在flutter中，当在脚手架/容器小部件中使用时，CustomPaint()的行为似乎很奇怪

class MyCustomLayout extends SingleChildLayoutDelegate {
  @override
  BoxConstraints getConstraintsForChild(BoxConstraints constraints) {
    // 定义子部件的约束条件
    return constraints.tighten(width: 200, height: 200);
  }

  @override
  Offset getPositionForChild(Size size, Size childSize) {
    // 定义子部件的位置
    return Offset(50, 50);
  }

  @override
  bool shouldRelayout(covariant SingleChildLayoutDelegate oldDelegate) {
    // 判断是否需要重新布局
    return false;
  }

  @override
  Size getSize(BoxConstraints constraints) {
    // 定义布局的大小
    return Size(300, 300);
  }
}

class MyCustomWidget extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return CustomSingleChildLayout(
      delegate: MyCustomLayout(),
      child: CustomPaint(
        painter: MyCustomPainter(),
      ),
    );
  }
}

class MyCustomPainter extends CustomPainter {
  @override
  void paint(Canvas canvas, Size size) {
    // 在画布上绘制自定义的UI元素
    // ...
  }

  @override
  bool shouldRepaint(covariant CustomPainter oldDelegate) {
    // 判断是否需要重新绘制
    return false;
  }
}