Our application is a WPF (4.0) AppBar docked to the top of the desktop. Various buttons in the AppBar produce Popups containing various data (in ListBoxes). I am trying to implement drag and drop of items between 2 Popups. It's pretty standard boiler plate stuff using Josh Smith's draggable ListBox that I've used half a dozen times before. Drag is initiating correctly, but the target Popup is not receiving the DragOver or Drop. Is this a focus issue? Or is there something else at work here?