Add a failing test for Offscreen in StrictMode

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

@@ -0,0 +1,82 @@
+let React;
+let Offscreen;
+let ReactDOMClient;
+let act;
+
+describe('ReactOffscreenStrictMode', () => {
+  let log;
+
+  beforeEach(() => {
+    jest.resetModules();
+    log = [];
+
+    React = require('react');
+    Offscreen = React.unstable_Offscreen;
+    ReactDOMClient = require('react-dom/client');
+    act = require('jest-react').act;
+
+    const ReactFeatureFlags = require('shared/ReactFeatureFlags');
+    ReactFeatureFlags.enableStrictEffects = __DEV__;
+  });
+
+  function Component({ label }) {
+    React.useEffect(() => {
+      log.push(`${label}: useEffect mount`);
+      return () => log.push(`${label}: useEffect unmount`);
+    });
+
+    React.useLayoutEffect(() => {
+      log.push(`${label}: useLayoutEffect mount`);
+      return () => log.push(`${label}: useLayoutEffect unmount`);
+    });
+
+    log.push(`${label}: render`);
+
+    return null;
+  }
+
+  if (__DEV__) {
+    // @gate enableOffscreen
+    it('should trigger strict effects when offscreen is visible', () => {
+      const container = document.createElement('div');
+      const root = ReactDOMClient.createRoot(container, {
+        unstable_strictMode: true,
+      });
+      act(() => {
+        root.render(
+          <Offscreen mode="visible">
+            <Component label="A" />
+          </Offscreen>
+        );
+      });
+
+      expect(log).toEqual([
+        'A: render',
+        'A: render',
+        'A: useLayoutEffect mount',
+        'A: useEffect mount',
+        'A: useLayoutEffect unmount',
+        'A: useEffect unmount',
+        'A: useLayoutEffect mount',
+        'A: useEffect mount',
+      ]);
+    });
+
+    // @gate enableOffscreen
+    it('should not trigger strict effects when offscreen is hidden', () => {
+      const container = document.createElement('div');
+      const root = ReactDOMClient.createRoot(container, {
+        unstable_strictMode: true,
+      });
+      act(() => {
+        root.render(
+          <Offscreen mode="hidden">
+            <Component label="A" />
+          </Offscreen>
+        );
+      });
+
+      expect(log).toEqual([]);
+    });
+  }
+});