diff --git a/widget/src/cocoa/nsChildView.h b/widget/src/cocoa/nsChildView.h
index f91e38b8850d..11a1449bf52a 100644
--- a/widget/src/cocoa/nsChildView.h
+++ b/widget/src/cocoa/nsChildView.h
@@ -364,9 +364,11 @@ public:
   static void MouseExitedWindow(NSEvent* aEvent);
   static void MouseEnteredWindow(NSEvent* aEvent);
   static void ReEvaluateMouseEnterState(NSEvent* aEvent = nil);
+  static void ResendLastMouseMoveEvent();
   static ChildView* ViewForEvent(NSEvent* aEvent);
 
   static ChildView* sLastMouseEventView;
+  static NSEvent* sLastMouseMoveEvent;
   static NSWindow* sWindowUnderMouse;
 };
 
diff --git a/widget/src/cocoa/nsChildView.mm b/widget/src/cocoa/nsChildView.mm
index e4ca94991207..8b50f1236933 100644
--- a/widget/src/cocoa/nsChildView.mm
+++ b/widget/src/cocoa/nsChildView.mm
@@ -126,6 +126,7 @@ PRBool gChildViewMethodsSwizzled = PR_FALSE;
 extern nsISupportsArray *gDraggedTransferables;
 
 ChildView* ChildViewMouseTracker::sLastMouseEventView = nil;
+NSEvent* ChildViewMouseTracker::sLastMouseMoveEvent = nil;
 NSWindow* ChildViewMouseTracker::sWindowUnderMouse = nil;
 
 #ifdef INVALIDATE_DEBUGGING
@@ -4900,6 +4901,8 @@ ChildViewMouseTracker::OnDestroyView(ChildView* aView)
 {
   if (sLastMouseEventView == aView) {
     sLastMouseEventView = nil;
+    [sLastMouseMoveEvent release];
+    sLastMouseMoveEvent = nil;
   }
 }
 
@@ -4945,11 +4948,23 @@ ChildViewMouseTracker::ReEvaluateMouseEnterState(NSEvent* aEvent)
   }
 }
 
+void
+ChildViewMouseTracker::ResendLastMouseMoveEvent()
+{
+  if (sLastMouseMoveEvent) {
+    MouseMoved(sLastMouseMoveEvent);
+  }
+}
+
 void
 ChildViewMouseTracker::MouseMoved(NSEvent* aEvent)
 {
   MouseEnteredWindow(aEvent);
   [sLastMouseEventView handleMouseMoved:aEvent];
+  if (sLastMouseMoveEvent != aEvent) {
+    [sLastMouseMoveEvent release];
+    sLastMouseMoveEvent = [aEvent retain];
+  }
 }
 
 ChildView*
diff --git a/widget/src/cocoa/nsCocoaWindow.mm b/widget/src/cocoa/nsCocoaWindow.mm
index 1b1944846522..11f87c17ccc0 100644
--- a/widget/src/cocoa/nsCocoaWindow.mm
+++ b/widget/src/cocoa/nsCocoaWindow.mm
@@ -2301,6 +2301,14 @@ static const NSString* kStateShowsToolbarButton = @"showsToolbarButton";
       // Re-layout our contents.
       geckoWindow->ReportSizeEvent();
     }
+
+    // Resizing the content area causes a reflow which would send a synthesized
+    // mousemove event to the old mouse position relative to the top left
+    // corner of the content area. But the mouse has shifted relative to the
+    // content area, so that event would have wrong position information. So
+    // we'll send a mouse move event with the correct new position.
+    ChildViewMouseTracker::ResendLastMouseMoveEvent();
+
     [self setTitlebarNeedsDisplayInRect:[self titlebarRect]];
   }
 }