diff --git a/packages/react-dom/src/events/DOMLegacyEventPluginSystem.js b/packages/react-dom/src/events/DOMLegacyEventPluginSystem.js
index 4ab66536dd..bf46b68474 100644
--- a/packages/react-dom/src/events/DOMLegacyEventPluginSystem.js
+++ b/packages/react-dom/src/events/DOMLegacyEventPluginSystem.js
@@ -15,6 +15,7 @@ import type {Fiber} from 'react-reconciler/src/ReactInternalTypes';
 import type {PluginModule} from 'legacy-events/PluginModuleType';
 import type {ReactSyntheticEvent} from 'legacy-events/ReactSyntheticEventType';
 import type {TopLevelType} from 'legacy-events/TopLevelEventTypes';
+import forEachAccumulated from 'legacy-events/forEachAccumulated';
 
 import {
   HostRoot,
@@ -45,6 +46,7 @@ import {
 } from './DOMTopLevelEventTypes';
 import {addTrappedEventListener} from './ReactDOMEventListener';
 import {batchedEventUpdates} from './ReactDOMUpdateBatching';
+import getListener from './getListener';
 
 /**
  * Summary of `DOMEventPluginSystem` event handling:
@@ -396,3 +398,203 @@ export function legacyTrapCapturedEvent(
   );
   listenerMap.set(topLevelType, {passive: undefined, listener});
 }
+
+function getParent(inst: Object | null): Object | null {
+  if (!inst) {
+    return null;
+  }
+  do {
+    inst = inst.return;
+    // TODO: If this is a HostRoot we might want to bail out.
+    // That is depending on if we want nested subtrees (layers) to bubble
+    // events to their parent. We could also go through parentNode on the
+    // host node but that wouldn't work for React Native and doesn't let us
+    // do the portal feature.
+  } while (inst && inst.tag !== HostComponent);
+  if (inst) {
+    return inst;
+  }
+  return null;
+}
+
+/**
+ * Simulates the traversal of a two-phase, capture/bubble event dispatch.
+ */
+function traverseTwoPhase(
+  inst: Object,
+  fn: Function,
+  arg: ReactSyntheticEvent,
+) {
+  const path = [];
+  while (inst) {
+    path.push(inst);
+    inst = getParent(inst);
+  }
+  let i;
+  for (i = path.length; i-- > 0; ) {
+    fn(path[i], 'captured', arg);
+  }
+  for (i = 0; i < path.length; i++) {
+    fn(path[i], 'bubbled', arg);
+  }
+}
+
+function listenerAtPhase(inst, event, propagationPhase) {
+  const registrationName =
+    event.dispatchConfig.phasedRegistrationNames[propagationPhase];
+  return getListener(inst, registrationName);
+}
+
+/**
+ * Return the lowest common ancestor of A and B, or null if they are in
+ * different trees.
+ */
+export function getLowestCommonAncestor(
+  instA: Object,
+  instB: Object,
+): Object | null {
+  let depthA = 0;
+  for (let tempA = instA; tempA; tempA = getParent(tempA)) {
+    depthA++;
+  }
+  let depthB = 0;
+  for (let tempB = instB; tempB; tempB = getParent(tempB)) {
+    depthB++;
+  }
+
+  // If A is deeper, crawl up.
+  while (depthA - depthB > 0) {
+    instA = getParent(instA);
+    depthA--;
+  }
+
+  // If B is deeper, crawl up.
+  while (depthB - depthA > 0) {
+    instB = getParent(instB);
+    depthB--;
+  }
+
+  // Walk in lockstep until we find a match.
+  let depth = depthA;
+  while (depth--) {
+    if (instA === instB || instA === instB.alternate) {
+      return instA;
+    }
+    instA = getParent(instA);
+    instB = getParent(instB);
+  }
+  return null;
+}
+
+/**
+ * Traverses the ID hierarchy and invokes the supplied `cb` on any IDs that
+ * should would receive a `mouseEnter` or `mouseLeave` event.
+ *
+ * Does not invoke the callback on the nearest common ancestor because nothing
+ * "entered" or "left" that element.
+ */
+export function traverseEnterLeave(
+  from: Object,
+  to: Object,
+  fn: Function,
