CustomPainter together with Paint()..blendMode gives weird blinking effect while scrolling - flutter

Question

After launching the application, the green circle is not visible. In order for the circle to appear, you need to scroll up. Then if you scroll down, the circle starts blinking as long as the scrolloffset is within about 20 pixels. If the scrolloffset is larger than 20 pixels, the circle disappears and appears only when the scrolloffset is 0.
If wrap the widget in RepaintBoundary, then the circle appears on any scroll and then does not disappear.
import 'package:flutter/material.dart';
void main() {
runApp(MyApp());
}
class DemoPainter extends CustomPainter {
#override
void paint(Canvas canvas, Size size) {
canvas.save();
final path = Path()..addOval(Rect.fromCircle(center: Offset.zero, radius: 50));
final paint = Paint()..color = Colors.green;
paint.blendMode = BlendMode.dstOver;
canvas.drawPath(path, paint);
canvas.restore();
}
#override
bool shouldRepaint(covariant CustomPainter oldDelegate) {
return true;
}
}
class TestWidget extends StatelessWidget {
const TestWidget({Key? key}) : super(key: key);
#override
Widget build(BuildContext context) {
return SizedBox(
height: 80,
child: Center(
child: CustomPaint(
painter: DemoPainter(),
child: Text(
"Child widget",
style: TextStyle(color: Colors.blue[200], fontWeight: FontWeight.bold, fontSize: 20),
),
),
));
}
}
class MyApp extends StatelessWidget {
#override
Widget build(BuildContext context) {
return MaterialApp(
theme: ThemeData(primarySwatch: Colors.blue),
home: Scaffold(
body: SingleChildScrollView(
child: Column(
children: [
const TestWidget(),
const TestWidget(),
const TestWidget(),
const TestWidget(),
const RepaintBoundary(child: TestWidget()),
Container(
height: 2000,
color: Colors.red,
)
],
),
),
),
);
}
}
The link to Dartpad, but i can't reproduce the effect on the web platform.
What could be the reason for such effect and how to get rid of it?