diff --git a/dom/base/nsContentUtils.cpp b/dom/base/nsContentUtils.cpp
index 47cc430a29ce..24eda39cf2c0 100644
--- a/dom/base/nsContentUtils.cpp
+++ b/dom/base/nsContentUtils.cpp
@@ -2540,17 +2540,6 @@ nsINode* nsContentUtils::GetCommonAncestorUnderInteractiveContent(
   return parent;
 }
 
-/* static */
-BrowserParent* nsContentUtils::GetCommonBrowserParentAncestor(
-    BrowserParent* aBrowserParent1, BrowserParent* aBrowserParent2) {
-  return GetCommonAncestorInternal(
-      aBrowserParent1, aBrowserParent2, [](BrowserParent* aBrowserParent) {
-        return aBrowserParent->GetBrowserBridgeParent()
-                   ? aBrowserParent->GetBrowserBridgeParent()->Manager()
-                   : nullptr;
-      });
-}
-
 /* static */
 template <typename FPT, typename FRT, typename SPT, typename SRT>
 Maybe<int32_t> nsContentUtils::ComparePoints(
@@ -7831,7 +7820,7 @@ nsresult nsContentUtils::SendMouseEvent(
   if (!widget) return NS_ERROR_FAILURE;
 
   EventMessage msg;
-  Maybe<WidgetMouseEvent::ExitFrom> exitFrom;
+  WidgetMouseEvent::ExitFrom exitFrom = WidgetMouseEvent::eChild;
   bool contextMenuKey = false;
   if (aType.EqualsLiteral("mousedown")) {
     msg = eMouseDown;
@@ -7843,11 +7832,9 @@ nsresult nsContentUtils::SendMouseEvent(
     msg = eMouseEnterIntoWidget;
   } else if (aType.EqualsLiteral("mouseout")) {
     msg = eMouseExitFromWidget;
-    exitFrom = Some(WidgetMouseEvent::eChild);
   } else if (aType.EqualsLiteral("mousecancel")) {
     msg = eMouseExitFromWidget;
-    exitFrom = Some(XRE_IsParentProcess() ? WidgetMouseEvent::eTopLevel
-                                          : WidgetMouseEvent::ePuppet);
+    exitFrom = WidgetMouseEvent::eTopLevel;
   } else if (aType.EqualsLiteral("mouselongtap")) {
     msg = eMouseLongTap;
   } else if (aType.EqualsLiteral("contextmenu")) {
diff --git a/dom/base/nsContentUtils.h b/dom/base/nsContentUtils.h
index 16c127704d0f..2448ca3eff1b 100644
--- a/dom/base/nsContentUtils.h
+++ b/dom/base/nsContentUtils.h
@@ -452,14 +452,6 @@ class nsContentUtils {
   static nsINode* GetCommonAncestorUnderInteractiveContent(nsINode* aNode1,
                                                            nsINode* aNode2);
 
-  /**
-   * Returns the common BrowserParent ancestor, if any, for two given
-   * BrowserParent.
-   */
-  static mozilla::dom::BrowserParent* GetCommonBrowserParentAncestor(
-      mozilla::dom::BrowserParent* aBrowserParent1,
-      mozilla::dom::BrowserParent* aBrowserParent2);
-
   /**
    * Returns true if aNode1 is before aNode2 in the same connected
    * tree.
diff --git a/dom/events/EventStateManager.cpp b/dom/events/EventStateManager.cpp
index 1fca3ee11243..8e3ec9258e47 100644
--- a/dom/events/EventStateManager.cpp
+++ b/dom/events/EventStateManager.cpp
@@ -646,11 +646,10 @@ nsresult EventStateManager::PreHandleEvent(nsPresContext* aPresContext,
       // above, at eMouseEnterIntoWidget case.
       aEvent->StopCrossProcessForwarding();
 
