diff --git a/dom/base/FragmentOrElement.cpp b/dom/base/FragmentOrElement.cpp
index f75bb0b490ad..372788bb41e6 100644
--- a/dom/base/FragmentOrElement.cpp
+++ b/dom/base/FragmentOrElement.cpp
@@ -1091,7 +1091,11 @@ nsIContent::GetEventTargetParent(EventChainPreVisitor& aVisitor)
             // point to a node in Shadow DOM.
             aVisitor.mEvent->mTarget = aVisitor.mTargetInKnownToBeHandledScope;
             return;
-          } else {
+          } else if (targetInKnownToBeHandledScope) {
+            // Note, if targetInKnownToBeHandledScope is null,
+            // mTargetInKnownToBeHandledScope could be Window object in content
+            // page and we're in chrome document in the same process.
+
             // Step 11.6
             aVisitor.mRetargetedRelatedTarget = retargetedRelatedTarget;
           }
diff --git a/dom/events/EventDispatcher.cpp b/dom/events/EventDispatcher.cpp
index eda2a5f31a5c..255bf616fe9f 100644
--- a/dom/events/EventDispatcher.cpp
+++ b/dom/events/EventDispatcher.cpp
@@ -656,6 +656,26 @@ MayRetargetToChromeIfCanNotHandleEvent(
   return nullptr;
 }
 
+static bool
+ShouldClearTargets(WidgetEvent* aEvent)
+{
+  nsCOMPtr<nsIContent> finalTarget;
+  nsCOMPtr<nsIContent> finalRelatedTarget;
+  if ((finalTarget = do_QueryInterface(aEvent->mTarget)) &&
+      finalTarget->SubtreeRoot()->IsShadowRoot()) {
+    return true;
+  }
+
+  if ((finalRelatedTarget =
+         do_QueryInterface(aEvent->mRelatedTarget)) &&
+      finalRelatedTarget->SubtreeRoot()->IsShadowRoot()) {
+    return true;
+  }
+  //XXXsmaug Check also all the touch objects.
+
+  return false;
+}
+
 /* static */ nsresult
 EventDispatcher::Dispatch(nsISupports* aTarget,
                           nsPresContext* aPresContext,
@@ -836,6 +856,8 @@ EventDispatcher::Dispatch(nsISupports* aTarget,
 
   aEvent->mOriginalRelatedTarget = aEvent->mRelatedTarget;
 
+  bool clearTargets = false;
+
   nsCOMPtr<nsIContent> content = do_QueryInterface(aEvent->mOriginalTarget);
   bool isInAnon = content && content->IsInAnonymousSubtree();
 
@@ -859,6 +881,8 @@ EventDispatcher::Dispatch(nsISupports* aTarget,
     for (uint32_t i = 0; i < chain.Length(); ++i) {
       chain[i].PreHandleEvent(preVisitor);
     }
+
+    clearTargets = ShouldClearTargets(aEvent);
   } else {
     // At least the original target can handle the event.
     // Setting the retarget to the |target| simplifies retargeting code.
@@ -927,6 +951,9 @@ EventDispatcher::Dispatch(nsISupports* aTarget,
         for (uint32_t i = 0; i < chain.Length(); ++i) {
           chain[i].PreHandleEvent(preVisitor);
         }
+
+        clearTargets = ShouldClearTargets(aEvent);
+
         // Handle the chain.
         EventChainPostVisitor postVisitor(preVisitor);
         MOZ_RELEASE_ASSERT(!aEvent->mPath);
@@ -951,15 +978,16 @@ EventDispatcher::Dispatch(nsISupports* aTarget,
   aEvent->mFlags.mDispatchedAtLeastOnce = true;
 
   // https://dom.spec.whatwg.org/#concept-event-dispatch
-  // Step 18
-  // "If target's root is a shadow root, then set event's target attribute and
-  //  event's relatedTarget to null."
-  nsCOMPtr<nsIContent> finalTarget = do_QueryInterface(aEvent->mTarget);
-  if (finalTarget && finalTarget->SubtreeRoot()->IsShadowRoot()) {
+  // step 10. If clearTargets, then:
+  //          1. Set event's target to null.
+  //          2. Set event's relatedTarget to null.
+  //          3. Set event's touch target list to the empty list.
+  if (clearTargets) {
     aEvent->mTarget = nullptr;
     aEvent->mOriginalTarget = nullptr;
     aEvent->mRelatedTarget = nullptr;
     aEvent->mOriginalRelatedTarget = nullptr;
+    //XXXsmaug Check also all the touch objects.
   }
 
   if (!externalDOMEvent && preVisitor.mDOMEvent) {
diff --git a/testing/web-platform/meta/dom/events/relatedTarget.window.js.ini b/testing/web-platform/meta/dom/events/relatedTarget.window.js.ini
index cbe3c35452cd..222634b6cc93 100644
--- a/testing/web-platform/meta/dom/events/relatedTarget.window.js.ini
+++ b/testing/web-platform/meta/dom/events/relatedTarget.window.js.ini
@@ -1,4 +1,5 @@
 [relatedTarget.window.html]
-  [Untitled]
+  prefs: [dom.webcomponents.shadowdom.enabled:true]
+  [Reset targets before activation behavior]
     expected: FAIL
 
diff --git a/testing/web-platform/tests/dom/events/relatedTarget.window.js b/testing/web-platform/tests/dom/events/relatedTarget.window.js
index d3632bb9e040..4f68550ad513 100644
--- a/testing/web-platform/tests/dom/events/relatedTarget.window.js
+++ b/testing/web-platform/tests/dom/events/relatedTarget.window.js
@@ -41,13 +41,15 @@ async_test(t => {
 
 async_test(t => {
   const shadowChild = shadow.appendChild(document.createElement("div"));
-  shadowChild.addEventListener("demo", t.step_func(() => document.body.appendChild(shadowChild)));
+  const shadowChild2 = shadow.appendChild(document.createElement("div"));
+  shadowChild2.addEventListener("demo", t.step_func(() => document.body.appendChild(shadowChild)));
   const event = new FocusEvent("demo", { relatedTarget: shadowChild });
-  document.body.dispatchEvent(event);
+  shadowChild2.dispatchEvent(event);
   assert_equals(shadowChild.parentNode, document.body);
   assert_equals(event.target, null);
   assert_equals(event.relatedTarget, null);
   shadowChild.remove();
+  shadowChild2.remove();
   t.done();
 }, "Reset if relatedTarget pointed to a shadow tree pre-dispatch");