+  argFrom: ReactSyntheticEvent,
+  argTo: ReactSyntheticEvent,
+) {
+  const common = from && to ? getLowestCommonAncestor(from, to) : null;
+  const pathFrom = [];
+  while (true) {
+    if (!from) {
+      break;
+    }
+    if (from === common) {
+      break;
+    }
+    const alternate = from.alternate;
+    if (alternate !== null && alternate === common) {
+      break;
+    }
+    pathFrom.push(from);
+    from = getParent(from);
+  }
+  const pathTo = [];
+  while (true) {
+    if (!to) {
+      break;
+    }
+    if (to === common) {
+      break;
+    }
+    const alternate = to.alternate;
+    if (alternate !== null && alternate === common) {
+      break;
+    }
+    pathTo.push(to);
+    to = getParent(to);
+  }
+  for (let i = 0; i < pathFrom.length; i++) {
+    fn(pathFrom[i], 'bubbled', argFrom);
+  }
+  for (let i = pathTo.length; i-- > 0; ) {
+    fn(pathTo[i], 'captured', argTo);
+  }
+}
+
+function accumulateDirectionalDispatches(inst, phase, event) {
+  if (__DEV__) {
+    if (!inst) {
+      console.error('Dispatching inst must not be null');
+    }
+  }
+  const listener = listenerAtPhase(inst, event, phase);
+  if (listener) {
+    event._dispatchListeners = accumulateInto(
+      event._dispatchListeners,
+      listener,
+    );
+    event._dispatchInstances = accumulateInto(event._dispatchInstances, inst);
+  }
+}
+
+function accumulateTwoPhaseDispatchesSingle(event) {
+  if (event && event.dispatchConfig.phasedRegistrationNames) {
+    traverseTwoPhase(event._targetInst, accumulateDirectionalDispatches, event);
+  }
+}
+
+/**
+ * Accumulates without regard to direction, does not look for phased
+ * registration names. Same as `accumulateDirectDispatchesSingle` but without
+ * requiring that the `dispatchMarker` be the same as the dispatched ID.
+ */
+function accumulateDispatches(
+  inst: Object,
+  ignoredDirection: ?boolean,
+  event: Object,
+): void {
+  if (inst && event && event.dispatchConfig.registrationName) {
+    const registrationName = event.dispatchConfig.registrationName;
+    const listener = getListener(inst, registrationName);
+    if (listener) {
+      event._dispatchListeners = accumulateInto(
+        event._dispatchListeners,
+        listener,
+      );
+      event._dispatchInstances = accumulateInto(event._dispatchInstances, inst);
+    }
+  }
+}
+
+export function accumulateTwoPhaseDispatches(
+  events: ReactSyntheticEvent | Array<ReactSyntheticEvent>,
+): void {
+  forEachAccumulated(events, accumulateTwoPhaseDispatchesSingle);
+}
+
+export function accumulateEnterLeaveDispatches(
+  leave: ReactSyntheticEvent,
+  enter: ReactSyntheticEvent,
+  from: Fiber,
+  to: Fiber,
+) {
+  traverseEnterLeave(from, to, accumulateDispatches, leave, enter);
+}
diff --git a/packages/react-dom/src/events/plugins/LegacyBeforeInputEventPlugin.js b/packages/react-dom/src/events/plugins/LegacyBeforeInputEventPlugin.js
index 96685f7c81..3c866c6749 100644
--- a/packages/react-dom/src/events/plugins/LegacyBeforeInputEventPlugin.js
+++ b/packages/react-dom/src/events/plugins/LegacyBeforeInputEventPlugin.js
@@ -28,7 +28,7 @@ import {
 } from '../FallbackCompositionState';
 import SyntheticCompositionEvent from '../SyntheticCompositionEvent';
 import SyntheticInputEvent from '../SyntheticInputEvent';
-import accumulateTwoPhaseListeners from '../accumulateTwoPhaseListeners';
+import {accumulateTwoPhaseDispatches} from '../DOMLegacyEventPluginSystem';
 
 const END_KEYCODES = [9, 13, 27, 32]; // Tab, Return, Esc, Space
 const START_KEYCODE = 229;
@@ -276,7 +276,7 @@ function extractCompositionEvent(
     }
   }
 
