Add regression tests for StrictMode transition unmount cleanup

packages/react-dom/src/__tests__/ReactDOMFiberAsync-test.js

@@ -638,6 +638,61 @@ describe('ReactDOMFiberAsync', () => {
     });
   });
 
+  it('calls effect cleanup when unmounting in a transition inside StrictMode', async () => {
+    let handlerCalls = 0;
+    let cleanupCalls = 0;
+    let setShow;
+
+    function Child() {
+      React.useEffect(() => {
+        const onResize = () => {
+          handlerCalls++;
+        };
+        window.addEventListener('resize', onResize);
+        return () => {
+          cleanupCalls++;
+          window.removeEventListener('resize', onResize);
+        };
+      }, []);
+
+      return <div>Child</div>;
+    }
+
+    function App() {
+      const [show, _setShow] = React.useState(true);
+      setShow = _setShow;
+      return <>{show ? <Child /> : null}</>;
+    }
+
+    const root = ReactDOMClient.createRoot(container);
+    await act(async () => {
+      root.render(
+        <React.StrictMode>
+          <App />
+        </React.StrictMode>,
+      );
+    });
+
+    await act(async () => {
+      React.startTransition(() => {
+        setShow(false);
+      });
+    });
+
+    // Cleanup should remove the listener before any post-unmount event.
+    window.dispatchEvent(new Event('resize'));
+    expect(handlerCalls).toBe(0);
+    if (__DEV__) {
+      expect(cleanupCalls).toBe(2);
+    } else {
+      expect(cleanupCalls).toBe(1);
+    }
+
+    await act(async () => {
+      root.unmount();
+    });
+  });
+
   it('Should not flush transition lanes if there is no transition scheduled in popState', async () => {
     let setHasNavigated;
     function App() {

packages/react-reconciler/src/__tests__/ActivityStrictMode-test.js

@@ -127,6 +127,35 @@ describe('Activity StrictMode', () => {
     ]);
   });
 
+  // @gate __DEV__
+  it('calls passive cleanup when unmounting in a transition', async () => {
+    const root = ReactNoop.createRoot();
+
+    await act(() => {
+      root.render(
+        <React.StrictMode>
+          <Activity mode="visible">
+            <Component label="A" />
+          </Activity>
+        </React.StrictMode>,
+      );
+    });
+
+    log = [];
+
+    await act(() => {
+      React.startTransition(() => {
+        root.render(
+          <React.StrictMode>
+            <Activity mode="visible" />
+          </React.StrictMode>,
+        );
+      });
+    });
+
+    expect(log).toContain('A: useEffect unmount');
+  });
+
   it('should not cause infinite render loop when StrictMode is used with Suspense and synchronous set states', async () => {
     // This is a regression test, see https://github.com/facebook/react/pull/25179 for more details.
     function App() {