diff --git a/dom/base/FragmentOrElement.cpp b/dom/base/FragmentOrElement.cpp
index 38be731811..6aea6820ef 100644
--- a/dom/base/FragmentOrElement.cpp
+++ b/dom/base/FragmentOrElement.cpp
@@ -824,6 +824,10 @@ nsIContent::GetEventTargetParent(EventChainPreVisitor& aVisitor)
   aVisitor.mCanHandle = true;
   aVisitor.mMayHaveListenerManager = HasListenerManager();
 
+  if (IsInShadowTree()) {
+    aVisitor.mItemInShadowTree = true;
+  }
+
   // Don't propagate mouseover and mouseout events when mouse is moving
   // inside chrome access only content.
   bool isAnonForEvents = IsRootOfChromeAccessOnlySubtree();
diff --git a/dom/base/nsGlobalWindow.cpp b/dom/base/nsGlobalWindow.cpp
index b53ad29769..c5ed38dc27 100644
--- a/dom/base/nsGlobalWindow.cpp
+++ b/dom/base/nsGlobalWindow.cpp
@@ -909,7 +909,8 @@ nsPIDOMWindow<T>::nsPIDOMWindow(nsPIDOMWindowOuter *aOuterWindow)
   mOuterWindow(aOuterWindow),
   // Make sure no actual window ends up with mWindowID == 0
   mWindowID(NextWindowID()), mHasNotifiedGlobalCreated(false),
-  mMarkedCCGeneration(0), mServiceWorkersTestingEnabled(false)
+  mMarkedCCGeneration(0), mServiceWorkersTestingEnabled(false),
+  mEvent(nullptr)
  {}
 
 template<class T>
@@ -5245,6 +5246,16 @@ nsGlobalWindow::SetOpener(JSContext* aCx, JS::Handle<JS::Value> aOpener,
   SetOpenerWindow(outer, false);
 }
 
+void
+nsGlobalWindow::GetEvent(JSContext* aCx, JS::MutableHandle<JS::Value> aRetval)
+{
+  if (mEvent) {
+    Unused << nsContentUtils::WrapNative(aCx, mEvent, aRetval);
+  } else {
+    aRetval.setUndefined();
+  }
+}
+
 void
 nsGlobalWindow::GetStatusOuter(nsAString& aStatus)
 {
diff --git a/dom/base/nsGlobalWindow.h b/dom/base/nsGlobalWindow.h
index bc989ea2e4..63bb574dd4 100644
--- a/dom/base/nsGlobalWindow.h
+++ b/dom/base/nsGlobalWindow.h
@@ -846,6 +846,7 @@ public:
   already_AddRefed<nsPIDOMWindowOuter> GetOpener() override;
   void SetOpener(JSContext* aCx, JS::Handle<JS::Value> aOpener,
                  mozilla::ErrorResult& aError);
+  void GetEvent(JSContext* aCx, JS::MutableHandle<JS::Value> aRetval);
   already_AddRefed<nsPIDOMWindowOuter> GetParentOuter();
   already_AddRefed<nsPIDOMWindowOuter> GetParent(mozilla::ErrorResult& aError);
   already_AddRefed<nsPIDOMWindowOuter> GetParent() override;
diff --git a/dom/base/nsPIDOMWindow.h b/dom/base/nsPIDOMWindow.h
index 21eb4cff7f..ffebf6570d 100644
--- a/dom/base/nsPIDOMWindow.h
+++ b/dom/base/nsPIDOMWindow.h
@@ -194,6 +194,10 @@ protected:
   // we have what it takes to do so.
   void MaybeCreateDoc();
 
+  // The event dispatch code sets and unsets this while keeping
+  // the event object alive.
+  nsIDOMEvent* mEvent;
+
 public:
   inline bool IsLoadingOrRunningTimeout() const;
 
@@ -789,6 +793,17 @@ public:
     return mInnerObjectsFreed;
   }
 
+  // Sets the event for window.event. Does NOT take ownership, so
+  // the caller is responsible for clearing the event before the
+  // event gets deallocated. Pass nullptr to set window.event to
+  // undefined. Returns the previous value.
+  nsIDOMEvent* SetEvent(nsIDOMEvent* aEvent)
+  {
+    nsIDOMEvent* old = mEvent;
+    mEvent = aEvent;
+    return old;
+  }
+
   /**
    * Check whether this window is a secure context.
    */
diff --git a/dom/events/EventDispatcher.cpp b/dom/events/EventDispatcher.cpp
index e2b5699241..001b458cf3 100644
--- a/dom/events/EventDispatcher.cpp
+++ b/dom/events/EventDispatcher.cpp
@@ -264,6 +264,16 @@ public:
     return mFlags.mRootOfClosedTree;
   }
 
