[Fiber] Double invoke Effects in Strict Mode during Hydration (#35961)

eps1lon · web-flow · commit 67e47593b607 · 2026-04-17T18:09:37.000+02:00
diff --git a/packages/react-dom/src/__tests__/ReactDOMServerPartialHydration-test.internal.js b/packages/react-dom/src/__tests__/ReactDOMServerPartialHydration-test.internal.js
@@ -20,6 +20,7 @@ let Scheduler;
 let Suspense;
 let SuspenseList;
 let useSyncExternalStore;
+let use;
 let act;
 let IdleEventPriority;
 let waitForAll;
@@ -116,6 +117,7 @@ describe('ReactDOMServerPartialHydration', () => {
     Activity = React.Activity;
     Suspense = React.Suspense;
     useSyncExternalStore = React.useSyncExternalStore;
+    use = React.use;
     if (gate(flags => flags.enableSuspenseList)) {
       SuspenseList = React.unstable_SuspenseList;
     }
@@ -256,6 +258,77 @@ describe('ReactDOMServerPartialHydration', () => {
     expect(container.textContent).toBe('HelloHello');
   });
 
+  it('replays effects when a suspended boundary hydrates in StrictMode', async () => {
+    const log = [];
+    let suspend = false;
+    let resolve;
+    const promise = new Promise(resolvePromise => (resolve = resolvePromise));
+
+    function EffectfulChild() {
+      React.useLayoutEffect(() => {
+        log.push('layout mount');
+        return () => log.push('layout unmount');
+      }, []);
+      React.useEffect(() => {
+        log.push('effect mount');
+        return () => log.push('effect unmount');
+      }, []);
+      return 'Hello';
+    }
+
+    function Child() {
+      if (suspend) {
+        use(promise);
+      }
+      return <EffectfulChild />;
+    }
+
+    function App() {
+      return (
+        <Suspense fallback="Loading...">
+          <Child />
+        </Suspense>
+      );
+    }
+
+    const element = (
+      <React.StrictMode>
+        <App />
+      </React.StrictMode>
+    );
+
+    suspend = false;
+    const finalHTML = ReactDOMServer.renderToString(element);
+    const container = document.createElement('div');
+    container.innerHTML = finalHTML;
+    expect(container.textContent).toBe('Hello');
+
+    suspend = true;
+    ReactDOMClient.hydrateRoot(container, element);
+    await waitForAll([]);
+    expect(log).toEqual([]);
+    expect(container.textContent).toBe('Hello');
+
+    suspend = false;
+    resolve();
+    await promise;
+    await waitForAll([]);
+
+    expect(container.textContent).toBe('Hello');
+    if (__DEV__) {
+      expect(log).toEqual([
+        'layout mount',
+        'effect mount',
+        'layout unmount',
+        'effect unmount',
+        'layout mount',
+        'effect mount',
+      ]);
+    } else {
+      expect(log).toEqual(['layout mount', 'effect mount']);
+    }
+  });
+
   it('falls back to client rendering boundary on mismatch', async () => {
     let client = false;
     let suspend = false;
diff --git a/packages/react-dom/src/__tests__/ReactServerRenderingHydration-test.js b/packages/react-dom/src/__tests__/ReactServerRenderingHydration-test.js
@@ -392,6 +392,56 @@ describe('ReactDOMServerHydration', () => {
     expect(element.textContent).toBe('Hi');
   });
 
