--- a/widget/windows/nsWindow.cpp
+++ b/widget/windows/nsWindow.cpp
@@ -2668,16 +2668,33 @@ void nsWindow::UpdateOpaqueRegion(const 
 
   // Only update glass area if there are changes
   if (memcmp(&mGlassMargins, &margins, sizeof mGlassMargins)) {
     mGlassMargins = margins;
     UpdateGlass();
   }
 }
 
+/**************************************************************
+*
+* SECTION: nsIWidget::UpdateWindowDraggingRegion
+*
+* For setting the draggable titlebar region from CSS
+* with -moz-window-dragging: drag.
+*
+**************************************************************/
+
+void
+nsWindow::UpdateWindowDraggingRegion(const nsIntRegion& aRegion)
+{
+  if (mDraggableRegion != aRegion) {
+    mDraggableRegion = aRegion;
+  }
+}
+
 void nsWindow::UpdateGlass()
 {
   MARGINS margins = mGlassMargins;
 
   // DWMNCRP_USEWINDOWSTYLE - The non-client rendering area is
   //                          rendered based on the window style.
   // DWMNCRP_ENABLED        - The non-client area rendering is
   //                          enabled; the window style is ignored.
@@ -5577,38 +5594,39 @@ nsWindow::ClientMarginHitTestPoint(int32
       testResult = HTBORDER;
   }
 
   if (!sIsInMouseCapture && allowContentOverride) {
     POINT pt = { mx, my };
     ::ScreenToClient(mWnd, &pt);
     if (pt.x == mCachedHitTestPoint.x && pt.y == mCachedHitTestPoint.y &&
         TimeStamp::Now() - mCachedHitTestTime < TimeDuration::FromMilliseconds(HITTEST_CACHE_LIFETIME_MS)) {
-      testResult = mCachedHitTestResult;
+      return mCachedHitTestResult;
+    } else if (mDraggableRegion.Contains(pt.x, pt.y)) {
+      testResult = HTCAPTION;
     } else {
       WidgetMouseEvent event(true, NS_MOUSE_MOZHITTEST, this,
                              WidgetMouseEvent::eReal,
                              WidgetMouseEvent::eNormal);
       event.refPoint = LayoutDeviceIntPoint(pt.x, pt.y);
       event.inputSource = MOUSE_INPUT_SOURCE();
       event.mFlags.mOnlyChromeDispatch = true;
       bool result = ConvertStatus(DispatchInputEvent(&event));
       if (result) {
         // The mouse is over a blank area
         testResult = testResult == HTCLIENT ? HTCAPTION : testResult;
-
       } else {
         // There's content over the mouse pointer. Set HTCLIENT
         // to possibly override a resizer border.
         testResult = HTCLIENT;
       }
-      mCachedHitTestPoint = pt;
-      mCachedHitTestTime = TimeStamp::Now();
-      mCachedHitTestResult = testResult;
-    }
+    }
+    mCachedHitTestPoint = pt;
+    mCachedHitTestTime = TimeStamp::Now();
+    mCachedHitTestResult = testResult;
   }
 
   return testResult;
 }
 
 TimeStamp
 nsWindow::GetMessageTimeStamp(LONG aEventTime)
 {

--- a/widget/windows/nsWindow.h
+++ b/widget/windows/nsWindow.h
@@ -189,16 +189,17 @@ public:
   virtual void            UpdateOpaqueRegion(const nsIntRegion& aOpaqueRegion);
 #endif // MOZ_XUL
   virtual nsIMEUpdatePreference GetIMEUpdatePreference();
   NS_IMETHOD              GetNonClientMargins(nsIntMargin &margins);
   NS_IMETHOD              SetNonClientMargins(nsIntMargin &margins);
   void                    SetDrawsInTitlebar(bool aState);
   mozilla::TemporaryRef<mozilla::gfx::DrawTarget> StartRemoteDrawing() override;
   virtual void            EndRemoteDrawing() override;
+  virtual void UpdateWindowDraggingRegion(const nsIntRegion& aRegion) override;
 
   virtual void            UpdateThemeGeometries(const nsTArray<ThemeGeometry>& aThemeGeometries) override;
   virtual uint32_t        GetMaxTouchPoints() const override;
 
   /**
    * Event helpers
    */
   virtual bool            DispatchMouseEvent(uint32_t aEventType, WPARAM wParam,
@@ -514,16 +515,19 @@ protected:
   // Cached copy of L&F's resize border  
   int32_t               mHorResizeMargin;
   int32_t               mVertResizeMargin;
   // Height of the caption plus border
   int32_t               mCaptionHeight;
 
   nsCOMPtr<nsIIdleServiceInternal> mIdleService;
 
+  // Draggable titlebar region maintained by UpdateWindowDraggingRegion
+  nsIntRegion mDraggableRegion;
+
   // Hook Data Memebers for Dropdowns. sProcessHook Tells the
   // hook methods whether they should be processing the hook
   // messages.
   static HHOOK          sMsgFilterHook;
   static HHOOK          sCallProcHook;
   static HHOOK          sCallMouseHook;
   static bool           sProcessHook;
   static UINT           sRollupMsgId;