental.async: true`

[lemmon]

Describe the bug

When using experimental: { async: true } in Svelte config, components rendered inside svelte:boundary pending snippets have their DOM elements created, but lifecycle events (onMount, $effect, use: actions, @attach) never execute.

Components render visually but any initialization logic dependent on these lifecycle hooks fails silently.

Expected Behavior

Lifecycle events should execute normally for components inside svelte:boundary pending snippets, regardless of the experimental.async setting.

Actual Behavior

Components inside pending snippets:

• ✅ Render to DOM (elements are visible)
• ❌ onMount() callbacks don't fire
• ❌ $effect() callbacks don't fire
• ❌ use: actions don't initialize
• ❌ @attach() callbacks don't fire

Reproduction

Setup

1. Enable async mode in svelte.config.js:

compilerOptions: {
  experimental: {
    async: true,
  },
}

2. Create a component with lifecycle hooks (e.g., Shimmer.svelte):

<script>
  import { onMount } from 'svelte';

  let { delay = 0 } = $props();
  let visible = $state(false);

  onMount(() => {
    console.log('onMount fired'); // Never logs
  });

  $effect(() => {
    console.log('$effect fired'); // Never logs
    if (delay > 0) {
      setTimeout(() => { visible = true; }, delay);
    }
  });
</script>

{#if visible}
  <div>Shimmer content</div>
{/if}

3. Use the component inside a svelte:boundary pending snippet:

<svelte:boundary>
  {@const data = await fetchData()}

  {#snippet pending()}
    <Shimmer delay={500} />
  {/snippet}
</svelte:boundary>

Expected Result

• Component renders
• onMount callback executes
• $effect callback executes
• Component initializes properly (e.g., delay timer starts)

Actual Result

• Component renders to DOM
• No lifecycle callbacks execute
• Component remains uninitialized (e.g., delay timer never starts, visible stays false)

Environment

• Svelte version: 5.x (with experimental.async: true)
• Framework: SvelteKit
• Browser: All browsers (browser-agnostic issue)

Additional Context

This prevents implementing delayed loading states (e.g., showing shimmer only after 500ms delay) when components rely on lifecycle hooks for initialization. The workaround is to move lifecycle logic outside the component or avoid using svelte:boundary pending snippets with components that need initialization.

Logs

System Info

System:
    OS: macOS 15.6
    CPU: (8) arm64 Apple M2
    Memory: 188.53 MB / 8.00 GB
    Shell: 5.9 - /bin/zsh
  Binaries:
    Node: 22.13.1 - /opt/homebrew/bin/node
    Yarn: 1.22.22 - /opt/homebrew/bin/yarn
    npm: 11.7.0 - /opt/homebrew/bin/npm
    pnpm: 10.23.0 - /opt/homebrew/bin/pnpm
  Browsers:
    Firefox: 145.0
    Safari: 18.6
  npmPackages:
    svelte: ~5.45 => 5.45.6

Severity

blocking an upgrade