-  accumulateTwoPhaseListeners(event);
+  accumulateTwoPhaseDispatches(event);
   return event;
 }
 
@@ -437,7 +437,7 @@ function extractBeforeInputEvent(
   );
 
   event.data = chars;
-  accumulateTwoPhaseListeners(event);
+  accumulateTwoPhaseDispatches(event);
   return event;
 }
 
diff --git a/packages/react-dom/src/events/plugins/LegacyChangeEventPlugin.js b/packages/react-dom/src/events/plugins/LegacyChangeEventPlugin.js
index dbe3685f36..e9518bb1a2 100644
--- a/packages/react-dom/src/events/plugins/LegacyChangeEventPlugin.js
+++ b/packages/react-dom/src/events/plugins/LegacyChangeEventPlugin.js
@@ -27,13 +27,9 @@ import {updateValueIfChanged} from '../../client/inputValueTracking';
 import {setDefaultValue} from '../../client/ReactDOMInput';
 import {enqueueStateRestore} from '../ReactDOMControlledComponent';
 
-import {
-  disableInputAttributeSyncing,
-  enableModernEventSystem,
-} from 'shared/ReactFeatureFlags';
-import accumulateTwoPhaseListeners from '../accumulateTwoPhaseListeners';
+import {disableInputAttributeSyncing} from 'shared/ReactFeatureFlags';
 import {batchedUpdates} from '../ReactDOMUpdateBatching';
-import {dispatchEventsInBatch} from '../DOMModernPluginEventSystem';
+import {accumulateTwoPhaseDispatches} from '../DOMLegacyEventPluginSystem';
 
 const eventTypes = {
   change: {
@@ -64,7 +60,7 @@ function createAndAccumulateChangeEvent(inst, nativeEvent, target) {
   event.type = 'change';
   // Flag this event loop as needing state restore.
   enqueueStateRestore(target);
-  accumulateTwoPhaseListeners(event);
+  accumulateTwoPhaseDispatches(event);
   return event;
 }
 /**
@@ -105,11 +101,7 @@ function manualDispatchChangeEvent(nativeEvent) {
 }
 
 function runEventInBatch(event) {
-  if (enableModernEventSystem) {
-    dispatchEventsInBatch([event]);
-  } else {
-    runEventsInBatch(event);
-  }
+  runEventsInBatch(event);
 }
 
 function getInstIfValueChanged(targetInst) {
diff --git a/packages/react-dom/src/events/plugins/LegacyEnterLeaveEventPlugin.js b/packages/react-dom/src/events/plugins/LegacyEnterLeaveEventPlugin.js
index cb51ed4546..9b23efd888 100644
--- a/packages/react-dom/src/events/plugins/LegacyEnterLeaveEventPlugin.js
+++ b/packages/react-dom/src/events/plugins/LegacyEnterLeaveEventPlugin.js
@@ -23,8 +23,7 @@ import {
 } from '../../client/ReactDOMComponentTree';
 import {HostComponent, HostText} from 'react-reconciler/src/ReactWorkTags';
 import {getNearestMountedFiber} from 'react-reconciler/src/ReactFiberTreeReflection';
-import {enableModernEventSystem} from 'shared/ReactFeatureFlags';
-import accumulateEnterLeaveListeners from '../accumulateEnterLeaveListeners';
+import {accumulateEnterLeaveDispatches} from '../DOMLegacyEventPluginSystem';
 
 const eventTypes = {
   mouseEnter: {
@@ -67,26 +66,16 @@ const EnterLeaveEventPlugin = {
     const isOutEvent =
       topLevelType === TOP_MOUSE_OUT || topLevelType === TOP_POINTER_OUT;
 
