add failing test for #14268

packages/svelte/tests/runtime-runes/samples/proxy-nullish-coalescing-assignment/_config.js

@@ -0,0 +1,16 @@
+import { flushSync } from 'svelte';
+import { test } from '../../test';
+
+export default test({
+	html: `<button>items: null</button>`,
+
+	test({ assert, target }) {
+		const btn = target.querySelector('button');
+
+		flushSync(() => btn?.click());
+		assert.htmlEqual(target.innerHTML, `<button>items: [0]</button>`);
+
+		flushSync(() => btn?.click());
+		assert.htmlEqual(target.innerHTML, `<button>items: [0,1]</button>`);
+	}
+});

packages/svelte/tests/runtime-runes/samples/proxy-nullish-coalescing-assignment/main.svelte

@@ -0,0 +1,7 @@
+<script>
+	let items = $state(null);
+</script>
+
+<button onclick={() => (items ??= []).push(items.length)}>
+	items: {JSON.stringify(items)}
+</button>