title: Returning nil from addLocalMonitorForEventsMatchingMask: block doesn't suppress events
body: <p>The following code doesn't suppress mouse drags in my app:</p>
  
  <pre><code>[NSEvent addLocalMonitorForEventsMatchingMask:NSLeftMouseDraggedMask handler:^(NSEvent *event) {
     return (NSEvent *)nil;
  }];
  </code></pre>
  
  <p>I'm still able to drag UI elements inside the app. As far as I know, if you return <code>nil</code> inside the block, the event won't be dispatched to the app. What am I missing?</p>