Sortable - Grid

Tiles in a wrapping grid, each in a drop target with Edges="BdndEdges.Horizontal": a grid is a horizontal list that wraps, so a tile dragged over another lands on the side the pointer is closest to, whatever row it is in. Within one list the drop is a reorder: BdndList.ReorderIndex works out where the tile ends up - one place less when it came from before the tile it was dropped on, since taking it out shifts the rest - and BdndList.Reorder moves it there. With the keyboard, Left and Right step through the gaps between the tiles, and Up and Down go a row up or down - to the gap closest across, so a tile moved down keeps its column - rather than one tile on. (BdndEdges.All would report the nearest of all four edges, for tiles where above and below mean something of their own.)