title: Multiple WPF popups in app
body: <p>I'm developing a WPF app and I'm having an issue with popups.  I understand the issue that a popup's zindex is higher than everything else.  The problem I'm having is my app can have situations that there are more than one popups open at a time.  When this happens, the last popup that opens is always on top of the other.  I would really like the action to be, whichever popup gets clicked last is on top.  I have a thumb on the popup so it can be dragged and I have a behaviour to control the dragging.  I have noticed that I can call
  popup.isopen = false and then popup.isopen = true in the thumb_drag event and the clicked popup shows on top, but it produces an undesirable flash when the popup closes and opens.  I've dug through reflector and tried several thing (coecevalue on the IsOpenProperty, InvalidateVisual, etc.) and am having no luck.  Has anyone had this same problem and been able to fix it?</p>
  
  <p>Thanks</p>