Where the gesture was grabbed, so a delta starts at the press rather than the first move.
Current pointer position: container media x, pane-local media y.
Every position the pointer passed through since the previous drag,
oldest first, the last one at point. Browsers deliver moves at frame
rate and fold the positions between frames into the event; a freehand
stroke drawn from point alone is a polyline of frame-rate corners.
Payload of the
dragevent: a draggable primitive being moved.