--- a/widget/windows/nsWindow.cpp
+++ b/widget/windows/nsWindow.cpp
@@ -265,16 +265,19 @@ LONG            nsWindow::sLastMouseDown
 LONG            nsWindow::sLastClickCount         = 0L;
 BYTE            nsWindow::sLastMouseButton        = 0;
 
 // Trim heap on minimize. (initialized, but still true.)
 int             nsWindow::sTrimOnMinimize         = 2;
 
 TriStateBool nsWindow::sHasBogusPopupsDropShadowOnMultiMonitor = TRI_UNKNOWN;
 
+WPARAM nsWindow::sMouseExitwParam = 0;
+LPARAM nsWindow::sMouseExitlParamScreen = 0;
+
 static SystemTimeConverter<DWORD>&
 TimeConverter() {
   static SystemTimeConverter<DWORD> timeConverterSingleton;
   return timeConverterSingleton;
 }
 
 namespace mozilla {
 
@@ -3266,16 +3269,27 @@ nsWindow::MakeFullScreen(bool aFullScree
     taskbarInfo->PrepareFullScreenHWND(mWnd, FALSE);
   }
 
   if (mWidgetListener) {
     mWidgetListener->SizeModeChanged(mSizeMode);
     mWidgetListener->FullscreenChanged(aFullScreen);
   }
 
+  // Send a eMouseEnterIntoWidget event since Windows has already sent
+  // a WM_MOUSELEAVE that caused us to send a eMouseExitFromWidget event.
+  if (aFullScreen && !sCurrentWindow) {
+    sCurrentWindow = this;
+    LPARAM pos = sCurrentWindow->lParamToClient(sMouseExitlParamScreen);
+    sCurrentWindow->DispatchMouseEvent(eMouseEnterIntoWidget,
+                                       sMouseExitwParam, pos, false,
+                                       WidgetMouseEvent::eLeftButton,
+                                       MOUSE_INPUT_SOURCE(), MOUSE_POINTERID());
+  }
+
   return NS_OK;
 }
 
 /**************************************************************
  *
  * SECTION: Native data storage
  *
  * nsIWidget::GetNativeData
@@ -4243,16 +4257,18 @@ nsWindow::DispatchMouseEvent(EventMessag
             sCurrentWindow->DispatchMouseEvent(eMouseEnterIntoWidget,
                                                wParam, pos, false,
                                                WidgetMouseEvent::eLeftButton,
                                                aInputSource, aPointerId);
           }
         }
       }
     } else if (aEventMessage == eMouseExitFromWidget) {
+      sMouseExitwParam = wParam;
+      sMouseExitlParamScreen = lParamToScreen(lParam);
       if (sCurrentWindow == this) {
         sCurrentWindow = nullptr;
       }
     }
 
     result = ConvertStatus(DispatchInputEvent(&event));
 
     // Release the widget with NS_IF_RELEASE() just in case

--- a/widget/windows/nsWindow.h
+++ b/widget/windows/nsWindow.h
@@ -642,16 +642,23 @@ protected:
   int32_t mCachedHitTestResult;
 
   RefPtr<mozilla::widget::WinCompositorWidget> mBasicLayersSurface;
 
   static bool sNeedsToInitMouseWheelSettings;
   static void InitMouseWheelScrollData();
 
   double mSizeConstraintsScale; // scale in effect when setting constraints
+
+  // Used to remember the wParam (i.e. currently pressed modifier keys)
+  // and lParam (i.e. last mouse position) in screen coordinates from
+  // the previous mouse-exit.  Static since it is not
+  // associated with a particular widget (since we exited the widget).
+  static WPARAM sMouseExitwParam;
+  static LPARAM sMouseExitlParamScreen;
 };
 
 /**
  * A child window is a window with different style.
  */
 class ChildWindow : public nsWindow {
 
 public: