Nesting - Tree

A folder tree built from nested drop targets. Each row has Edges="BdndEdges.Vertical": a node dropped on its top half goes before it, on its bottom half after it. Below each row, the list of its children is a drop target too: a node dropped there - the indented area, which grows while a node is dragged - becomes its last child. Only the innermost target under the pointer shows an edge, and a node can never be dropped into its own subtree. With the keyboard, the arrows visit every place: before, after and into every node.