title: How can I use NSVisualEffectView in window's title bar?
body: <p>In OS X Yosemite, Apple introduced a new class <code>NSVisualEffectView</code>. Currently, this class is undocumented, but we can use it in Interface Builder.</p>
  
  <p>How can I use <code>NSVisualEffectView</code> in the window's title bar?</p>
  
  <p>Here's example: in Safari, when you scroll, the content appears under the toolbar and titlebar with a vibrance and blur effect.</p>
  
  <p><img src="https://i.stack.imgur.com/pcnlH.png" alt="enter image description here"></p>