fix(unmount): Flush useEffect cleanup functions syncronously (#746)

src/__tests__/render.js

@@ -87,3 +87,17 @@ test('renders options.wrapper around node', () => {
 </div>
 `)
 })
+
+test('flushes useEffect cleanup functions sync on unmount()', () => {
+  const spy = jest.fn()
+  function Component() {
+    React.useEffect(() => spy, [])
+    return null
+  }
+  const {unmount} = render(<Component />)
+  expect(spy).toHaveBeenCalledTimes(0)
+
+  unmount()
+
+  expect(spy).toHaveBeenCalledTimes(1)
+})

src/pure.js

@@ -74,7 +74,11 @@ function render(
           el.forEach(e => console.log(prettyDOM(e, maxLength, options)))
         : // eslint-disable-next-line no-console,
           console.log(prettyDOM(el, maxLength, options)),
-    unmount: () => ReactDOM.unmountComponentAtNode(container),
+    unmount: () => {
+      act(() => {
+        ReactDOM.unmountComponentAtNode(container)
+      })
+    },
     rerender: rerenderUi => {
       render(wrapUiIfNeeded(rerenderUi), {container, baseElement})
       // Intentionally do not return anything to avoid unnecessarily complicating the API.