check 380192 in again, b=388384

widget/src/cocoa/nsCocoaWindow.mm

@@ -780,8 +780,10 @@ NS_IMETHODIMP nsCocoaWindow::Resize(PRInt32 aWidth, PRInt32 aHeight, PRBool aRep
     StopResizing();
   }
 
-  mBounds.width  = aWidth;
-  mBounds.height = aHeight;
+  // report the actual size of the window because it can be restricted
+  NSRect finalWindowFrame = [mWindow contentRectForFrameRect:[mWindow frame]];
+  mBounds.width  = (nscoord)finalWindowFrame.size.width;
+  mBounds.height = (nscoord)finalWindowFrame.size.height;
 
   // tell gecko to update all the child widgets
   ReportSizeEvent();