title: virtualizingstackpanel and height
body: <p>If I have a virtualizingstackpanel inside of my scrollviewer then how does my scrollviewer know what height the scrollbar needs to be?</p>
  
  <p>For example if I have 10,000 items how does it know that I have only scrolled 5 percent down the list?</p>
  
  <p>What if I do not know the size of the items?</p>