diff --git a/gfx/src/nsRegion.h b/gfx/src/nsRegion.h
index 92a005536e7e..023aa763a1f7 100644
--- a/gfx/src/nsRegion.h
+++ b/gfx/src/nsRegion.h
@@ -68,6 +68,10 @@ public:
   {
     return IsEqual(aRgn);
   }
+  bool operator!=(const nsRegion& aRgn) const
+  {
+    return !(*this == aRgn);
+  }
 
   friend std::ostream& operator<<(std::ostream& stream, const nsRegion& m);
 
@@ -464,6 +468,10 @@ public:
   {
     return IsEqual(aRgn);
   }
+  bool operator!=(const nsIntRegion& aRgn) const
+  {
+    return !(*this == aRgn);
+  }
 
   friend std::ostream& operator<<(std::ostream& stream, const nsIntRegion& m) {
     return stream << m.mImpl;
diff --git a/widget/cocoa/nsChildView.h b/widget/cocoa/nsChildView.h
index 721f3cc4665b..bf57575f6968 100644
--- a/widget/cocoa/nsChildView.h
+++ b/widget/cocoa/nsChildView.h
@@ -310,8 +310,6 @@ typedef NSInteger NSEventGestureAxis;
 
 - (void)handleMouseMoved:(NSEvent*)aEvent;
 
-- (void)updateWindowDraggableStateOnMouseMove:(NSEvent*)theEvent;
-
 - (void)sendMouseEnterOrExitEvent:(NSEvent*)aEvent
                             enter:(BOOL)aEnter
                              type:(mozilla::WidgetMouseEvent::exitType)aType;
@@ -548,6 +546,9 @@ public:
 
   virtual void UpdateThemeGeometries(const nsTArray<ThemeGeometry>& aThemeGeometries);
 
+  virtual void UpdateWindowDraggingRegion(const nsIntRegion& aRegion) MOZ_OVERRIDE;
+  const nsIntRegion& GetDraggableRegion() { return mDraggableRegion; }
+
   void              HidePlugin();
   void              UpdatePluginPort();
 
@@ -679,6 +680,8 @@ protected:
   // uploaded to to mTitlebarImage. Main thread only.
   nsIntRegion           mDirtyTitlebarRegion;
 
+  nsIntRegion           mDraggableRegion;
+
   // Cached value of [mView backingScaleFactor], to avoid sending two obj-c
   // messages (respondsToSelector, backingScaleFactor) every time we need to
   // use it.
diff --git a/widget/cocoa/nsChildView.mm b/widget/cocoa/nsChildView.mm
index 1bd8777a3c44..4661ae19daca 100644
--- a/widget/cocoa/nsChildView.mm
+++ b/widget/cocoa/nsChildView.mm
@@ -121,6 +121,7 @@ extern "C" {
 
   typedef CFTypeRef CGSRegionObj;
   CGError CGSNewRegionWithRect(const CGRect *rect, CGSRegionObj *outRegion);
+  CGError CGSNewRegionWithRectList(const CGRect *rects, int rectCount, CGSRegionObj *outRegion);
 }
 
 // defined in nsMenuBarX.mm
@@ -203,6 +204,7 @@ static uint32_t gNumberOfWidgetsNeedingEventThread = 0;
 - (APZCTreeManager*)apzctm;
 
 - (BOOL)inactiveWindowAcceptsMouseEvent:(NSEvent*)aEvent;
+- (void)updateWindowDraggableState;
 
 @end
 
@@ -221,6 +223,10 @@ static uint32_t gNumberOfWidgetsNeedingEventThread = 0;
 - (float)roundedCornerRadius;
 @end
 
+@interface NSView(DraggableRegion)
+- (CGSRegionObj)_regionForOpaqueDescendants:(NSRect)aRect forMove:(BOOL)aForMove;
+@end
+
 // Starting with 10.7 the bottom corners of all windows are rounded.
 // Unfortunately, the standard rounding that OS X applies to OpenGL views
 // does not use anti-aliasing and looks very crude. Since we want a smooth,
@@ -2387,15 +2393,15 @@ nsChildView::UpdateTitlebarCGContext()
 
   CGContextSaveGState(ctx);
 
-  std::vector<CGRect> rects;
+  nsTArray<CGRect> rects;
   nsIntRegionRectIterator iter(dirtyTitlebarRegion);
   for (;;) {
     const nsIntRect* r = iter.Next();
     if (!r)
       break;
-    rects.push_back(CGRectMake(r->x, r->y, r->width, r->height));
+    rects.AppendElement(CGRectMake(r->x, r->y, r->width, r->height));
   }
-  CGContextClipToRects(ctx, rects.data(), rects.size());
+  CGContextClipToRects(ctx, rects.Elements(), rects.Length());
 
   CGContextClearRect(ctx, CGRectMake(0, 0, texSize.width, texSize.height));
 
@@ -2748,6 +2754,15 @@ nsChildView::DoRemoteComposition(const nsIntRect& aRenderRect)
   [(ChildView*)mView postRender:mGLPresenter->GetNSOpenGLContext()];
 }
 