-      // If the event is not a top-level window or puppet widget exit, then it's
-      // not really an exit --- we may have traversed widget boundaries but
-      // we're still in our toplevel window or puppet widget.
-      if (mouseEvent->mExitFrom.value() != WidgetMouseEvent::eTopLevel &&
-          mouseEvent->mExitFrom.value() != WidgetMouseEvent::ePuppet) {
+      // If the event is not a top-level window exit, then it's not
+      // really an exit --- we may have traversed widget boundaries but
+      // we're still in our toplevel window.
+      if (mouseEvent->mExitFrom != WidgetMouseEvent::eTopLevel) {
         // Treat it as a synthetic move so we don't generate spurious
         // "exit" or "move" events.  Any necessary "out" or "over" events
         // will be generated by GenerateMouseEnterExit
@@ -658,14 +657,10 @@ nsresult EventStateManager::PreHandleEvent(nsPresContext* aPresContext,
         mouseEvent->mReason = WidgetMouseEvent::eSynthesized;
         // then fall through...
       } else {
-        MOZ_ASSERT_IF(XRE_IsParentProcess(), mouseEvent->mExitFrom.value() ==
-                                                 WidgetMouseEvent::eTopLevel);
-        MOZ_ASSERT_IF(XRE_IsContentProcess(), mouseEvent->mExitFrom.value() ==
-                                                  WidgetMouseEvent::ePuppet);
         // We should synthetize corresponding pointer events
         GeneratePointerEnterExit(ePointerLeave, mouseEvent);
         GenerateMouseEnterExit(mouseEvent);
-        // This is really an exit and should stop here
+        // This is a window level mouse exit event and should stop here
         aEvent->mMessage = eVoidEvent;
         break;
       }
@@ -1284,40 +1279,6 @@ bool EventStateManager::WalkESMTreeToHandleAccessKey(
   return false;
 }  // end of HandleAccessKey
 
-static BrowserParent* GetBrowserParentAncestor(BrowserParent* aBrowserParent) {
-  MOZ_ASSERT(aBrowserParent);
-
-  BrowserBridgeParent* bbp = aBrowserParent->GetBrowserBridgeParent();
-  if (!bbp) {
-    return nullptr;
-  }
-
-  return bbp->Manager();
-}
-
-static void DispatchCrossProcessMouseExitEvents(WidgetMouseEvent* aMouseEvent,
-                                                BrowserParent* aRemoteTarget,
-                                                BrowserParent* aStopAncestor,
-                                                bool aIsReallyExit) {
-  MOZ_ASSERT(aMouseEvent);
-  MOZ_ASSERT(aRemoteTarget);
-  MOZ_ASSERT(aRemoteTarget != aStopAncestor);
-  MOZ_ASSERT_IF(aStopAncestor, nsContentUtils::GetCommonBrowserParentAncestor(
-                                   aRemoteTarget, aStopAncestor));
-
-  while (aRemoteTarget != aStopAncestor) {
-    UniquePtr<WidgetMouseEvent> mouseExitEvent =
-        CreateMouseOrPointerWidgetEvent(aMouseEvent, eMouseExitFromWidget,
-                                        aMouseEvent->mRelatedTarget);
-    mouseExitEvent->mExitFrom =
-        Some(aIsReallyExit ? WidgetMouseEvent::ePuppet
-                           : WidgetMouseEvent::ePuppetParentToPuppetChild);
-    aRemoteTarget->SendRealMouseEvent(*mouseExitEvent);
-
-    aRemoteTarget = GetBrowserParentAncestor(aRemoteTarget);
-  }
-}
-
 void EventStateManager::DispatchCrossProcessEvent(WidgetEvent* aEvent,
                                                   BrowserParent* aRemoteTarget,
                                                   nsEventStatus* aStatus) {
@@ -1351,27 +1312,6 @@ void EventStateManager::DispatchCrossProcessEvent(WidgetEvent* aEvent,
 
