title: How can I prevent UI raycast from going through another Canvas?
body: <p>I have a two canvas in a Scene and all <code>Canvas</code> are set to Screen Space mode camera render mode. The first <code>Canvas</code> is the main <code>Canvas</code> and has some buttons.The 'order in layer' property is 0.</p>
  
  <p>The 2nd <code>Canvas</code> covers the whole area of the 1st <code>Canvas</code> . When you click some button on the first <code>Canvas</code>,the second <code>Canvas</code> will popup over the first <code>Canvas</code> (order in layer is 1). </p>
  
  <p>The problem is that when I touch over 2nd canvas, the raycast also hit the 1st <code>Canvas</code>. How can I prevent the raycast from hitting the first <code>Canvas</code> while the 2nd <code>Canvas</code> (popup) is displayed on the screen?</p>