debugPaintSize method
In debug mode, paints a border around this render box.
Called for every RenderBox when debugPaintSizeEnabled is true.
Implementation
@protected
@visibleForTesting
void debugPaintSize(PaintingContext context, Offset offset) {
  assert(() {
    final Paint paint = Paint()
      ..style = PaintingStyle.stroke
      ..strokeWidth = 1.0
      ..color = const Color(0xFF00FFFF);
    context.canvas.drawRect((offset & size).deflate(0.5), paint);
    return true;
  }());
}