   switch (aEvent->mClass) {
     case eMouseEventClass: {
-      BrowserParent* oldRemote = BrowserParent::GetLastMouseRemoteTarget();
-
-      // If this is a eMouseExitFromWidget event, need to redirect the event to
-      // the last remote and and notify all its ancestors about the exit, if
-      // any.
-      if (mouseEvent->mMessage == eMouseExitFromWidget) {
-        MOZ_ASSERT(mouseEvent->mExitFrom.value() == WidgetMouseEvent::ePuppet);
-        MOZ_ASSERT(mouseEvent->mReason == WidgetMouseEvent::eReal);
-        MOZ_ASSERT(!mouseEvent->mLayersId.IsValid());
-        MOZ_ASSERT(remote->GetBrowserHost());
-
-        if (oldRemote && oldRemote != remote) {
-          MOZ_ASSERT(nsContentUtils::GetCommonBrowserParentAncestor(
-                         remote, oldRemote) == remote);
-          remote = oldRemote;
-        }
-
-        DispatchCrossProcessMouseExitEvents(mouseEvent, remote, nullptr, true);
-        return;
-      }
-
       // If a mouse is over a remote target A, and then moves to
       // remote target B, we'd deliver the event directly to remote target B
       // after the moving, A would never get notified that the mouse left.
@@ -1380,37 +1320,14 @@ void EventStateManager::DispatchCrossProcessEvent(WidgetEvent* aEvent,
       // process directly (see
       // https://bugzilla.mozilla.org/show_bug.cgi?id=1549355), we probably
       // don't need to check mReason then.
+      BrowserParent* oldRemote = BrowserParent::GetLastMouseRemoteTarget();
       if (mouseEvent->mReason == WidgetMouseEvent::eReal &&
           remote != oldRemote) {
-        MOZ_ASSERT(mouseEvent->mMessage != eMouseExitFromWidget);
         if (oldRemote) {
-          BrowserParent* commonAncestor =
-              nsContentUtils::GetCommonBrowserParentAncestor(remote, oldRemote);
-          if (commonAncestor == oldRemote) {
-            // Mouse moves to the inner OOP frame, it is not a really exit.
-            DispatchCrossProcessMouseExitEvents(
-                mouseEvent, GetBrowserParentAncestor(remote),
-                GetBrowserParentAncestor(commonAncestor), false);
-          } else if (commonAncestor == remote) {
-            // Mouse moves to the outer OOP frame, it is a really exit.
-            DispatchCrossProcessMouseExitEvents(mouseEvent, oldRemote,
-                                                commonAncestor, true);
-          } else {
-            // Mouse moves to OOP frame in other subtree, it is a really exit,
-            // need to notify all its ancestors before common ancestor about the
-            // exit.
-            DispatchCrossProcessMouseExitEvents(mouseEvent, oldRemote,
-                                                commonAncestor, true);
-            if (commonAncestor) {
-              UniquePtr<WidgetMouseEvent> mouseExitEvent =
-                  CreateMouseOrPointerWidgetEvent(mouseEvent,
-                                                  eMouseExitFromWidget,
-                                                  mouseEvent->mRelatedTarget);
-              mouseExitEvent->mExitFrom =
-                  Some(WidgetMouseEvent::ePuppetParentToPuppetChild);
-              commonAncestor->SendRealMouseEvent(*mouseExitEvent);
-            }
-          }
+          UniquePtr<WidgetMouseEvent> mouseExitEvent =
+              CreateMouseOrPointerWidgetEvent(mouseEvent, eMouseExitFromWidget,
+                                              mouseEvent->mRelatedTarget);
+          oldRemote->SendRealMouseEvent(*mouseExitEvent);
         }
 
         if (mouseEvent->mMessage != eMouseExitFromWidget &&
@@ -4302,11 +4219,11 @@ nsIFrame* EventStateManager::DispatchMouseOrPointerEvent(
     // event to the remote frame.
     if (IsTopLevelRemoteTarget(targetContent)) {
       if (aMessage == eMouseOut) {
-        // For remote content, send a puppet widget mouse exit event.
+        // For remote content, send a "top-level" widget mouse exit event.
         UniquePtr<WidgetMouseEvent> remoteEvent =
             CreateMouseOrPointerWidgetEvent(aMouseEvent, eMouseExitFromWidget,
                                             relatedContent);
-        remoteEvent->mExitFrom = Some(WidgetMouseEvent::ePuppet);
+        remoteEvent->mExitFrom = WidgetMouseEvent::eTopLevel;
 
