Remove flushDiscreteUpdates from end of event (#21223)

We don't need this anymore because we flush in a microtask.

This should allow us to remove the logic in the event system that
tracks nested event dispatches.

I added a test to confirm that nested event dispatches don't triggger
a synchronous flush, like they would if we wrapped them `flushSync`. It
already passed; I added it to prevent a regression.

packages/react-dom/src/__tests__/ReactDOMNestedEvents-test.js

@@ -0,0 +1,78 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @emails react-core
+ */
+
+'use strict';
+
+describe('ReactDOMNestedEvents', () => {
+  let React;
+  let ReactDOM;
+  let Scheduler;
+  let TestUtils;
+  let act;
+  let useState;
+
+  beforeEach(() => {
+    jest.resetModules();
+    React = require('react');
+    ReactDOM = require('react-dom');
+    Scheduler = require('scheduler');
+    TestUtils = require('react-dom/test-utils');
+    act = TestUtils.unstable_concurrentAct;
+    useState = React.useState;
+  });
+
+  // @gate experimental
+  test('nested event dispatches should not cause updates to flush', async () => {
+    const buttonRef = React.createRef(null);
+    function App() {
+      const [isClicked, setIsClicked] = useState(false);
+      const [isFocused, setIsFocused] = useState(false);
+      const onClick = () => {
+        setIsClicked(true);
+        const el = buttonRef.current;
+        el.focus();
+        // The update triggered by the focus event should not have flushed yet.
+        // Nor the click update. They would have if we had wrapped the focus
+        // call in `flushSync`, though.
+        Scheduler.unstable_yieldValue(
+          'Value right after focus call: ' + el.innerHTML,
+        );
+      };
+      const onFocus = () => {
+        setIsFocused(true);
+      };
+      return (
+        <>
+          <button ref={buttonRef} onFocus={onFocus} onClick={onClick}>
+            {`Clicked: ${isClicked}, Focused: ${isFocused}`}
+          </button>
+        </>
+      );
+    }
+
+    const container = document.createElement('div');
+    document.body.appendChild(container);
+    const root = ReactDOM.unstable_createRoot(container);
+
+    await act(async () => {
+      root.render(<App />);
+    });
+    expect(buttonRef.current.innerHTML).toEqual(
+      'Clicked: false, Focused: false',
+    );
+
+    await act(async () => {
+      buttonRef.current.click();
+    });
+    expect(Scheduler).toHaveYielded([
+      'Value right after focus call: Clicked: false, Focused: false',
+    ]);
+    expect(buttonRef.current.innerHTML).toEqual('Clicked: true, Focused: true');
+  });
+});

packages/react-dom/src/events/ReactDOMEventListener.js

@@ -25,21 +25,13 @@ import {
   getSuspenseInstanceFromFiber,
 } from 'react-reconciler/src/ReactFiberTreeReflection';
 import {HostRoot, SuspenseComponent} from 'react-reconciler/src/ReactWorkTags';
-import {
-  type EventSystemFlags,
-  IS_CAPTURE_PHASE,
-  IS_LEGACY_FB_SUPPORT_MODE,
-} from './EventSystemFlags';
+import {type EventSystemFlags, IS_CAPTURE_PHASE} from './EventSystemFlags';
 
 import getEventTarget from './getEventTarget';
 import {getClosestInstanceFromNode} from '../client/ReactDOMComponentTree';
 
-import {enableLegacyFBSupport} from 'shared/ReactFeatureFlags';
 import {dispatchEventForPluginEventSystem} from './DOMPluginEventSystem';
-import {
-  flushDiscreteUpdatesIfNeeded,
-  discreteUpdates,
-} from './ReactDOMUpdateBatching';
+import {discreteUpdates} from './ReactDOMUpdateBatching';
 
 import {
   getCurrentPriorityLevel as getCurrentSchedulerPriorityLevel,
@@ -120,14 +112,6 @@ function dispatchDiscreteEvent(
   container,
   nativeEvent,
 ) {
-  if (
-    !enableLegacyFBSupport ||
-    // If we are in Legacy FB support mode, it means we've already
-    // flushed for this event and we don't need to do it again.
-    (eventSystemFlags & IS_LEGACY_FB_SUPPORT_MODE) === 0
-  ) {
-    flushDiscreteUpdatesIfNeeded(nativeEvent.timeStamp);
-  }
   discreteUpdates(
     dispatchEvent,
     domEventName,

packages/react-dom/src/events/ReactDOMUpdateBatching.js

@@ -88,13 +88,6 @@ export function discreteUpdates(fn, a, b, c, d) {
   }
 }
 
-// TODO: Replace with flushSync
-export function flushDiscreteUpdatesIfNeeded(timeStamp: number) {
-  if (!isInsideEventHandler) {
-    flushDiscreteUpdatesImpl();
-  }
-}
-
 export function setBatchingImplementation(
   _batchedUpdatesImpl,
   _discreteUpdatesImpl,

packages/react-dom/src/events/__tests__/DOMPluginEventSystem-test.internal.js

@@ -656,7 +656,7 @@ describe('DOMPluginEventSystem', () => {
           document.body.removeChild(parentContainer);
         });
 
