The widget to display instead of child when one or more drags are under way.
If this is null, then this widget will always display child (and so the drag source representation will not change while a drag is under way).
The feedback widget is shown under the pointer when a drag is under way.
To limit the number of simultaneous drags on multitouch devices, see maxSimultaneousDrags.
final Widget? childWhenDragging;