-    if (isOverEvent && (eventSystemFlags & IS_REPLAYED) === 0) {
-      const related = nativeEvent.relatedTarget || nativeEvent.fromElement;
-      if (related) {
-        if (enableModernEventSystem) {
-          // Due to the fact we don't add listeners to the document with the
-          // modern event system and instead attach listeners to roots, we
-          // need to handle the over event case. To ensure this, we just need to
-          // make sure the node that we're coming from is managed by React.
-          const inst = getClosestInstanceFromNode(related);
-          if (inst !== null) {
-            return null;
-          }
-        } else {
-          // If this is an over event with a target, then we've already dispatched
-          // the event in the out event of the other target. If this is replayed,
-          // then it's because we couldn't dispatch against this target previously
-          // so we have to do it now instead.
-          return null;
-        }
-      }
+    if (
+      isOverEvent &&
+      (eventSystemFlags & IS_REPLAYED) === 0 &&
+      (nativeEvent.relatedTarget || nativeEvent.fromElement)
+    ) {
+      // If this is an over event with a target, then we've already dispatched
+      // the event in the out event of the other target. If this is replayed,
+      // then it's because we couldn't dispatch against this target previously
+      // so we have to do it now instead.
+      return null;
     }
 
     if (!isOutEvent && !isOverEvent) {
@@ -174,15 +163,13 @@ const EnterLeaveEventPlugin = {
     enter.target = toNode;
     enter.relatedTarget = fromNode;
 
-    accumulateEnterLeaveListeners(leave, enter, from, to);
+    accumulateEnterLeaveDispatches(leave, enter, from, to);
 
-    if (!enableModernEventSystem) {
-      // If we are not processing the first ancestor, then we
-      // should not process the same nativeEvent again, as we
-      // will have already processed it in the first ancestor.
-      if ((eventSystemFlags & IS_FIRST_ANCESTOR) === 0) {
-        return [leave];
-      }
+    // If we are not processing the first ancestor, then we
+    // should not process the same nativeEvent again, as we
+    // will have already processed it in the first ancestor.
+    if ((eventSystemFlags & IS_FIRST_ANCESTOR) === 0) {
+      return [leave];
     }
 
     return [leave, enter];
diff --git a/packages/react-dom/src/events/plugins/LegacySelectEventPlugin.js b/packages/react-dom/src/events/plugins/LegacySelectEventPlugin.js
index 0d1266b0bc..74d1d9fb74 100644
--- a/packages/react-dom/src/events/plugins/LegacySelectEventPlugin.js
+++ b/packages/react-dom/src/events/plugins/LegacySelectEventPlugin.js
@@ -26,7 +26,7 @@ import {getNodeFromInstance} from '../../client/ReactDOMComponentTree';
 import {hasSelectionCapabilities} from '../../client/ReactInputSelection';
 import {DOCUMENT_NODE} from '../../shared/HTMLNodeType';
 import {isListeningToAllDependencies} from '../DOMEventListenerMap';
-import accumulateTwoPhaseListeners from '../accumulateTwoPhaseListeners';
+import {accumulateTwoPhaseDispatches} from '../DOMLegacyEventPluginSystem';
 
 const skipSelectionChangeEvent =
   canUseDOM && 'documentMode' in document && document.documentMode <= 11;
@@ -135,7 +135,7 @@ function constructSelectEvent(nativeEvent, nativeEventTarget) {
     syntheticEvent.type = 'select';
     syntheticEvent.target = activeElement;
 
-    accumulateTwoPhaseListeners(syntheticEvent);
+    accumulateTwoPhaseDispatches(syntheticEvent);
 
     return syntheticEvent;
   }
@@ -166,16 +166,11 @@ const SelectEventPlugin = {
     nativeEvent,
     nativeEventTarget,
     eventSystemFlags,
-    container,
   ) {
-    const containerOrDoc =
-      container || getEventTargetDocument(nativeEventTarget);
+    const doc = getEventTargetDocument(nativeEventTarget);
     // Track whether all listeners exists for this plugin. If none exist, we do
     // not extract events. See #3639.
-    if (
-      !containerOrDoc ||
-      !isListeningToAllDependencies('onSelect', containerOrDoc)
-    ) {
+    if (!doc || !isListeningToAllDependencies('onSelect', doc)) {
       return null;
     }
 
