From be603f5a51be33027193d9e5ae35a0e572d8ad5d Mon Sep 17 00:00:00 2001
From: Dominic Gannaway <trueadm@users.noreply.github.com>
Date: Fri, 13 Dec 2019 17:03:35 +0000
Subject: [PATCH] [react-events] Remove lastNativeEvent in favor of SystemFlags
 (#17585)

---
 packages/legacy-events/EventSystemFlags.js        |  1 +
 .../react-dom/src/events/EnterLeaveEventPlugin.js | 15 +++++----------
 .../react-dom/src/events/ReactDOMEventListener.js |  9 ++++++++-
 3 files changed, 14 insertions(+), 11 deletions(-)

diff --git a/packages/legacy-events/EventSystemFlags.js b/packages/legacy-events/EventSystemFlags.js
index 1ad8aa01c2518..a3d51f3197b75 100644
--- a/packages/legacy-events/EventSystemFlags.js
+++ b/packages/legacy-events/EventSystemFlags.js
@@ -15,3 +15,4 @@ export const IS_PASSIVE = 1 << 2;
 export const IS_ACTIVE = 1 << 3;
 export const PASSIVE_NOT_SUPPORTED = 1 << 4;
 export const IS_REPLAYED = 1 << 5;
+export const IS_FIRST_ANCESTOR = 1 << 6;
diff --git a/packages/react-dom/src/events/EnterLeaveEventPlugin.js b/packages/react-dom/src/events/EnterLeaveEventPlugin.js
index 300772410c603..de9b130462fb0 100644
--- a/packages/react-dom/src/events/EnterLeaveEventPlugin.js
+++ b/packages/react-dom/src/events/EnterLeaveEventPlugin.js
@@ -13,7 +13,7 @@ import {
   TOP_POINTER_OUT,
   TOP_POINTER_OVER,
 } from './DOMTopLevelEventTypes';
-import {IS_REPLAYED} from 'legacy-events/EventSystemFlags';
+import {IS_REPLAYED, IS_FIRST_ANCESTOR} from 'legacy-events/EventSystemFlags';
 import SyntheticMouseEvent from './SyntheticMouseEvent';
 import SyntheticPointerEvent from './SyntheticPointerEvent';
 import {
@@ -42,12 +42,6 @@ const eventTypes = {
   },
 };
 
-// We track the lastNativeEvent to ensure that when we encounter
-// cases where we process the same nativeEvent multiple times,
-// which can happen when have multiple ancestors, that we don't
-// duplicate enter
-let lastNativeEvent;
-
 const EnterLeaveEventPlugin = {
   eventTypes: eventTypes,
 
@@ -169,11 +163,12 @@ const EnterLeaveEventPlugin = {
 
     accumulateEnterLeaveDispatches(leave, enter, from, to);
 
-    if (nativeEvent === lastNativeEvent) {
-      lastNativeEvent = null;
+    // 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];
     }
-    lastNativeEvent = nativeEvent;
 
     return [leave, enter];
   },
diff --git a/packages/react-dom/src/events/ReactDOMEventListener.js b/packages/react-dom/src/events/ReactDOMEventListener.js
index 08c8962ec4187..bb7450c45a5e1 100644
--- a/packages/react-dom/src/events/ReactDOMEventListener.js
+++ b/packages/react-dom/src/events/ReactDOMEventListener.js
@@ -49,6 +49,7 @@ import {
   IS_PASSIVE,
   IS_ACTIVE,
   PASSIVE_NOT_SUPPORTED,
+  IS_FIRST_ANCESTOR,
 } from 'legacy-events/EventSystemFlags';
 
 import {
@@ -175,13 +176,19 @@ function handleTopLevel(bookKeeping: BookKeepingInstance) {
     const eventTarget = getEventTarget(bookKeeping.nativeEvent);
     const topLevelType = ((bookKeeping.topLevelType: any): DOMTopLevelEventType);
     const nativeEvent = ((bookKeeping.nativeEvent: any): AnyNativeEvent);
+    let eventSystemFlags = bookKeeping.eventSystemFlags;
+
+    // If this is the first ancestor, we mark it on the system flags
+    if (i === 0) {
+      eventSystemFlags |= IS_FIRST_ANCESTOR;
+    }
 
     runExtractedPluginEventsInBatch(
       topLevelType,
       targetInst,
       nativeEvent,
       eventTarget,
-      bookKeeping.eventSystemFlags,
+      eventSystemFlags,
     );
   }
 }