Sortable - Virtualized list

A thousand rows in Virtualize: only the rows in view (and a few around them) are rendered, each a drop target with Edges="BdndEdges.Vertical" around a draggable. Rows scrolled out unregister, rows scrolled in register - and the row being dragged stays registered until its drag ends. The rows drag with the pointer (PointerDrag="BdndPointerDrag.All"), so holding a row near the top or bottom of the list scrolls it, and the copy that follows the pointer keeps to the list's axis (PreviewAxis="BdndAxis.Vertical"); with the keyboard, each move scrolls the place it lands on into view.