         // mCurrentTarget is set to the new target, so we must reset it to the
         // old target and then dispatch a cross-process event. (mCurrentTarget
diff --git a/dom/events/test/file_mouse_enterleave.html b/dom/events/test/file_mouse_enterleave.html
index 9e3d1f1d8b04..75fa7bda3ce3 100644
--- a/dom/events/test/file_mouse_enterleave.html
+++ b/dom/events/test/file_mouse_enterleave.html
@@ -15,7 +15,7 @@
 <div id="reflow"></div>
 <script>
 function listener(e) {
-  parent.postMessage({ eventType: e.type, targetName: e.target.localName }, "*");
+  parent.postMessage(e.type, "*");
 }
 
 window.addEventListener("message", function(aEvent) {
@@ -31,10 +31,6 @@ window.addEventListener("message", function(aEvent) {
 let target = document.getElementById("target");
 target.addEventListener("mouseenter", listener);
 target.addEventListener("mouseleave", listener);
-
-let root = document.documentElement;
-root.addEventListener("mouseenter", listener);
-root.addEventListener("mouseleave", listener);
 </script>
 </body>
 </html>
diff --git a/dom/events/test/mochitest.ini b/dom/events/test/mochitest.ini
index 51a22859b87b..60f5bbd29d1e 100644
--- a/dom/events/test/mochitest.ini
+++ b/dom/events/test/mochitest.ini
@@ -193,7 +193,6 @@ support-files =
 [test_mouse_enterleave_iframe.html]
 support-files =
   file_mouse_enterleave.html
-skip-if = verify # Bug 1659940
 [test_moz_mouse_pixel_scroll_event.html]
 [test_offsetxy.html]
 [test_onerror_handler_args.html]
diff --git a/dom/events/test/test_mouse_enterleave_iframe.html b/dom/events/test/test_mouse_enterleave_iframe.html
index ee4b4f6fc13e..d6196ed6763f 100644
--- a/dom/events/test/test_mouse_enterleave_iframe.html
+++ b/dom/events/test/test_mouse_enterleave_iframe.html
@@ -10,14 +10,13 @@
   height: 30px;
 }
 
-#target, #target2 {
-  width: 150px;
-  height: 150px;
+#target {
+  width: 300px;
+  height: 300px;
   background-color: #fcc;
-  display: inline-block;
 }
 
-#frame, #frame2 {
+#frame {
   height: 100%;
   width: 100%;
 }
@@ -33,9 +32,6 @@
 <div id="target">
   <iframe id="frame" frameborder="0" scrolling="no" src="http://example.com/tests/dom/events/test/file_mouse_enterleave.html"></iframe>
 </div>
-<div id="target2">
-  <iframe id="frame2" frameborder="0" scrolling="no" src="http://example.com/tests/dom/events/test/file_mouse_enterleave.html"></iframe>
-</div>
 <div id="reflow"></div>
 <script>
 
@@ -47,25 +43,17 @@ function reflow() {
   div.getBoundingClientRect();
 }
 
-function waitForMessage(aRemoteTarget, aEventType, aTargetName) {
+function waitForMessage(aRemoteTarget, aMessage) {
   return new Promise(function(aResolve, aReject) {
     window.addEventListener("message", function listener(aEvent) {
-      if (aEvent.source != aRemoteTarget.contentWindow) {
-        return;
-      }
-
-      if (aEvent.data.eventType !== aEventType) {
+      if (aEvent.data !== aMessage) {
         window.removeEventListener("message", listener);
-        ok(false, `receive unexpected message ${JSON.stringify(aEvent.data)}`);
-        aReject(new Error(`receive unexpected message ${JSON.stringify(aEvent.data)}`));
+        ok(false, `receive unexpected message ${aEvent.data}`);
+        aReject(new Error(`receive unexpected message ${aEvent.data}`));
         return;
       }
 
-      if (aEvent.data.targetName !== aTargetName) {
-        return;
-      }
-
-      ok(true, `receive message ${JSON.stringify(aEvent.data)}`);
+      ok(true, `receive message ${aEvent.data}`);
       // Trigger a reflow which will generate synthesized mouse move event.
       aRemoteTarget.contentWindow.postMessage("reflow", "*");
       // Now wait a bit to see if there is any unexpected message fired.
@@ -77,15 +65,6 @@ function waitForMessage(aRemoteTarget, aEventType, aTargetName) {
   });
 }
 
