Re-lan d "Flush discrete passive effects before paint (facebook#21150)"

This re-lands commit 2e7acee.

packages/react-reconciler/src/ReactFiberWorkLoop.new.js

@@ -1976,6 +1976,21 @@ function commitRootImpl(root, renderPriorityLevel) {
     return null;
   }
 
+  // If the passive effects are the result of a discrete render, flush them
+  // synchronously at the end of the current task so that the result is
+  // immediately observable. Otherwise, we assume that they are not
+  // order-dependent and do not need to be observed by external systems, so we
+  // can wait until after paint.
+  // TODO: We can optimize this by not scheduling the callback earlier. Since we
+  // currently schedule the callback in multiple places, will wait until those
+  // are consolidated.
+  if (
+    includesSomeLane(pendingPassiveEffectsLanes, SyncLane) &&
+    root.tag !== LegacyRoot
+  ) {
+    flushPassiveEffects();
+  }
+
   // If layout work was scheduled, flush it now.
   flushSyncCallbacks();
 

packages/react-reconciler/src/ReactFiberWorkLoop.old.js

@@ -1976,6 +1976,21 @@ function commitRootImpl(root, renderPriorityLevel) {
     return null;
   }
 
+  // If the passive effects are the result of a discrete render, flush them
+  // synchronously at the end of the current task so that the result is
+  // immediately observable. Otherwise, we assume that they are not
+  // order-dependent and do not need to be observed by external systems, so we
+  // can wait until after paint.
+  // TODO: We can optimize this by not scheduling the callback earlier. Since we
+  // currently schedule the callback in multiple places, will wait until those
+  // are consolidated.
+  if (
+    includesSomeLane(pendingPassiveEffectsLanes, SyncLane) &&
+    root.tag !== LegacyRoot
+  ) {
+    flushPassiveEffects();
+  }
+
   // If layout work was scheduled, flush it now.
   flushSyncCallbacks();
 

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

@@ -61,4 +61,73 @@ describe('ReactFlushSync', () => {
     });
     expect(root).toMatchRenderedOutput('1, 1');
   });
+
+  test('flushes passive effects synchronously when they are the result of a sync render', async () => {
+    function App() {
+      useEffect(() => {
+        Scheduler.unstable_yieldValue('Effect');
+      }, []);
+      return <Text text="Child" />;
+    }
+
+    const root = ReactNoop.createRoot();
+    await ReactNoop.act(async () => {
+      ReactNoop.flushSync(() => {
+        root.render(<App />);
+      });
+      expect(Scheduler).toHaveYielded([
+        'Child',
+        // Because the pending effect was the result of a sync update, calling
+        // flushSync should flush it.
+        'Effect',
+      ]);
+      expect(root).toMatchRenderedOutput('Child');
+    });
+  });
+
+  test('do not flush passive effects synchronously in legacy mode', async () => {
+    function App() {
+      useEffect(() => {
+        Scheduler.unstable_yieldValue('Effect');
+      }, []);
+      return <Text text="Child" />;
+    }
+
+    const root = ReactNoop.createLegacyRoot();
+    await ReactNoop.act(async () => {
+      ReactNoop.flushSync(() => {
+        root.render(<App />);
+      });
+      expect(Scheduler).toHaveYielded([
+        'Child',
+        // Because we're in legacy mode, we shouldn't have flushed the passive
+        // effects yet.
+      ]);
+      expect(root).toMatchRenderedOutput('Child');
+    });
+    // Effect flushes after paint.
+    expect(Scheduler).toHaveYielded(['Effect']);
+  });
+
+  test("do not flush passive effects synchronously when they aren't the result of a sync render", async () => {
+    function App() {
+      useEffect(() => {
+        Scheduler.unstable_yieldValue('Effect');
+      }, []);
+      return <Text text="Child" />;
+    }
+
+    const root = ReactNoop.createRoot();
+    await ReactNoop.act(async () => {
+      root.render(<App />);
+      expect(Scheduler).toFlushUntilNextPaint([
+        'Child',
+        // Because the passive effect was not the result of a sync update, it
+        // should not flush before paint.
+      ]);
+      expect(root).toMatchRenderedOutput('Child');
+    });
+    // Effect flushes after paint.
+    expect(Scheduler).toHaveYielded(['Effect']);
+  });
 });

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

@@ -32,6 +32,7 @@ let useDeferredValue;
 let forwardRef;
 let memo;
 let act;
+let ContinuousEventPriority;
 
 describe('ReactHooksWithNoopRenderer', () => {
   beforeEach(() => {
@@ -55,6 +56,8 @@ describe('ReactHooksWithNoopRenderer', () => {
     useDeferredValue = React.unstable_useDeferredValue;
     Suspense = React.Suspense;
     act = ReactNoop.act;
+    ContinuousEventPriority = require('react-reconciler/constants')
+      .ContinuousEventPriority;
 
     textCache = new Map();
 
@@ -1397,10 +1400,10 @@ describe('ReactHooksWithNoopRenderer', () => {
         expect(Scheduler).toFlushAndYieldThrough(['Child one render']);
 
         // Schedule unmount for the parent that unmounts children with pending update.
-        ReactNoop.flushSync(() => {
+        ReactNoop.unstable_runWithPriority(ContinuousEventPriority, () => {
           setParentState(false);
         });
-        expect(Scheduler).toHaveYielded([
+        expect(Scheduler).toFlushUntilNextPaint([
           'Parent false render',
           'Parent false commit',
         ]);