-        it('handle click events on dynamic portals', () => {
+        it('handle click events on dynamic portals', async () => {
           const log = [];
 
           function Parent() {
@@ -670,7 +670,7 @@ describe('DOMPluginEventSystem', () => {
                   ref.current,
                 ),
               );
-            });
+            }, []);
 
             return (
               <div ref={ref} onClick={() => log.push('parent')} id="parent">
@@ -679,25 +679,33 @@ describe('DOMPluginEventSystem', () => {
             );
           }
 
-          ReactDOM.render(<Parent />, container);
+          await act(async () => {
+            ReactDOM.render(<Parent />, container);
+          });
 
           const parent = container.lastChild;
           expect(parent.id).toEqual('parent');
-          dispatchClickEvent(parent);
+
+          await act(async () => {
+            dispatchClickEvent(parent);
+          });
 
           expect(log).toEqual(['parent']);
 
           const child = parent.lastChild;
           expect(child.id).toEqual('child');
-          dispatchClickEvent(child);
+
+          await act(async () => {
+            dispatchClickEvent(child);
+          });
 
           // we add both 'child' and 'parent' due to bubbling
           expect(log).toEqual(['parent', 'child', 'parent']);
         });
 
         // Slight alteration to the last test, to catch
         // a subtle difference in traversal.
-        it('handle click events on dynamic portals #2', () => {
+        it('handle click events on dynamic portals #2', async () => {
           const log = [];
 
           function Parent() {
@@ -711,7 +719,7 @@ describe('DOMPluginEventSystem', () => {
                   ref.current,
                 ),
               );
-            });
+            }, []);
 
             return (
               <div ref={ref} onClick={() => log.push('parent')} id="parent">
@@ -720,17 +728,25 @@ describe('DOMPluginEventSystem', () => {
             );
           }
 
-          ReactDOM.render(<Parent />, container);
+          await act(async () => {
+            ReactDOM.render(<Parent />, container);
+          });
 
           const parent = container.lastChild;
           expect(parent.id).toEqual('parent');
-          dispatchClickEvent(parent);
+
+          await act(async () => {
+            dispatchClickEvent(parent);
+          });
 
           expect(log).toEqual(['parent']);
 
           const child = parent.lastChild;
           expect(child.id).toEqual('child');
-          dispatchClickEvent(child);
+
+          await act(async () => {
+            dispatchClickEvent(child);
+          });
 
           // we add both 'child' and 'parent' due to bubbling
           expect(log).toEqual(['parent', 'child', 'parent']);

packages/react-native-renderer/src/ReactFabric.js

@@ -19,7 +19,6 @@ import {
   batchedEventUpdates,
   batchedUpdates as batchedUpdatesImpl,
   discreteUpdates,
-  flushDiscreteUpdates,
   createContainer,
   updateContainer,
   injectIntoDevTools,
@@ -242,7 +241,6 @@ function createPortal(
 setBatchingImplementation(
   batchedUpdatesImpl,
   discreteUpdates,
-  flushDiscreteUpdates,
   batchedEventUpdates,
 );
 

packages/react-native-renderer/src/ReactNativeRenderer.js

@@ -19,7 +19,6 @@ import {
   batchedUpdates as batchedUpdatesImpl,
   batchedEventUpdates,
   discreteUpdates,
-  flushDiscreteUpdates,
   createContainer,
   updateContainer,
   injectIntoDevTools,
@@ -241,7 +240,6 @@ function createPortal(
 setBatchingImplementation(
   batchedUpdatesImpl,
   discreteUpdates,
-  flushDiscreteUpdates,
   batchedEventUpdates,
 );
 

packages/react-native-renderer/src/legacy-events/ReactGenericBatching.js

@@ -18,7 +18,6 @@ let batchedUpdatesImpl = function(fn, bookkeeping) {
 let discreteUpdatesImpl = function(fn, a, b, c, d) {
   return fn(a, b, c, d);
 };
-let flushDiscreteUpdatesImpl = function() {};
 let batchedEventUpdatesImpl = batchedUpdatesImpl;
 
 let isInsideEventHandler = false;
@@ -59,25 +58,15 @@ export function discreteUpdates(fn, a, b, c, d) {
     return discreteUpdatesImpl(fn, a, b, c, d);
   } finally {
     isInsideEventHandler = prevIsInsideEventHandler;
-    if (!isInsideEventHandler) {
-    }
-  }
-}
-
-export function flushDiscreteUpdatesIfNeeded() {
-  if (!isInsideEventHandler) {
-    flushDiscreteUpdatesImpl();
   }
 }
 
 export function setBatchingImplementation(
   _batchedUpdatesImpl,
   _discreteUpdatesImpl,
-  _flushDiscreteUpdatesImpl,
   _batchedEventUpdatesImpl,
 ) {
   batchedUpdatesImpl = _batchedUpdatesImpl;
   discreteUpdatesImpl = _discreteUpdatesImpl;
-  flushDiscreteUpdatesImpl = _flushDiscreteUpdatesImpl;
   batchedEventUpdatesImpl = _batchedEventUpdatesImpl;
 }