+  void SetItemInShadowTree(bool aSet)
+  {
+    mFlags.mItemInShadowTree = aSet;
+  }
+
+  bool IsItemInShadowTree()
+  {
+    return mFlags.mItemInShadowTree;
+  }
+
   void SetIsSlotInClosedTree(bool aSet)
   {
     mFlags.mIsSlotInClosedTree = aSet;
@@ -351,7 +361,8 @@ public:
       mManager->HandleEvent(aVisitor.mPresContext, aVisitor.mEvent,
                             &aVisitor.mDOMEvent,
                             CurrentTarget(),
-                            &aVisitor.mEventStatus);
+                            &aVisitor.mEventStatus,
+                            IsItemInShadowTree());
       NS_ASSERTION(aVisitor.mEvent->mCurrentTarget == nullptr,
                    "CurrentTarget should be null!");
     }
@@ -384,6 +395,7 @@ private:
     bool mWantsPreHandleEvent : 1;
     bool mPreHandleEventOnly : 1;
     bool mRootOfClosedTree : 1;
+    bool mItemInShadowTree : 1;
     bool mIsSlotInClosedTree : 1;
     bool mIsChromeHandler : 1;
   private:
@@ -433,6 +445,7 @@ EventTargetChainItem::GetEventTargetParent(EventChainPreVisitor& aVisitor)
   SetWantsPreHandleEvent(aVisitor.mWantsPreHandleEvent);
   SetPreHandleEventOnly(aVisitor.mWantsPreHandleEvent && !aVisitor.mCanHandle);
   SetRootOfClosedTree(aVisitor.mRootOfClosedTree);
+  SetItemInShadowTree(aVisitor.mItemInShadowTree);
   SetRetargetedRelatedTarget(aVisitor.mRetargetedRelatedTarget);
   mItemFlags = aVisitor.mItemFlags;
   mItemData = aVisitor.mItemData;
diff --git a/dom/events/EventDispatcher.h b/dom/events/EventDispatcher.h
index 403c472c5c..9eaa124137 100644
--- a/dom/events/EventDispatcher.h
+++ b/dom/events/EventDispatcher.h
@@ -125,6 +125,7 @@ public:
     , mMayHaveListenerManager(true)
     , mWantsPreHandleEvent(false)
     , mRootOfClosedTree(false)
+    , mItemInShadowTree(false)
     , mParentIsSlotInClosedTree(false)
     , mParentIsChromeHandler(false)
     , mRelatedTargetRetargetedInCurrentScope(false)
@@ -147,6 +148,7 @@ public:
     mMayHaveListenerManager = true;
     mWantsPreHandleEvent = false;
     mRootOfClosedTree = false;
+    mItemInShadowTree = false;
     mParentIsSlotInClosedTree = false;
     mParentIsChromeHandler = false;
     // Note, we don't clear mRelatedTargetRetargetedInCurrentScope explicitly,
@@ -236,6 +238,12 @@ public:
    */
   bool mRootOfClosedTree;
 
+  /**
+   * If target is node and its root is a shadow root.
+   * https://dom.spec.whatwg.org/#event-path-item-in-shadow-tree
+   */
+  bool mItemInShadowTree;
+
   /**
    * True if mParentTarget is HTMLSlotElement in a closed shadow tree and the
    * current target is assigned to that slot.
diff --git a/dom/events/EventListenerManager.cpp b/dom/events/EventListenerManager.cpp
index 221b464773..af268f24bb 100644
--- a/dom/events/EventListenerManager.cpp
+++ b/dom/events/EventListenerManager.cpp
@@ -1120,6 +1120,38 @@ EventListenerManager::GetLegacyEventMessage(EventMessage aEventMessage) const
   }
 }
 
