CustomPainter Path.lineTo() shadow

[Dshah]

I am using CustomPainter where I need to draw line

class ShapesPainter extends CustomPainter {

  @override

  void paint(Canvas canvas, Size size) {

    final Paint firstPaint = Paint();

    firstPaint.color = const Color.fromARGB(255, 236, 0, 140);

    final Path firstPath = Path();

    firstPath.lineTo(size.width, 0);

    firstPath.lineTo(0, size.height * 0.10);

    firstPath.close();

    canvas.drawShadow(firstPath, Colors.black87, 2.0, false);

    canvas.drawPath(firstPath, firstPaint);

 }

}

I need to leave margin around the screen so I am using margin in container:

........

Container(

          color: Colors.white,

          margin:

              EdgeInsets.only(top: 60.0, bottom: 20.0, left: 15.0, right: 15.0),

          child: Container(

            child: CustomPaint(

              painter: ShapesPainter(),

              child: Container(

.......

I need to draw shadow under my custom path which I used anvas.drawShadow method in my build Widget, But there is a little shadow also coming over the left side, Please see the image below I pointed an error, Couldn't find any solution for this.

Thanks for any guidance.