diff --git a/packages/react-dom/src/events/plugins/LegacySimpleEventPlugin.js b/packages/react-dom/src/events/plugins/LegacySimpleEventPlugin.js
index cd61625518..532dbec15c 100644
--- a/packages/react-dom/src/events/plugins/LegacySimpleEventPlugin.js
+++ b/packages/react-dom/src/events/plugins/LegacySimpleEventPlugin.js
@@ -14,10 +14,8 @@ import type {
 import type {ReactSyntheticEvent} from 'legacy-events/ReactSyntheticEventType';
 import type {Fiber} from 'react-reconciler/src/ReactInternalTypes';
 import type {PluginModule} from 'legacy-events/PluginModuleType';
-import type {EventSystemFlags} from '../EventSystemFlags';
 
 import SyntheticEvent from 'legacy-events/SyntheticEvent';
-import {IS_TARGET_PHASE_ONLY} from '../EventSystemFlags';
 
 import * as DOMTopLevelEventTypes from '../DOMTopLevelEventTypes';
 import {
@@ -37,10 +35,7 @@ import SyntheticTransitionEvent from '../SyntheticTransitionEvent';
 import SyntheticUIEvent from '../SyntheticUIEvent';
 import SyntheticWheelEvent from '../SyntheticWheelEvent';
 import getEventCharCode from '../getEventCharCode';
-import accumulateTwoPhaseListeners from '../accumulateTwoPhaseListeners';
-import accumulateEventTargetListeners from '../accumulateEventTargetListeners';
-
-import {enableUseEventAPI} from 'shared/ReactFeatureFlags';
+import {accumulateTwoPhaseDispatches} from '../DOMLegacyEventPluginSystem';
 
 // Only used in DEV for exhaustiveness validation.
 const knownHTMLTopLevelTypes: Array<DOMTopLevelEventType> = [
@@ -86,8 +81,6 @@ const SimpleEventPlugin: PluginModule<MouseEvent> = {
     targetInst: null | Fiber,
     nativeEvent: MouseEvent,
     nativeEventTarget: null | EventTarget,
-    eventSystemFlags?: EventSystemFlags,
-    targetContainer?: null | EventTarget,
   ): null | ReactSyntheticEvent {
     const dispatchConfig = topLevelEventsToDispatchConfig.get(topLevelType);
     if (!dispatchConfig) {
@@ -109,8 +102,6 @@ const SimpleEventPlugin: PluginModule<MouseEvent> = {
         break;
       case DOMTopLevelEventTypes.TOP_BLUR:
       case DOMTopLevelEventTypes.TOP_FOCUS:
-      case DOMTopLevelEventTypes.TOP_BEFORE_BLUR:
-      case DOMTopLevelEventTypes.TOP_AFTER_BLUR:
         EventConstructor = SyntheticFocusEvent;
         break;
       case DOMTopLevelEventTypes.TOP_CLICK:
@@ -179,10 +170,7 @@ const SimpleEventPlugin: PluginModule<MouseEvent> = {
         break;
       default:
         if (__DEV__) {
-          if (
-            knownHTMLTopLevelTypes.indexOf(topLevelType) === -1 &&
-            dispatchConfig.customEvent !== true
-          ) {
+          if (knownHTMLTopLevelTypes.indexOf(topLevelType) === -1) {
             console.error(
               'SimpleEventPlugin: Unhandled event type, `%s`. This warning ' +
                 'is likely caused by a bug in React. Please file an issue.',
@@ -201,23 +189,7 @@ const SimpleEventPlugin: PluginModule<MouseEvent> = {
       nativeEvent,
       nativeEventTarget,
     );
-
-    // For TargetEvent only accumulation, we do not traverse through
-    // the React tree looking for managed React DOM elements that have
-    // events. Instead we only check the EventTarget Store Map to see
-    // if the container has listeners for the particular phase we're
-    // interested in. This is because we attach the native event listener
-    // only in the given phase.
-    if (
-      enableUseEventAPI &&
-      eventSystemFlags !== undefined &&
-      eventSystemFlags & IS_TARGET_PHASE_ONLY &&
-      targetContainer != null
-    ) {
-      accumulateEventTargetListeners(event, targetContainer);
-    } else {
-      accumulateTwoPhaseListeners(event, true);
-    }
+    accumulateTwoPhaseDispatches(event);
     return event;
   },
 };