title: How does VirtualizingStackPanel decide when to unload (dispose?) of virtualized controls?
body: <p>I'm working with an ItemsControl using a VirtualizingStackPanel in its ControlTemplate.  I've got the virtualization working, to some extent.  I've put debugging statements in the Loaded and Unloaded event handlers for the controls in my DataTemplate for the items, but they don't get unloaded immediately after the controls go out of view.  Is there any way to force this behaviour ?</p>