performPaint method
RenderLayoutBox real paint things after basiclly paint box model. Override which to paint layout or intrinsic things. Used by RenderReplaced, RenderFlowLayout, RenderFlexLayout.
Implementation
@override
void performPaint(PaintingContext context, Offset offset) {
  offset += Offset(renderStyle.paddingLeft.computedValue, renderStyle.paddingTop.computedValue);
  offset +=
      Offset(renderStyle.effectiveBorderLeftWidth.computedValue, renderStyle.effectiveBorderTopWidth.computedValue);
  if (child != null) {
    context.paintChild(child!, offset);
  }
}