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

@@ -163,7 +163,7 @@ import {
   DefaultEventPriority,
   getCurrentUpdatePriority,
   setCurrentUpdatePriority,
-  higherEventPriority,
+  lowerEventPriority,
   lanesToEventPriority,
 } from './ReactEventPriorities.new';
 import {requestCurrentTransition, NoTransition} from './ReactFiberTransition';
@@ -2008,10 +2008,8 @@ 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 previousPriority = getCurrentUpdatePriority();
     try {
       setCurrentUpdatePriority(priority);

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

@@ -163,7 +163,7 @@ import {
   DefaultEventPriority,
   getCurrentUpdatePriority,
   setCurrentUpdatePriority,
-  higherEventPriority,
+  lowerEventPriority,
   lanesToEventPriority,
 } from './ReactEventPriorities.old';
 import {requestCurrentTransition, NoTransition} from './ReactFiberTransition';
@@ -2008,10 +2008,8 @@ 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 previousPriority = getCurrentUpdatePriority();
     try {
       setCurrentUpdatePriority(priority);