--- a/widget/cocoa/nsChildView.mm
+++ b/widget/cocoa/nsChildView.mm
@@ -220,16 +220,17 @@ static uint32_t gNumberOfWidgetsNeedingE
 @end
 
 @interface NSView(NSThemeFrameCornerRadius)
 - (float)roundedCornerRadius;
 @end
 
 @interface NSView(DraggableRegion)
 - (CGSRegionObj)_regionForOpaqueDescendants:(NSRect)aRect forMove:(BOOL)aForMove;
+- (CGSRegionObj)_regionForOpaqueDescendants:(NSRect)aRect forMove:(BOOL)aForMove forUnderTitlebar:(BOOL)aForUnderTitlebar;
 @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,
 // anti-aliased curve, we'll draw it ourselves.
 // Additionally, we need to turn off the OS-supplied rounding because it
 // eats into our corner's curve. We do that by overriding an NSSurface method.
@@ -4953,16 +4954,33 @@ NewCGSRegionFromRegion(const nsIntRegion
   nsIntRegion opaqueRegion;
   opaqueRegion.Sub(boundingRect, mGeckoChild->GetDraggableRegion());
 
   return NewCGSRegionFromRegion(opaqueRegion, ^(const nsIntRect& r) {
     return [self convertToFlippedWindowCoordinates:mGeckoChild->DevPixelsToCocoaPoints(r)];
   });
 }
 
+// Starting with 10.10, in addition to the traditional
+// -[NSView _regionForOpaqueDescendants:forMove:] method, there's a new form with
+// an additional forUnderTitlebar argument, which is sometimes called instead of
+// the old form. We need to override the new variant as well.
+- (CGSRegionObj)_regionForOpaqueDescendants:(NSRect)aRect
+                                    forMove:(BOOL)aForMove
+                           forUnderTitlebar:(BOOL)aForUnderTitlebar
+{
+  if (!aForMove || !mGeckoChild) {
+    return [super _regionForOpaqueDescendants:aRect
+                                      forMove:aForMove
+                             forUnderTitlebar:aForUnderTitlebar];
+  }
+
+  return [self _regionForOpaqueDescendants:aRect forMove:aForMove];
+}
+
 - (void)handleMouseMoved:(NSEvent*)theEvent
 {
   NS_OBJC_BEGIN_TRY_ABORT_BLOCK;
 
   if (!mGeckoChild)
     return;
 
   WidgetMouseEvent geckoEvent(true, NS_MOUSE_MOVE, mGeckoChild,