Fix double-firing mouseenter (#19571)

* test: Simulate mouseover in browser

* Fix duplicate onMouseEnter event when relatedTarget is a root

* Test leave as well

Co-authored-by: Sebastian Silbermann <silbermann.sebastian@gmail.com>

Author: gaearon

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

@@ -989,6 +989,57 @@ describe('ReactDOMFiber', () => {
     }
   });
 
+  // Regression test for https://github.com/facebook/react/issues/19562
+  it('does not fire mouseEnter twice when relatedTarget is the root node', () => {
+    let ops = [];
+    let target = null;
+
+    function simulateMouseMove(from, to) {
+      if (from) {
+        from.dispatchEvent(
+          new MouseEvent('mouseout', {
+            bubbles: true,
+            cancelable: true,
+            relatedTarget: to,
+          }),
+        );
+      }
+      if (to) {
+        to.dispatchEvent(
+          new MouseEvent('mouseover', {
+            bubbles: true,
+            cancelable: true,
+            relatedTarget: from,
+          }),
+        );
+      }
+    }
+
+    ReactDOM.render(
+      <div
+        ref={n => (target = n)}
+        onMouseEnter={() => ops.push('enter')}
+        onMouseLeave={() => ops.push('leave')}
+      />,
+      container,
+    );
+
+    simulateMouseMove(null, container);
+    expect(ops).toEqual([]);
+
+    ops = [];
+    simulateMouseMove(container, target);
+    expect(ops).toEqual(['enter']);
+
+    ops = [];
+    simulateMouseMove(target, container);
+    expect(ops).toEqual(['leave']);
+
+    ops = [];
+    simulateMouseMove(container, null);
+    expect(ops).toEqual([]);
+  });
+
   it('should throw on bad createPortal argument', () => {
     expect(() => {
       ReactDOM.createPortal(<div>portal</div>, null);

packages/react-dom/src/events/plugins/EnterLeaveEventPlugin.js

@@ -22,6 +22,7 @@ import {
 import {
   getClosestInstanceFromNode,
   getNodeFromInstance,
+  isContainerMarkedAsRoot,
 } from '../../client/ReactDOMComponentTree';
 import {accumulateEnterLeaveTwoPhaseListeners} from '../DOMPluginEventSystem';
 
@@ -57,15 +58,19 @@ function extractEvents(
     domEventName === 'mouseout' || domEventName === 'pointerout';
 
   if (isOverEvent && (eventSystemFlags & IS_REPLAYED) === 0) {
+    // If this is an over event with a target, we might have 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.
     const related =
       (nativeEvent: any).relatedTarget || (nativeEvent: any).fromElement;
     if (related) {
-      // 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) {
+      // If the related node is managed by React, we can assume that we have
+      // already dispatched the corresponding events during its mouseout.
+      if (
+        getClosestInstanceFromNode(related) ||
+        isContainerMarkedAsRoot(related)
+      ) {
         return;
       }
     }