From d03951293deb8b1a63e57e24b06a3120b46f42bf Mon Sep 17 00:00:00 2001
From: Clay Tercek <code@claytercek.com>
Date: Fri, 14 Aug 2020 08:05:53 -0400
Subject: [PATCH] fix event.relatedTarget fallback logic for firefox (#19607)

* fix event.relatedTarget fallback logic for firefox

* check if relatedTarget is undefined for fallback
---
 packages/react-dom/src/events/SyntheticEvent.js | 10 +++++-----
 1 file changed, 5 insertions(+), 5 deletions(-)

diff --git a/packages/react-dom/src/events/SyntheticEvent.js b/packages/react-dom/src/events/SyntheticEvent.js
index a6c7e60411198..e226e95833cae 100644
--- a/packages/react-dom/src/events/SyntheticEvent.js
+++ b/packages/react-dom/src/events/SyntheticEvent.js
@@ -170,12 +170,12 @@ export const MouseEventInterface = {
   button: 0,
   buttons: 0,
   relatedTarget: function(event) {
-    return (
-      event.relatedTarget ||
-      (event.fromElement === event.srcElement
+    if (event.relatedTarget === undefined)
+      return event.fromElement === event.srcElement
         ? event.toElement
-        : event.fromElement)
-    );
+        : event.fromElement;
+
+    return event.relatedTarget;
   },
   movementX: function(event) {
     if ('movementX' in event) {