title: Clipping rounded corners on a NSScrollView
body: <p>I have a simple custom borderless NSWindow subclass which has a rounded rectangle shape.</p>
  
  <p>In the content view of this window, I've added an NSScrollView.</p>
  
  <p>How do I get the NSScrollView to clip its document view to the rounded rectangle shape of the NSWindow?</p>
  
  <p>I've tried subclassing the NSScrollView, overriding drawRect: and adding a clipping path before calling super.  I've also tried subclassing the document view and the clip view with the same technique but I cannot get it to clip.</p>
  
  <p>BTW, this is on Lion with the elastic scrolling behaviour.</p>