+void
+nsChildView::UpdateWindowDraggingRegion(const nsIntRegion& aRegion)
+{
+  if (mDraggableRegion != aRegion) {
+    mDraggableRegion = aRegion;
+    [(ChildView*)mView updateWindowDraggableState];
+  }
+}
+
 #ifdef ACCESSIBILITY
 already_AddRefed<a11y::Accessible>
 nsChildView::GetDocumentAccessible()
@@ -3602,7 +3617,9 @@ NSEvent* gLastDragMouseDownEvent = nil;
 
 - (BOOL)mouseDownCanMoveWindow
 {
-  return [[self window] isMovableByWindowBackground];
+  // Return YES so that _regionForOpaqueDescendants gets called, where the
+  // actual draggable region will be assembled.
+  return YES;
 }
 
 -(void)updateGLContext
@@ -4885,24 +4902,60 @@ NSEvent* gLastDragMouseDownEvent = nil;
   mGeckoChild->DispatchEvent(&event, status);
 }
 
-- (void)updateWindowDraggableStateOnMouseMove:(NSEvent*)theEvent
+- (void)updateWindowDraggableState
 {
-  if (!theEvent || !mGeckoChild) {
-    return;
+  // Trigger update to the window server.
+  [[self window] setMovableByWindowBackground:NO];
+  [[self window] setMovableByWindowBackground:YES];
+}
+
+// aRect is in view coordinates relative to this NSView.
+- (CGRect)convertToFlippedWindowCoordinates:(NSRect)aRect
+{
+  // First, convert the rect to regular window coordinates...
+  NSRect inWindowCoords = [self convertRect:aRect toView:nil];
+  // ... and then flip it again because window coordinates have their origin
+  // in the bottom left corner, and we need it to be in the top left corner.
+  inWindowCoords.origin.y = [[self window] frame].size.height - NSMaxY(inWindowCoords);
+  return NSRectToCGRect(inWindowCoords);
+}
+
+static CGSRegionObj
+NewCGSRegionFromRegion(const nsIntRegion& aRegion,
+                       CGRect (^aRectConverter)(const nsIntRect&))
+{
+  nsTArray<CGRect> rects;
+  nsIntRegionRectIterator iter(aRegion);
+  for (;;) {
+    const nsIntRect* r = iter.Next();
+    if (!r)
+      break;
+    rects.AppendElement(aRectConverter(*r));
   }
 
-  nsCocoaWindow* windowWidget = mGeckoChild->GetXULWindowWidget();
-  if (!windowWidget) {
-    return;
+  CGSRegionObj region;
+  CGSNewRegionWithRectList(rects.Elements(), rects.Length(), &region);
+  return region;
+}
+
+// This function is called with forMove:YES to calculate the draggable region
+// of the window which will be submitted to the window server. Window dragging
+// is handled on the window server without calling back into our process, so it
+// also works while our app is unresponsive.
+- (CGSRegionObj)_regionForOpaqueDescendants:(NSRect)aRect forMove:(BOOL)aForMove
+{
+  if (!aForMove || !mGeckoChild) {
+    return [super _regionForOpaqueDescendants:aRect forMove:aForMove];
   }
 
-  // We assume later on that sending a hit test event won't cause widget destruction.
-  WidgetMouseEvent hitTestEvent(true, NS_MOUSE_MOZHITTEST, mGeckoChild,
-                                WidgetMouseEvent::eReal);
-  [self convertCocoaMouseEvent:theEvent toGeckoEvent:&hitTestEvent];
-  bool result = mGeckoChild->DispatchWindowEvent(hitTestEvent);
+  nsIntRect boundingRect = mGeckoChild->CocoaPointsToDevPixels(aRect);
 
-  [windowWidget->GetCocoaWindow() setMovableByWindowBackground:result];
+  nsIntRegion opaqueRegion;
+  opaqueRegion.Sub(boundingRect, mGeckoChild->GetDraggableRegion());
+
+  return NewCGSRegionFromRegion(opaqueRegion, ^(const nsIntRect& r) {
+    return [self convertToFlippedWindowCoordinates:mGeckoChild->DevPixelsToCocoaPoints(r)];
+  });
 }
 
 - (void)handleMouseMoved:(NSEvent*)theEvent
diff --git a/widget/cocoa/nsCocoaWindow.mm b/widget/cocoa/nsCocoaWindow.mm
index a7931c94edb9..435da5c1bf69 100644
--- a/widget/cocoa/nsCocoaWindow.mm
+++ b/widget/cocoa/nsCocoaWindow.mm
@@ -3269,9 +3269,6 @@ static const NSString* kStateShowsToolbarButton = @"showsToolbarButton";
       if (delegate && [delegate isKindOfClass:[WindowDelegate class]]) {
         nsCocoaWindow *widget = [(WindowDelegate *)delegate geckoWidget];
         if (widget) {
-          if (type == NSMouseMoved) {
-            [[self mainChildView] updateWindowDraggableStateOnMouseMove:anEvent];
-          }
           if (gGeckoAppModalWindowList && (widget != gGeckoAppModalWindowList->window))
             return;
           if (widget->HasModalDescendents())