Properly flush updates

test/utils/userEvent.ts

@@ -1,3 +1,4 @@
+import * as React from 'react';
 import { click, mouseDown, mouseUp } from './fireDiscreteEvent';
 import { act, fireEvent } from './createClientRender';
 
@@ -7,9 +8,16 @@ export function touch(target: Element): void {
 }
 
 export function mousePress(target: Element): void {
-  mouseDown(target);
-  mouseUp(target);
-  click(target);
-  // Flush scheduled effects
-  act(() => {});
+  if (typeof (React as any).unstable_act === 'function') {
+    (React as any).unstable_act(() => {
+      mouseDown(target);
+      mouseUp(target);
+      click(target);
+    });
+  } else {
+    mouseDown(target);
+    mouseUp(target);
+    click(target);
+    act(() => {});
+  }
 }