1. override
void paintStack(PaintingContext context, Offset offset)

Override in subclasses to customize how the stack paints.

By default, the stack uses defaultPaint. This function is called by paint after potentially applying a clip to contain visual overflow.

Source 

@override
void paintStack(PaintingContext context, Offset offset) {
  if (firstChild == null || index == null)
    return;
  RenderBox child = _childAtIndex();
  final StackParentData childParentData = child.parentData;
  context.paintChild(child, childParentData.offset + offset);
}