- the whole thing should sit in a Canvas
- create a UserWidget anchor (a border will do), ensure it’s transparent but Visible
- place 2 anchors for each missing piece
- when the drag OP is taking place, keep track of how many and which anchors drag enters & leaves
- if you detect 2 simultaneously, we’re hovering over the correct spot
- the anchors can be made smaller / larger to control difficulty / required accuracy
- once we have 2 anchors dragged over & drag is cancelled (or perhaps just detecting 2 anchors is enough), snap the dragged widget to the centre of the anchor in the upper left corner