Cover comma-joined duplicate search params

packages/svelte/src/reactivity/url-search-params.test.ts

@@ -74,6 +74,25 @@ test('URLSearchParams.set updates when duplicate values collapse to the same joi
 	cleanup();
 });
 
+test('URLSearchParams.set updates when duplicate values collapse to the same comma-joined string', () => {
+	const params = new SvelteURLSearchParams('a=a&a=b');
+	const log: any = [];
+
+	const cleanup = effect_root(() => {
+		render_effect(() => {
+			log.push(params.toString());
+		});
+	});
+
+	flushSync(() => {
+		params.set('a', 'a,b');
+	});
+
+	assert.deepEqual(log, ['a=a&a=b', 'a=a%2Cb']);
+
+	cleanup();
+});
+
 test('URLSearchParams.append', () => {
 	const params = new SvelteURLSearchParams();
 	const log: any = [];