Fix: Passive effect updates are never sync

I screwed this up in facebook#21082. Got confused by the < versus > thing again.

The helper functions are annoying, too, because I always forget the
intended order of the arguments. But they're still helpful because when
we refactor the type we only have the change the logic in one place.

Added a regression test.

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

@@ -53,6 +53,13 @@ export function higherEventPriority(
   return a !== 0 && a < b ? a : b;
 }
 
+export function lowerEventPriority(
+  a: EventPriority,
+  b: EventPriority,
+): EventPriority {
+  return a === 0 || a > b ? a : b;
+}
+
 export function isHigherEventPriority(
   a: EventPriority,
   b: EventPriority,

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

@@ -53,6 +53,13 @@ export function higherEventPriority(
   return a !== 0 && a < b ? a : b;
 }
 
+export function lowerEventPriority(
+  a: EventPriority,
+  b: EventPriority,
+): EventPriority {
+  return a === 0 || a > b ? a : b;
+}
+
 export function isHigherEventPriority(
   a: EventPriority,
   b: EventPriority,

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

@@ -167,7 +167,7 @@ import {
   IdleEventPriority,
   getCurrentUpdatePriority,
   setCurrentUpdatePriority,
-  higherEventPriority,
+  lowerEventPriority,
   lanesToEventPriority,
 } from './ReactEventPriorities.new';
 import {requestCurrentTransition, NoTransition} from './ReactFiberTransition';
@@ -2049,10 +2049,8 @@ function commitRootImpl(root, renderPriorityLevel) {
 export function flushPassiveEffects(): boolean {
   // Returns whether passive effects were flushed.
   if (pendingPassiveEffectsLanes !== NoLanes) {
-    const priority = higherEventPriority(
-      DefaultEventPriority,
-      lanesToEventPriority(pendingPassiveEffectsLanes),
-    );
+    const renderPriority = lanesToEventPriority(pendingPassiveEffectsLanes);
+    const priority = lowerEventPriority(DefaultEventPriority, renderPriority);
     const prevTransition = ReactCurrentBatchConfig.transition;
     const previousPriority = getCurrentUpdatePriority();
     try {

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

@@ -167,7 +167,7 @@ import {
   IdleEventPriority,
   getCurrentUpdatePriority,
   setCurrentUpdatePriority,
-  higherEventPriority,
+  lowerEventPriority,
   lanesToEventPriority,
 } from './ReactEventPriorities.old';
 import {requestCurrentTransition, NoTransition} from './ReactFiberTransition';
@@ -2049,10 +2049,8 @@ function commitRootImpl(root, renderPriorityLevel) {
 export function flushPassiveEffects(): boolean {
   // Returns whether passive effects were flushed.
   if (pendingPassiveEffectsLanes !== NoLanes) {
-    const priority = higherEventPriority(
-      DefaultEventPriority,
-      lanesToEventPriority(pendingPassiveEffectsLanes),
-    );
+    const renderPriority = lanesToEventPriority(pendingPassiveEffectsLanes);
+    const priority = lowerEventPriority(DefaultEventPriority, renderPriority);
     const prevTransition = ReactCurrentBatchConfig.transition;
     const previousPriority = getCurrentUpdatePriority();
     try {

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

@@ -166,4 +166,25 @@ describe('ReactFlushSync', () => {
     // Effect flushes after paint.
     expect(Scheduler).toHaveYielded(['Effect']);
   });
+
+  test('setState inside passive effect triggered by sync update is not sync', async () => {
+    const root = ReactNoop.createRoot();
+
+    function App() {
+      const [state, setState] = useState(1);
+      useEffect(() => {
+        setState(2);
+      }, []);
+      return <Text text={state} />;
+    }
+
+    await ReactNoop.act(async () => {
+      ReactNoop.flushSync(() => {
+        root.render(<App />);
+      });
+      // Should not have flushed the effect update yet
+      expect(Scheduler).toHaveYielded([1]);
+    });
+    expect(Scheduler).toHaveYielded([2]);
+  });
 });