Add test

packages/svelte/tests/runtime-runes/samples/array-push-in-effect/_config.js

@@ -0,0 +1,33 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	/**
+	 * Ensure that mutating an array with push inside an $effect
+	 * does not cause an infinite re-execution loop.
+	 */
+	test({ assert, target }) {
+		const button = target.querySelector('button');
+
+		// initial render — effect should have run once
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>inc</button>
+				<p>0</p>
+			`
+		);
+
+		// increment count; effect should append one new entry only
+		flushSync(() => button?.click());
+
+		assert.htmlEqual(
+			target.innerHTML,
+			`
+				<button>inc</button>
+				<p>0</p>
+				<p>1</p>
+			`
+		);
+	}
+}); 

packages/svelte/tests/runtime-runes/samples/array-push-in-effect/main.svelte

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+<script>
+	let count = $state(0);
+	let log = $state([]);
+
+	$effect(() => {
+		log.push(count);
+	});
+
+	function inc() {
+		count += 1;
+	}
+</script>
+
+<button on:click={inc}>inc</button>
+{#each log as item}
+	<p>{item}</p>
+{/each}