move method
Move the given child
in the child list to be after another child.
More efficient than removing and re-adding the child. Requires the child
to already be in the child list at some position. Pass null for after
to
move the child to the start of the child list.
Implementation
@override
void move(RenderBox child, {RenderBox? after}) {
super.move(child, after: after);
_paintingOrder = null;
}