title: Hide NSWindow titlebar background like Messages.app
body: <p>I'm relative new to OS X development (coming from iOS).</p>
  
  <p>How can I archive a similar window like in the messages app for OS X?</p>
  
  <p>I think I must create a NSWindow subclass and override something? Any idea what?</p>
  
  <p><img src="https://i.stack.imgur.com/Q3pXu.png" alt="Messages.app"></p>
  
  <p>OS X Yosemite is the deployment target. No need to support older OS.</p>