title: Calling setNeedsDisplay does not always triggers drawLayer:inContext:
body: <p>What can be the reason of that most of the time calling setNeedsDisplay triggers drawLayer:inContext: , but sometimes it doesn't?</p>
  
  <p>(CALayer's size is not zero; opacity is 1.0)</p>
  
  <p>Thanks for your help in advance!</p>