-function waitForLeaveEvent(aTarget) {
-  return new Promise(function(aResolve) {
-    aTarget.addEventListener("mouseleave", function(aEvent) {
-      ok(true, `receive ${aEvent.type}`);
-      aResolve();
-    }, { once: true });
-  });
-}
-
 function waitForEnterLeaveEvents(aEnterTarget, aLeaveTarget) {
   let expectedEvents = [{target: aEnterTarget, eventName: "mouseenter"}];
   if (aLeaveTarget) {
@@ -147,7 +126,6 @@ function moveMouseToInitialPosition() {
   return new Promise((aResolve) => {
     let start = document.getElementById("start");
     let startRect = start.getBoundingClientRect();
-    info("Mouse moves to initial position");
     synthesizeNativeMouseMove(start, startRect.width / 2, startRect.height / 2,
                               aResolve);
   });
@@ -175,15 +153,13 @@ add_task(async function testMouseEnterLeave() {
 
   info("Mouse moves into iframe");
   promise = Promise.all([waitForEnterLeaveEvents(target, div),
-                         waitForMessage(iframe, "mouseenter", "div"),
-                         waitForMessage(iframe, "mouseenter", "html")]);
+                         waitForMessage(iframe, "mouseenter")]);
   synthesizeNativeMouseMove(target, targetRect.width / 2, targetRect.height / 2);
   await promise;
 
   info("Mouse moves out from iframe to the div above iframe");
   promise = Promise.all([waitForEnterLeaveEvents(div, target),
-                         waitForMessage(iframe, "mouseleave", "div"),
-                         waitForMessage(iframe, "mouseleave", "html")]);
+                         waitForMessage(iframe, "mouseleave")]);
   synthesizeNativeMouseMove(div, divRect.width / 2, divRect.height / 2);
   await promise;
 
