Apply #20778 to new fork, too

packages/react-dom/src/__tests__/ReactDOMServerSelectiveHydration-test.internal.js

@@ -92,6 +92,21 @@ function dispatchClickEvent(target) {
   return target.dispatchEvent(mouseOutEvent);
 }
 
+// TODO: There's currently no React DOM API to opt into Idle priority updates,
+// and there's no native DOM event that maps to idle priority, so this is a
+// temporary workaround. Need something like ReactDOM.unstable_IdleUpdates.
+function TODO_scheduleIdleDOMSchedulerTask(fn) {
+  Scheduler.unstable_runWithPriority(Scheduler.unstable_IdlePriority, () => {
+    const prevEvent = window.event;
+    window.event = {type: 'message'};
+    try {
+      fn();
+    } finally {
+      window.event = prevEvent;
+    }
+  });
+}
+
 describe('ReactDOMServerSelectiveHydration', () => {
   beforeEach(() => {
     jest.resetModuleRegistry();
@@ -889,12 +904,10 @@ describe('ReactDOMServerSelectiveHydration', () => {
       expect(Scheduler).toFlushAndYieldThrough(['App', 'Commit']);
 
       // Render an update at Idle priority that needs to update A.
-      Scheduler.unstable_runWithPriority(
-        Scheduler.unstable_IdlePriority,
-        () => {
-          root.render(<App a="AA" />);
-        },
-      );
+
+      TODO_scheduleIdleDOMSchedulerTask(() => {
+        root.render(<App a="AA" />);
+      });
 
       // Start rendering. This will force the first boundary to hydrate
       // by scheduling it at one higher pri than Idle.

packages/react-noop-renderer/src/ReactNoop.js

@@ -41,6 +41,7 @@ export const {
   deferredUpdates,
   unbatchedUpdates,
   discreteUpdates,
+  idleUpdates,
   flushDiscreteUpdates,
   flushSync,
   flushPassiveEffects,

packages/react-noop-renderer/src/ReactNoopPersistent.js

@@ -41,6 +41,7 @@ export const {
   deferredUpdates,
   unbatchedUpdates,
   discreteUpdates,
+  idleUpdates,
   flushDiscreteUpdates,
   flushSync,
   flushPassiveEffects,

packages/react-noop-renderer/src/createReactNoop.js

@@ -27,6 +27,7 @@ import {
   LegacyRoot,
 } from 'react-reconciler/src/ReactRootTags';
 
+import {enableNativeEventPriorityInference} from 'shared/ReactFeatureFlags';
 import ReactSharedInternals from 'shared/ReactSharedInternals';
 import enqueueTask from 'shared/enqueueTask';
 const {IsSomeRendererActing} = ReactSharedInternals;
@@ -392,7 +393,7 @@ function createReactNoop(reconciler: Function, useMutation: boolean) {
     resetAfterCommit(): void {},
 
     getCurrentEventPriority() {
-      return NoopRenderer.DefaultEventPriority;
+      return currentEventPriority;
     },
 
     now: Scheduler.unstable_now,
@@ -587,6 +588,8 @@ function createReactNoop(reconciler: Function, useMutation: boolean) {
   const roots = new Map();
   const DEFAULT_ROOT_ID = '<default>';
 
+  let currentEventPriority = NoopRenderer.DefaultEventPriority;
+
   function childToJSX(child, text) {
     if (text !== null) {
       return text;
@@ -925,6 +928,23 @@ function createReactNoop(reconciler: Function, useMutation: boolean) {
 
     discreteUpdates: NoopRenderer.discreteUpdates,
 
+    idleUpdates<T>(fn: () => T): T {
+      if (enableNativeEventPriorityInference) {
+        const prevEventPriority = currentEventPriority;
+        currentEventPriority = NoopRenderer.IdleEventPriority;
+        try {
+          fn();
+        } finally {
+          currentEventPriority = prevEventPriority;
+        }
+      } else {
+        return Scheduler.unstable_runWithPriority(
+          Scheduler.unstable_IdlePriority,
+          fn,
+        );
+      }
+    },
+
     flushDiscreteUpdates: NoopRenderer.flushDiscreteUpdates,
 
     flushSync(fn: () => mixed) {

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

@@ -70,7 +70,7 @@ const RetryLanePriority: LanePriority = 5;
 const SelectiveHydrationLanePriority: LanePriority = 4;
 
 const IdleHydrationLanePriority: LanePriority = 3;
-const IdleLanePriority: LanePriority = 2;
+export const IdleLanePriority: LanePriority = 2;
 
 const OffscreenLanePriority: LanePriority = 1;
 
