chore: add regression test for $effect.pre rerun after flushSync

Closes #18209

packages/svelte/tests/signals/test.ts

@@ -1151,6 +1151,28 @@ describe('signals', () => {
 		};
 	});
 
+	it('pre effect that writes its own dependency reruns on every flushSync', () => {
+		const destroy = effect_root(() => {
+			const control = state('A');
+			const value = state(0);
+
+			user_pre_effect(() => {
+				$.get(value);
+				set(value, $.get(control) === 'A' ? 1 : 2);
+			});
+
+			set(control, 'B');
+			flushSync();
+			assert.equal($.get(value), 2);
+
+			set(control, 'A');
+			flushSync();
+			assert.equal($.get(value), 1);
+		});
+
+		destroy();
+	});
+
 	test('proxy version state does not trigger self-dependency guard', () => {
 		return () => {
 			const s = proxy({ a: { b: 1 } });