From bdea5464ffff485855dc935823479af3d9827058 Mon Sep 17 00:00:00 2001
From: Sebastian Sebbie Silbermann <sebastian.silbermann@vercel.com>
Date: Wed, 15 Apr 2026 19:14:15 +0200
Subject: [PATCH 1/2] [Fiber][enableYieldingBeforePassive] Enable in
 experimental builds again

---
 packages/shared/ReactFeatureFlags.js                   | 2 +-
 packages/shared/forks/ReactFeatureFlags.www-dynamic.js | 1 +
 packages/shared/forks/ReactFeatureFlags.www.js         | 3 +--
 3 files changed, 3 insertions(+), 3 deletions(-)

diff --git a/packages/shared/ReactFeatureFlags.js b/packages/shared/ReactFeatureFlags.js
index c1d12dfcfbd4..5276aa184576 100644
--- a/packages/shared/ReactFeatureFlags.js
+++ b/packages/shared/ReactFeatureFlags.js
@@ -67,7 +67,7 @@ export const enableLegacyFBSupport: boolean = false;
 
 // Yield to the browser event loop and not just the scheduler event loop before passive effects.
 // Fix gated tests that fail with this flag enabled before turning it back on.
-export const enableYieldingBeforePassive: boolean = false;
+export const enableYieldingBeforePassive: boolean = __EXPERIMENTAL__;
 
 // Experiment to intentionally yield less to block high framerate animations.
 export const enableThrottledScheduling: boolean = false;
diff --git a/packages/shared/forks/ReactFeatureFlags.www-dynamic.js b/packages/shared/forks/ReactFeatureFlags.www-dynamic.js
index e19b3314c19b..c3fb3736dcee 100644
--- a/packages/shared/forks/ReactFeatureFlags.www-dynamic.js
+++ b/packages/shared/forks/ReactFeatureFlags.www-dynamic.js
@@ -37,6 +37,7 @@ export const enableFragmentRefsScrollIntoView: boolean = __VARIANT__;
 export const enableFragmentRefsTextNodes: boolean = __VARIANT__;
 export const enableInternalInstanceMap: boolean = __VARIANT__;
 export const enableParallelTransitions: boolean = __VARIANT__;
+export const enableYieldingBeforePassive: boolean = __VARIANT__;
 
 export const enableEffectEventMutationPhase: boolean = __VARIANT__;
 
diff --git a/packages/shared/forks/ReactFeatureFlags.www.js b/packages/shared/forks/ReactFeatureFlags.www.js
index 740a3f3f845b..23f26ebfe0f3 100644
--- a/packages/shared/forks/ReactFeatureFlags.www.js
+++ b/packages/shared/forks/ReactFeatureFlags.www.js
@@ -35,6 +35,7 @@ export const {
   enableFragmentRefsTextNodes,
   enableInternalInstanceMap,
   enableParallelTransitions,
+  enableYieldingBeforePassive,
 } = dynamicFeatureFlags;
 
 // On WWW, __EXPERIMENTAL__ is used for a new modern build.
@@ -53,8 +54,6 @@ export const enableMoveBefore: boolean = false;
 export const disableInputAttributeSyncing: boolean = false;
 export const enableLegacyFBSupport: boolean = true;
 
-export const enableYieldingBeforePassive: boolean = false;
-
 export const enableThrottledScheduling: boolean = false;
 
 export const enableComponentPerformanceTrack: boolean = true;

From 4afd2b0c488a4f0fdd3f0c7bbc6029f67e317cfd Mon Sep 17 00:00:00 2001
From: Sebastian Sebbie Silbermann <sebastian.silbermann@vercel.com>
Date: Wed, 15 Apr 2026 19:50:54 +0200
Subject: [PATCH 2/2] Update tests that weren't updated during "Don't try to
 hydrate a hidden Offscreen tree"

---
 ...DOMServerPartialHydration-test.internal.js | 30 ++++++++++++++-----
 1 file changed, 22 insertions(+), 8 deletions(-)

diff --git a/packages/react-dom/src/__tests__/ReactDOMServerPartialHydration-test.internal.js b/packages/react-dom/src/__tests__/ReactDOMServerPartialHydration-test.internal.js
index 3e6af7bc9744..104e8fb57827 100644
--- a/packages/react-dom/src/__tests__/ReactDOMServerPartialHydration-test.internal.js
+++ b/packages/react-dom/src/__tests__/ReactDOMServerPartialHydration-test.internal.js
@@ -3713,11 +3713,19 @@ describe('ReactDOMServerPartialHydration', () => {
       return <span>Hidden</span>;
     }
 
+    function VisibleChild() {
+      Scheduler.log('VisibleChild');
+      React.useEffect(() => {
+        Scheduler.log('VisibleChild Effect');
+      });
+      return <span ref={visibleRef}>Visible</span>;
+    }
+
     function App() {
       Scheduler.log('App');
       return (
         <>
-          <span ref={visibleRef}>Visible</span>
+          <VisibleChild />
           <Activity mode="hidden">
             <HiddenChild />
           </Activity>
@@ -3733,7 +3741,7 @@ describe('ReactDOMServerPartialHydration', () => {
     // During server rendering, the Child component should not be evaluated,
     // because it's inside a hidden tree.
     const finalHTML = ReactDOMServer.renderToString(<App />);
-    assertLog(['App']);
+    assertLog(['App', 'VisibleChild']);
 
     const container = document.createElement('div');
     container.innerHTML = finalHTML;
@@ -3753,17 +3761,18 @@ describe('ReactDOMServerPartialHydration', () => {
 
     // The visible span successfully hydrates
     ReactDOMClient.hydrateRoot(container, <App />);
-    await waitForPaint(['App']);
+    await waitForPaint(['App', 'VisibleChild']);
     expect(visibleRef.current).toBe(visibleSpan);
 
+    // Subsequently, the hidden child is prerendered on the client
+    // along with hydrating the Suspense boundary outside the Activity.
     if (gate(flags => flags.enableYieldingBeforePassive)) {
-      // Passive effects.
-      await waitForPaint([]);
+      await waitForPaint(['VisibleChild Effect']);
+      await waitForPaint(['HiddenChild']);
+    } else {
+      await waitForPaint(['VisibleChild Effect', 'HiddenChild']);
     }
 
-    // Subsequently, the hidden child is prerendered on the client
-    // along with hydrating the Suspense boundary outside the Activity.
-    await waitForPaint(['HiddenChild']);
     expect(container).toMatchInlineSnapshot(`
       <div>
         <span>
@@ -3779,6 +3788,11 @@ describe('ReactDOMServerPartialHydration', () => {
       </div>
     `);
 
+    if (gate(flags => flags.enableYieldingBeforePassive)) {
+      // passive effects
+      await waitForPaint([]);
+    }
+
     // Next the child inside the Activity is hydrated.
     await waitForPaint(['HiddenChild']);