From 0d1fb7be6c4f028cba8361f11d8f2f865b6efae3 Mon Sep 17 00:00:00 2001
From: Clay Tercek <code@claytercek.com>
Date: Thu, 13 Aug 2020 14:21:25 -0400
Subject: [PATCH 1/2] fix event.relatedTarget fallback logic for firefox

---
 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 a6c7e6041119..e9bb4edf390a 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 && event.fromElement)
+      return event.fromElement === event.srcElement
         ? event.toElement
-        : event.fromElement)
-    );
+        : event.fromElement;
+
+    return event.relatedTarget;
   },
   movementX: function(event) {
     if ('movementX' in event) {

From ccdf9c2f249ff017ce9633fec430c8cd6a68d386 Mon Sep 17 00:00:00 2001
From: Clay Tercek <code@claytercek.com>
Date: Thu, 13 Aug 2020 15:24:30 -0400
Subject: [PATCH 2/2] check if relatedTarget is undefined for fallback

---
 packages/react-dom/src/events/SyntheticEvent.js | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/packages/react-dom/src/events/SyntheticEvent.js b/packages/react-dom/src/events/SyntheticEvent.js
index e9bb4edf390a..e226e95833ca 100644
--- a/packages/react-dom/src/events/SyntheticEvent.js
+++ b/packages/react-dom/src/events/SyntheticEvent.js
@@ -170,7 +170,7 @@ export const MouseEventInterface = {
   button: 0,
   buttons: 0,
   relatedTarget: function(event) {
-    if (!event.relatedTarget && event.fromElement)
+    if (event.relatedTarget === undefined)
       return event.fromElement === event.srcElement
         ? event.toElement
         : event.fromElement;