@@ -192,81 +168,4 @@ add_task(async function testMouseEnterLeave() {
   // multiple times. 
   await moveMouseToInitialPosition();
 });
-
-add_task(async function testMouseEnterLeaveBetweenIframe() {
-  let target = document.getElementById("target");
-  let targetRect = target.getBoundingClientRect();
-  let iframe = document.getElementById("frame");
-
-  info("Mouse moves into the first iframe");
-  let promise = Promise.all([waitForEnterLeaveEvents(target),
-                             waitForMessage(iframe, "mouseenter", "div"),
-                             waitForMessage(iframe, "mouseenter", "html")]);
-  synthesizeNativeMouseMove(target, targetRect.width / 2, targetRect.height / 2);
-  await promise;
-
-  let target2 = document.getElementById("target2");
-  let target2Rect = target2.getBoundingClientRect();
-  let iframe2 = document.getElementById("frame2");
-
-  info("Mouse moves out from the first iframe to the second iframe");
-  promise = Promise.all([waitForEnterLeaveEvents(target2, target),
-                         waitForMessage(iframe, "mouseleave", "div"),
-                         waitForMessage(iframe, "mouseleave", "html"),
-                         waitForMessage(iframe2, "mouseenter", "div"),
-                         waitForMessage(iframe2, "mouseenter", "html")]);
-  synthesizeNativeMouseMove(target2, target2Rect.width / 2, target2Rect.height / 2);
-  await promise;
-
-  info("Mouse moves out from the second iframe to the first iframe");
-  promise = Promise.all([waitForEnterLeaveEvents(target, target2),
-                         waitForMessage(iframe2, "mouseleave", "div"),
-                         waitForMessage(iframe2, "mouseleave", "html"),
-                         waitForMessage(iframe, "mouseenter", "div"),
-                         waitForMessage(iframe, "mouseenter", "html")]);
-  synthesizeNativeMouseMove(target, targetRect.width / 2, targetRect.height / 2);
-  await promise;
-
-  // Move mouse back to initial position.
-  await Promise.all([waitForLeaveEvent(target),
-                     waitForMessage(iframe, "mouseleave", "div"),
-                     waitForMessage(iframe, "mouseleave", "html"),
-                     moveMouseToInitialPosition()]);
-});
-
-add_task(async function testMouseEnterLeaveSwitchWindow() {
-  let target = document.getElementById("target");
-  let targetRect = target.getBoundingClientRect();
-  let iframe = document.getElementById("frame");
-
-  info("Mouse moves into iframe");
-  let promise = Promise.all([waitForEnterLeaveEvents(target),
-                             waitForMessage(iframe, "mouseenter", "div"),
-                             waitForMessage(iframe, "mouseenter", "html")]);
-  synthesizeNativeMouseMove(target, targetRect.width / 2, targetRect.height / 2);
-  await promise;
-
-  info("Open and switch to new window");
-  promise = Promise.all([waitForLeaveEvent(target),
-                         waitForMessage(iframe, "mouseleave", "div"),
-                         waitForMessage(iframe, "mouseleave", "html")]);
-  let win = window.open("http://example.com/tests/dom/events/test/file_mouse_enterleave.html");
-  // Trigger a reflow which will generate synthesized mouse move event.
-  win.postMessage("reflow", "*");
-  await promise;
-
-  info("Switch back to test window");
-  promise = Promise.all([waitForEnterLeaveEvents(target),
-                         waitForMessage(iframe, "mouseenter", "div"),
-                         waitForMessage(iframe, "mouseenter", "html")]);
-  win.close();
-  // Trigger a reflow which will generate synthesized mouse move event.
-  reflow();
-  synthesizeNativeMouseMove(target, targetRect.width / 2, targetRect.height / 2);
-  await promise;
-
-  // Move mouse back to initial position.
-  await Promise.all([waitForLeaveEvent(target),
-                     moveMouseToInitialPosition()]);
-});
 </script>
diff --git a/layout/base/PresShell.cpp b/layout/base/PresShell.cpp
index 7db70b11c563..db83c8cfb395 100644
--- a/layout/base/PresShell.cpp
+++ b/layout/base/PresShell.cpp
@@ -6982,9 +6982,7 @@ nsresult PresShell::EventHandler::HandleEventUsingCoordinates(
   WidgetMouseEvent* mouseEvent = aGUIEvent->AsMouseEvent();
   bool isWindowLevelMouseExit =
       (aGUIEvent->mMessage == eMouseExitFromWidget) &&
-      (mouseEvent &&
-       (mouseEvent->mExitFrom.value() == WidgetMouseEvent::eTopLevel ||
-        mouseEvent->mExitFrom.value() == WidgetMouseEvent::ePuppet));
+      (mouseEvent && mouseEvent->mExitFrom == WidgetMouseEvent::eTopLevel);
 
   // Get the frame at the event point. However, don't do this if we're
   // capturing and retargeting the event because the captured frame will
@@ -7606,8 +7604,7 @@ bool PresShell::EventHandler::MaybeDiscardOrDelayMouseEvent(
              (aGUIEvent->mMessage == eMouseUp ||
               // contextmenu is triggered after right mouseup on Windows and
               // right mousedown on other platforms.
-              aGUIEvent->mMessage == eContextMenu ||
-              aGUIEvent->mMessage == eMouseExitFromWidget)) {
+              aGUIEvent->mMessage == eContextMenu)) {
     UniquePtr<DelayedMouseEvent> delayedMouseEvent =
         MakeUnique<DelayedMouseEvent>(aGUIEvent->AsMouseEvent());
     PushDelayedEventIntoQueue(std::move(delayedMouseEvent));
diff --git a/widget/MouseEvents.h b/widget/MouseEvents.h
index 865526948875..8cc40e5b4583 100644
--- a/widget/MouseEvents.h
+++ b/widget/MouseEvents.h
@@ -190,18 +190,14 @@ class WidgetMouseEvent : public WidgetMouseEventBase,
     eControlClick
   };
 