+already_AddRefed<nsPIDOMWindowInner>
+EventListenerManager::WindowFromListener(Listener* aListener,
+                                         bool aItemInShadowTree)
+{
+  nsCOMPtr<nsPIDOMWindowInner> innerWindow;
+  if (!aItemInShadowTree) {
+    if (aListener->mListener.HasWebIDLCallback()) {
+      CallbackObject* callback = aListener->mListener.GetWebIDLCallback();
+      nsGlobalWindow* win;
+      if (callback) {
+        // Find the real underlying callback.
+        JSObject* realCallback =
+          js::UncheckedUnwrap(callback->CallbackPreserveColor());
+        // Get the global for this callback.
+        win = mIsMainThreadELM ?
+              xpc::WindowGlobalOrNull(realCallback) :
+              nullptr;
+      }
+      if (win && win->IsInnerWindow()) {
+        innerWindow = win->AsInner(); // Can be nullptr
+      }
+    } else {
+      // Can't get the global from
+      // listener->mListener.GetXPCOMCallback().
+      // In most cases, it would be the same as for
+      // the target, so let's do that.
+      innerWindow = GetInnerWindowForTarget(); // Can be nullptr
+    }
+  }
+  return innerWindow.forget();
+}
+
 /**
 * Causes a check for event listeners and processing by them if they exist.
 * @param an event listener
@@ -1130,7 +1162,8 @@ EventListenerManager::HandleEventInternal(nsPresContext* aPresContext,
                                           WidgetEvent* aEvent,
                                           nsIDOMEvent** aDOMEvent,
                                           EventTarget* aCurrentTarget,
-                                          nsEventStatus* aEventStatus)
+                                          nsEventStatus* aEventStatus,
+                                          bool aItemInShadowTree)
 {
   //Set the value of the internal PreventDefault flag properly based on aEventStatus
   if (!aEvent->DefaultPrevented() &&
@@ -1222,9 +1255,18 @@ EventListenerManager::HandleEventInternal(nsPresContext* aPresContext,
               listener = listenerHolder.ptr();
               hasRemovedListener = true;
             }
+            nsCOMPtr<nsPIDOMWindowInner> innerWindow =
+              WindowFromListener(listener, aItemInShadowTree);
+            nsIDOMEvent* oldWindowEvent = nullptr;
+            if (innerWindow) {
+              oldWindowEvent = innerWindow->SetEvent(*aDOMEvent);
+            }
             if (NS_FAILED(HandleEventSubType(listener, *aDOMEvent, aCurrentTarget))) {
               aEvent->mFlags.mExceptionWasRaised = true;
             }
+            if (innerWindow) {
+              Unused << innerWindow->SetEvent(oldWindowEvent);
+            }
             aEvent->mFlags.mInPassiveListener = false;
 
             if (needsEndEventMarker) {
diff --git a/dom/events/EventListenerManager.h b/dom/events/EventListenerManager.h
index 51373317d4..decd305f5e 100644
--- a/dom/events/EventListenerManager.h
+++ b/dom/events/EventListenerManager.h
@@ -350,7 +350,8 @@ public:
                    WidgetEvent* aEvent, 
                    nsIDOMEvent** aDOMEvent,
                    dom::EventTarget* aCurrentTarget,
-                   nsEventStatus* aEventStatus)
+                   nsEventStatus* aEventStatus,
+                   bool aItemInShadowTree)
   {
     if (mListeners.IsEmpty() || aEvent->PropagationStopped()) {
       return;
@@ -371,7 +372,7 @@ public:
       return;
     }
     HandleEventInternal(aPresContext, aEvent, aDOMEvent, aCurrentTarget,
-                        aEventStatus);
+                        aEventStatus, aItemInShadowTree);
   }
 
   /**
@@ -482,7 +483,8 @@ protected:
                            WidgetEvent* aEvent,
                            nsIDOMEvent** aDOMEvent,
                            dom::EventTarget* aCurrentTarget,
-                           nsEventStatus* aEventStatus);
+                           nsEventStatus* aEventStatus,
+                           bool aItemInShadowTree);
 
   nsresult HandleEventSubType(Listener* aListener,
                               nsIDOMEvent* aDOMEvent,
@@ -573,6 +575,10 @@ public:
     return typedHandler ? typedHandler->OnBeforeUnloadEventHandler() : nullptr;
   }
 
+private:
+  already_AddRefed<nsPIDOMWindowInner> WindowFromListener(Listener* aListener,
+                                                          bool aItemInShadowTree);
+
 protected:
   /**
    * Helper method for implementing the various Get*EventHandler above.  Will
diff --git a/dom/webidl/Window.webidl b/dom/webidl/Window.webidl
index 59cc9689c2..2ba9f4f124 100644
--- a/dom/webidl/Window.webidl
+++ b/dom/webidl/Window.webidl
@@ -50,6 +50,7 @@ interface nsIDOMCrypto;
   [Throws] void stop();
   [Throws, CrossOriginCallable, UnsafeInPrerendering] void focus();
   [Throws, CrossOriginCallable] void blur();
+  [Replaceable, Pref="dom.window.event.enabled"] readonly attribute any event;
 
   // other browsing contexts
   [Replaceable, Throws, CrossOriginReadable] readonly attribute WindowProxy frames;
diff --git a/modules/libpref/init/all.js b/modules/libpref/init/all.js
index 51296a21ac..aad7c119d9 100644
--- a/modules/libpref/init/all.js
+++ b/modules/libpref/init/all.js
@@ -5244,6 +5244,9 @@ pref("prompts.content_handling_dialog_modal.enabled", false);
 // Whether module scripts (<script type="module">) are enabled for content.
 pref("dom.moduleScripts.enabled", true);
 
+// Whether read-only Window property event is enabled for content.
+pref("dom.window.event.enabled", false);
+
 // Report details when a media source error occurs?
 // Enabled by default in debug builds, otherwise should be explicitly enabled
 // by the user to prevent XO leaking of the response status (CVE-2020-15666)