add test case that passes on old commit

Author: rbalicki2

packages/react-reconciler/src/__tests__/ReactOffscreenStrictMode-demo-test.js

@@ -0,0 +1,99 @@
+let React;
+let Offscreen;
+let ReactNoop;
+let act;
+let log;
+let ReactTestRenderer;
+
+let Parent;
+let Child;
+
+let shouldSuspend;
+let resolve;
+let suspensePromise;
+
+describe('ReactOffscreenStrictMode', () => {
+  beforeEach(() => {
+    shouldSuspend = true;
+    resolve;
+    suspensePromise = new Promise(_resolve => {
+      resolve = _resolve;
+    });
+    ReactTestRenderer = require('react-test-renderer');
+    jest.resetModules();
+    log = [];
+
+    React = require('react');
+    Offscreen = React.unstable_Offscreen;
+    ReactNoop = require('react-noop-renderer');
+    act = require('jest-react').act;
+
+    Parent = function Parent({children}) {
+      log.push('Parent rendered');
+      React.useEffect(() => {
+        log.push('Parent mount');
+        return () => log.push('Parent unmount');
+      });
+
+      return children;
+    }
+
+    Child = function Child() {
+      log.push('Child rendered');
+      React.useEffect(() => {
+        log.push('Child mount');
+        return () => log.push('Child unmount');
+      });
+      if (shouldSuspend) {
+        log.push('Child suspended');
+        throw suspensePromise;
+      }
+      return null;
+    }
+  });
+
+  it('child suspense double invoke test', async () => {
+    log.push('-------------------------------------------------');
+
+
+    act(() => {
+      ReactNoop.render(
+        <React.StrictMode>
+          <Offscreen mode="visible">
+            <Parent>
+              <React.Suspense fallback="fallback">
+                <Child />
+              </React.Suspense>
+            </Parent>
+          </Offscreen>
+        </React.StrictMode>,
+      );
+    });
+
+    log.push('------------------------------');
+
+    await act(async () => {
+      resolve();
+      shouldSuspend = false;
+      await suspensePromise;
+      jest.runAllImmediates();
+    });
+
+    expect(log).toEqual([
+      '-------------------------------------------------',
+      'Parent rendered',
+      'Parent rendered',
+      'Child rendered',
+      'Child suspended',
+      'Parent mount',
+      'Parent unmount',
+      'Parent mount',
+      '------------------------------',
+      'Child rendered',
+      'Child rendered',
+      'Child mount',
+      'Child unmount',
+      'Child mount',
+    ]);
+  });
+});