-  typedef uint8_t ExitFromType;
-  enum ExitFrom : ExitFromType {
-    eChild,
-    eTopLevel,
-    ePuppet,
-    ePuppetParentToPuppetChild
-  };
+  typedef bool ExitFromType;
+  enum ExitFrom : ExitFromType { eChild, eTopLevel };
 
  protected:
   WidgetMouseEvent()
       : mReason(eReal),
         mContextMenuTrigger(eNormal),
+        mExitFrom(eChild),
         mIgnoreRootScrollFrame(false),
         mClickCount(0),
         mUseLegacyNonPrimaryDispatch(false) {}
@@ -211,6 +207,7 @@ class WidgetMouseEvent : public WidgetMouseEventBase,
       : WidgetMouseEventBase(aIsTrusted, aMessage, aWidget, aEventClassID),
         mReason(aReason),
         mContextMenuTrigger(eNormal),
+        mExitFrom(eChild),
         mIgnoreRootScrollFrame(false),
         mClickCount(0),
         mUseLegacyNonPrimaryDispatch(false) {}
@@ -224,6 +221,7 @@ class WidgetMouseEvent : public WidgetMouseEventBase,
       : WidgetMouseEventBase(aIsTrusted, aMessage, aWidget, eMouseEventClass),
         mReason(aReason),
         mContextMenuTrigger(aContextMenuTrigger),
+        mExitFrom(eChild),
         mIgnoreRootScrollFrame(false),
         mClickCount(0),
         mUseLegacyNonPrimaryDispatch(false) {
@@ -271,10 +269,10 @@ class WidgetMouseEvent : public WidgetMouseEventBase,
   // other reasons (typically, a click of right mouse button).
   ContextMenuTrigger mContextMenuTrigger;
 
-  // mExitFrom contains a value only when mMessage is eMouseExitFromWidget.
-  // This indicates if the mouse cursor exits from a top level platform widget,
-  // a child widget or a puppet widget.
-  Maybe<ExitFrom> mExitFrom;
+  // mExitFrom is valid only when mMessage is eMouseExitFromWidget.
+  // This indicates if the mouse cursor exits from a top level widget or
+  // a child widget.
+  ExitFrom mExitFrom;
 
   // Whether the event should ignore scroll frame bounds during dispatch.
   bool mIgnoreRootScrollFrame;
@@ -292,7 +290,6 @@ class WidgetMouseEvent : public WidgetMouseEventBase,
     AssignMouseEventBaseData(aEvent, aCopyTargets);
     AssignPointerHelperData(aEvent, /* aCopyCoalescedEvents */ true);
 
-    mExitFrom = aEvent.mExitFrom;
     mIgnoreRootScrollFrame = aEvent.mIgnoreRootScrollFrame;
     mClickCount = aEvent.mClickCount;
     mUseLegacyNonPrimaryDispatch = aEvent.mUseLegacyNonPrimaryDispatch;
diff --git a/widget/cocoa/nsChildView.mm b/widget/cocoa/nsChildView.mm
index 589e69f8b5a9..7f80f6aa7396 100644
--- a/widget/cocoa/nsChildView.mm
+++ b/widget/cocoa/nsChildView.mm
@@ -3159,9 +3159,9 @@ NSEvent* gLastDragMouseDownEvent = nil;  // [strong]
   EventMessage msg = aEnter ? eMouseEnterIntoWidget : eMouseExitFromWidget;
   WidgetMouseEvent event(true, msg, mGeckoChild, WidgetMouseEvent::eReal);
   event.mRefPoint = mGeckoChild->CocoaPointsToDevPixels(localEventLocation);
-  if (event.mMessage == eMouseExitFromWidget) {
-    event.mExitFrom = Some(aExitFrom);
-  }
+
+  event.mExitFrom = aExitFrom;
+
   nsEventStatus status;  // ignored
   mGeckoChild->DispatchEvent(&event, status);
 }
diff --git a/widget/gtk/nsWindow.cpp b/widget/gtk/nsWindow.cpp
index 9f9dadd9a661..0807ad0fa344 100644
--- a/widget/gtk/nsWindow.cpp
+++ b/widget/gtk/nsWindow.cpp
@@ -3199,9 +3199,9 @@ void nsWindow::OnLeaveNotifyEvent(GdkEventCrossing* aEvent) {
   event.mRefPoint = GdkEventCoordsToDevicePixels(aEvent->x, aEvent->y);
   event.AssignEventTime(GetWidgetEventTime(aEvent->time));
 
