how to implement this overlay with flutter?

I belive you can use the paint widget for the same and later using the positioned widget you can place it on top of any container .

below is the example code to us the paint widget

 class TriangleBox extends CustomPainter {
    final Color strokeColor;
    final PaintingStyle paintingStyle;
    final double strokeWidth;

    TrianglePainter({
     required this.strokeColor,
     required this.paintingStyle,
     required this.strokeWidth,
    });

    @override
    void paint(Canvas canvas, Size size) {
    final Paint paint = Paint()
      ..color = strokeColor
      ..style = paintingStyle
      ..strokeWidth = strokeWidth;

    final Path path = Path();
    path.moveTo(size.width / 2, 0);
    path.lineTo(0, size.height);
    path.lineTo(size.width, size.height);
    path.close();

     canvas.drawPath(path, paint);
     }
  }