fix: defer effects in pending boundary

packages/svelte/tests/runtime-runes/samples/async-boundary-coordination/_config.js

@@ -0,0 +1,46 @@
+import { tick } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+async test({ assert, target }) {
+await tick();
+
+assert.htmlEqual(
+target.innerHTML,
+`
+<p>hello from server</p>
+<p>hello from server</p>
+<p>hello from server</p>
+<p>hello from server</p>
+`
+);
+
+const [button1, button2] = target.querySelectorAll('button');
+
+button1.click();
+await tick();
+
+assert.htmlEqual(
+target.innerHTML,
+`
+<p>hello from browser</p>
+<p>hello from browser</p>
+<p>hello from server</p>
+<p>hello from server</p>
+`
+);
+
+button2.click();
+await tick();
+
+assert.htmlEqual(
+target.innerHTML,
+`
+<p>hello from browser</p>
+<p>hello from browser</p>
+<p>hello from browser</p>
+<p>hello from browser</p>
+`
+);
+}
+});

packages/svelte/tests/runtime-runes/samples/async-boundary-coordination/main.svelte

@@ -0,0 +1,35 @@
+<script>
+let browser = typeof window !== 'undefined';
+
+let a = Promise.withResolvers();
+let b = Promise.withResolvers();
+
+if (!browser) {
+a.resolve(true);
+b.resolve(true);
+}
+</script>
+
+<button onclick={() => a.resolve(true)}>
+resolve a
+</button>
+
+<button onclick={() => b.resolve(true)}>
+resolve b
+</button>
+
+<svelte:boundary>
+{#if await a.promise}
+<p>hello from {browser ? 'browser' : 'server'}</p>
+{/if}
+
+<p>hello from {browser ? 'browser' : 'server'}</p>
+</svelte:boundary>
+
+<svelte:boundary>
+{#if await b.promise}
+<p>hello from {browser ? 'browser' : 'server'}</p>
+{/if}
+
+<p>hello from {browser ? 'browser' : 'server'}</p>
+</svelte:boundary>