@@ -275,6 +275,7 @@ export function getNextLanes(root: FiberRoot, wipLanes: Lanes): Lanes {
 
   // Check if any work has expired.
   if (expiredLanes !== NoLanes) {
+    // TODO: Should entangle with SyncLane
     nextLanes = expiredLanes;
     nextLanePriority = return_highestLanePriority = SyncLanePriority;
   } else {

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

@@ -70,7 +70,7 @@ const RetryLanePriority: LanePriority = 5;
 const SelectiveHydrationLanePriority: LanePriority = 4;
 
 const IdleHydrationLanePriority: LanePriority = 3;
-const IdleLanePriority: LanePriority = 2;
+export const IdleLanePriority: LanePriority = 2;
 
 const OffscreenLanePriority: LanePriority = 1;
 
@@ -275,6 +275,7 @@ export function getNextLanes(root: FiberRoot, wipLanes: Lanes): Lanes {
 
   // Check if any work has expired.
   if (expiredLanes !== NoLanes) {
+    // TODO: Should entangle with SyncLane
     nextLanes = expiredLanes;
     nextLanePriority = return_highestLanePriority = SyncLanePriority;
   } else {

packages/react-reconciler/src/ReactFiberReconciler.js

@@ -55,6 +55,7 @@ import {
   DefaultEventPriority as DefaultEventPriority_old,
   DiscreteEventPriority as DiscreteEventPriority_old,
   ContinuousEventPriority as ContinuousEventPriority_old,
+  IdleEventPriority as IdleEventPriority_old,
 } from './ReactFiberReconciler.old';
 
 import {
@@ -98,6 +99,7 @@ import {
   DefaultEventPriority as DefaultEventPriority_new,
   DiscreteEventPriority as DiscreteEventPriority_new,
   ContinuousEventPriority as ContinuousEventPriority_new,
+  IdleEventPriority as IdleEventPriority_new,
 } from './ReactFiberReconciler.new';
 
 export const createContainer = enableNewReconciler
@@ -183,6 +185,9 @@ export const DiscreteEventPriority = enableNewReconciler
 export const ContinuousEventPriority = enableNewReconciler
   ? ContinuousEventPriority_new
   : ContinuousEventPriority_old;
+export const IdleEventPriority = enableNewReconciler
+  ? IdleEventPriority_new
+  : IdleEventPriority_old;
 
 //TODO: "psuedo" is spelled "pseudo"
 export const createHasPsuedoClassSelector = enableNewReconciler

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

@@ -101,6 +101,7 @@ export {
   InputDiscreteLanePriority as DiscreteEventPriority,
   InputContinuousLanePriority as ContinuousEventPriority,
   DefaultLanePriority as DefaultEventPriority,
+  IdleLanePriority as IdleEventPriority,
 } from './ReactFiberLane.new';
 
 export {registerMutableSourceForHydration} from './ReactMutableSource.new';

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

@@ -101,6 +101,7 @@ export {
   InputDiscreteLanePriority as DiscreteEventPriority,
   InputContinuousLanePriority as ContinuousEventPriority,
   DefaultLanePriority as DefaultEventPriority,
+  IdleLanePriority as IdleEventPriority,
 } from './ReactFiberLane.old';
 
 export {registerMutableSourceForHydration} from './ReactMutableSource.new';

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

@@ -465,15 +465,7 @@ export function requestUpdateLane(fiber: Fiber): Lane {
   } else {
     if (enableNativeEventPriorityInference) {
       const eventLanePriority = getCurrentEventPriority();
-      if (eventLanePriority === DefaultLanePriority) {
-        // TODO: move this case into the ReactDOM host config.
-        const schedulerLanePriority = schedulerPriorityToLanePriority(
-          schedulerPriority,
-        );
-        lane = findUpdateLane(schedulerLanePriority, currentEventWipLanes);
-      } else {
-        lane = findUpdateLane(eventLanePriority, currentEventWipLanes);
-      }
+      lane = findUpdateLane(eventLanePriority, currentEventWipLanes);
     } else {
       const schedulerLanePriority = schedulerPriorityToLanePriority(
         schedulerPriority,