+  it('replays effects when hydrating a StrictMode subtree', async () => {
+    const log = [];
+    function Child() {
+      React.useLayoutEffect(() => {
+        log.push('layout mount');
+        return () => log.push('layout unmount');
+      }, []);
+      React.useEffect(() => {
+        log.push('effect mount');
+        return () => log.push('effect unmount');
+      }, []);
+      return <span>Hello</span>;
+    }
+
+    function App() {
+      return (
+        <div>
+          <Child />
+        </div>
+      );
+    }
+
+    const markup = (
+      <React.StrictMode>
+        <App />
+      </React.StrictMode>
+    );
+
+    const element = document.createElement('div');
+    element.innerHTML = ReactDOMServer.renderToString(markup);
+    expect(element.textContent).toBe('Hello');
+
+    await act(() => {
+      ReactDOMClient.hydrateRoot(element, markup);
+    });
+
+    if (__DEV__) {
+      expect(log).toEqual([
+        'layout mount',
+        'effect mount',
+        'layout unmount',
+        'effect unmount',
+        'layout mount',
+        'effect mount',
+      ]);
+    } else {
+      expect(log).toEqual(['layout mount', 'effect mount']);
+    }
+  });
+
   it('should be able to render and hydrate forwardRef components', async () => {
     const FunctionComponent = ({label, forwardedRef}) => (
       <div ref={forwardedRef}>{label}</div>
diff --git a/packages/react-reconciler/src/ReactFiberBeginWork.js b/packages/react-reconciler/src/ReactFiberBeginWork.js
@@ -88,6 +88,7 @@ import {
   NoFlags,
   PerformedWork,
   Placement,
+  PlacementDEV,
   Hydrating,
   Callback,
   ContentReset,
@@ -1080,7 +1081,8 @@ function updateDehydratedActivityComponent(
       // Conceptually this is similar to Placement in that a new subtree is
       // inserted into the React tree here. It just happens to not need DOM
       // mutations because it already exists.
-      primaryChildFragment.flags |= Hydrating;
+      // We should still treat it as a newly inserted Fiber to double invoke Strict Effects.
+      primaryChildFragment.flags |= Hydrating | PlacementDEV;
       return primaryChildFragment;
     }
   } else {
@@ -1899,7 +1901,8 @@ function updateHostRoot(
         // Conceptually this is similar to Placement in that a new subtree is
         // inserted into the React tree here. It just happens to not need DOM
         // mutations because it already exists.
-        node.flags = (node.flags & ~Placement) | Hydrating;
+        // We should still treat it as a newly inserted Fiber to double invoke Strict Effects.
+        node.flags = (node.flags & ~Placement) | Hydrating | PlacementDEV;
         node = node.sibling;
       }
     }
@@ -3104,7 +3107,8 @@ function updateDehydratedSuspenseComponent(
       // Conceptually this is similar to Placement in that a new subtree is
       // inserted into the React tree here. It just happens to not need DOM
       // mutations because it already exists.
-      primaryChildFragment.flags |= Hydrating;
+      // We should still treat it as a newly inserted Fiber to double invoke Strict Effects.
+      primaryChildFragment.flags |= Hydrating | PlacementDEV;
       return primaryChildFragment;
     }
   } else {
diff --git a/packages/react-reconciler/src/ReactFiberWorkLoop.js b/packages/react-reconciler/src/ReactFiberWorkLoop.js
@@ -5312,9 +5312,11 @@ function doubleInvokeEffectsInDEVIfNecessary(
   if (fiber.memoizedState === null) {
     // Only consider Offscreen that is visible.
     // TODO (Offscreen) Handle manual mode.
-    if (isInStrictMode && fiber.flags & Visibility) {
-      // Double invoke effects on Offscreen's subtree only
+    if (isInStrictMode && fiber.flags & (Visibility | PlacementDEV)) {
+      // Double invoke effects on Offscreen's subtree
       // if it is visible and its visibility has changed.
+      // However, we also need to consider newly hydrated Offscreen because their
+      // visibility flags might not have changed.
       runWithFiberInDEV(fiber, doubleInvokeEffectsOnFiber, root, fiber);
     } else if (fiber.subtreeFlags & PlacementDEV) {
       // Something in the subtree could have been suspended.
diff --git a/packages/react-reconciler/src/__tests__/ActivityStrictMode-test.js b/packages/react-reconciler/src/__tests__/ActivityStrictMode-test.js
@@ -230,4 +230,127 @@ describe('Activity StrictMode', () => {
       'Child mount',
     ]);
   });
+
+  // @gate __DEV__
+  it('should double invoke effects on newly inserted children while Activity becomes visible', async () => {
+    function Parent({children}) {
+      log.push('Parent rendered');
+      React.useEffect(() => {
+        log.push('Parent mount');
+        return () => {
+          log.push('Parent unmount');
+        };
+      });
+
+      return <div>{children}</div>;
+    }
+
+    function Child({name}) {
+      log.push(`Child ${name} rendered`);
+      React.useEffect(() => {
+        log.push(`Child ${name} mount`);
+        return () => {
+          log.push(`Child ${name} unmount`);
+        };
+      });
+
+      return null;
+    }
+
+    await act(() => {
+      ReactNoop.render(
+        <React.StrictMode>
+          <Activity mode="hidden">
+            <Parent />
+          </Activity>
+        </React.StrictMode>,
+      );
+    });
+
+    expect(log).toEqual(['Parent rendered', 'Parent rendered']);
+
+    log.length = 0;
+    await act(() => {
+      ReactNoop.render(
+        <React.StrictMode>
+          <Activity mode="visible">
+            <Parent>
+              <Child name="one" />
+            </Parent>
+          </Activity>
+        </React.StrictMode>,
+      );
+    });
+
+    expect(log).toEqual([
+      'Parent rendered',
+      'Parent rendered',
+      'Child one rendered',
+      'Child one rendered',
+      'Child one mount',
+      'Parent mount',
+      // StrictMode double invocation
+      'Parent unmount',
+      'Child one unmount',
+      'Child one mount',
+      'Parent mount',
+    ]);
+
+    log.length = 0;
+    await act(() => {
+      ReactNoop.render(
+        <React.StrictMode>
+          <Activity mode="visible">
+            <Parent>
+              <Child name="one" />
+            </Parent>
+          </Activity>
+        </React.StrictMode>,
+      );
+    });
+
+    expect(log).toEqual([
+      'Parent rendered',
+      'Parent rendered',
+      'Child one rendered',
+      'Child one rendered',
+      // single Effect invocation. No double invocation on update.
+      'Child one unmount',
+      'Parent unmount',
+      'Child one mount',
+      'Parent mount',
+    ]);
+
+    log.length = 0;
+    await act(() => {
+      ReactNoop.render(
+        <React.StrictMode>
+          <Activity mode="visible">
+            <Parent>
+              <Child name="one" />
+              <Child name="two" />
+            </Parent>
+          </Activity>
+        </React.StrictMode>,
+      );
+    });
+
+    expect(log).toEqual([
+      'Parent rendered',
+      'Parent rendered',
+      'Child one rendered',
+      'Child one rendered',
+      'Child two rendered',
+      'Child two rendered',
+      // single Effect invocation for existing Components.
+      'Child one unmount',
+      'Parent unmount',
+      'Child one mount',
+      'Child two mount',
+      'Parent mount',
+      // Double Effect invocation for new Component "two"
+      'Child two unmount',
+      'Child two mount',
+    ]);
+  });
 });