-  event.mExitFrom = Some(is_top_level_mouse_exit(mGdkWindow, aEvent)
-                             ? WidgetMouseEvent::eTopLevel
-                             : WidgetMouseEvent::eChild);
+  event.mExitFrom = is_top_level_mouse_exit(mGdkWindow, aEvent)
+                        ? WidgetMouseEvent::eTopLevel
+                        : WidgetMouseEvent::eChild;
 
   LOG(("OnLeaveNotify: %p\n", (void*)this));
 
diff --git a/widget/nsGUIEventIPC.h b/widget/nsGUIEventIPC.h
index a5c8e5c66232..75f12d691c02 100644
--- a/widget/nsGUIEventIPC.h
+++ b/widget/nsGUIEventIPC.h
@@ -244,11 +244,7 @@ struct ParamTraits<mozilla::WidgetMouseEvent> {
     WriteParam(aMsg, static_cast<paramType::ReasonType>(aParam.mReason));
     WriteParam(aMsg, static_cast<paramType::ContextMenuTriggerType>(
                          aParam.mContextMenuTrigger));
-    WriteParam(aMsg, aParam.mExitFrom.isSome());
-    if (aParam.mExitFrom.isSome()) {
-      WriteParam(
-          aMsg, static_cast<paramType::ExitFromType>(aParam.mExitFrom.value()));
-    }
+    WriteParam(aMsg, static_cast<paramType::ExitFromType>(aParam.mExitFrom));
     WriteParam(aMsg, aParam.mClickCount);
   }
 
@@ -257,24 +253,20 @@ struct ParamTraits<mozilla::WidgetMouseEvent> {
     bool rv;
     paramType::ReasonType reason = 0;
     paramType::ContextMenuTriggerType contextMenuTrigger = 0;
-    bool hasExitFrom = false;
+    paramType::ExitFromType exitFrom = 0;
     rv = ReadParam(aMsg, aIter,
                    static_cast<mozilla::WidgetMouseEventBase*>(aResult)) &&
          ReadParam(aMsg, aIter,
                    static_cast<mozilla::WidgetPointerHelper*>(aResult)) &&
          ReadParam(aMsg, aIter, &aResult->mIgnoreRootScrollFrame) &&
          ReadParam(aMsg, aIter, &reason) &&
-         ReadParam(aMsg, aIter, &contextMenuTrigger);
+         ReadParam(aMsg, aIter, &contextMenuTrigger) &&
+         ReadParam(aMsg, aIter, &exitFrom) &&
+         ReadParam(aMsg, aIter, &aResult->mClickCount);
     aResult->mReason = static_cast<paramType::Reason>(reason);
     aResult->mContextMenuTrigger =
         static_cast<paramType::ContextMenuTrigger>(contextMenuTrigger);
-    rv = rv && ReadParam(aMsg, aIter, &hasExitFrom);
-    if (hasExitFrom) {
-      paramType::ExitFromType exitFrom = 0;
-      rv = rv && ReadParam(aMsg, aIter, &exitFrom);
-      aResult->mExitFrom = Some(static_cast<paramType::ExitFrom>(exitFrom));
-    }
-    rv = rv && ReadParam(aMsg, aIter, &aResult->mClickCount);
+    aResult->mExitFrom = static_cast<paramType::ExitFrom>(exitFrom);
     return rv;
   }
 };
diff --git a/widget/windows/nsWindow.cpp b/widget/windows/nsWindow.cpp
index 37c1e18ea3da..78b9e2cf7237 100644
--- a/widget/windows/nsWindow.cpp
+++ b/widget/windows/nsWindow.cpp
@@ -4555,9 +4555,8 @@ bool nsWindow::DispatchMouseEvent(EventMessage aEventMessage, WPARAM wParam,
       }
       break;
     case eMouseExitFromWidget:
-      event.mExitFrom =
-          Some(IsTopLevelMouseExit(mWnd) ? WidgetMouseEvent::eTopLevel
-                                         : WidgetMouseEvent::eChild);
+      event.mExitFrom = IsTopLevelMouseExit(mWnd) ? WidgetMouseEvent::eTopLevel
+                                                  : WidgetMouseEvent::eChild;
       break;
     default:
       break;