Canvas: shadowBlur is either missing or rough for small values

import QtQuick 2.0

Item {
    id: root
    width: 300
    height: 300

    function degToRad(degrees) {
        return degrees * (Math.PI / 180);
    }

    Canvas {
        id: contentCanvas
        width: parent.width
        height: parent.height

        onPaint: {
            var ctx = getContext("2d");
            ctx.lineWidth = 8;
            ctx.strokeStyle = "blue";
            ctx.shadowBlur = 1.9;
//            ctx.shadowBlur = 3.9;
//            ctx.shadowBlur = 5.9;
            ctx.shadowColor = "blue";

            ctx.beginPath();
            ctx.arc(width / 2, height / 2, 50, degToRad(0), degToRad(360), false);
            ctx.stroke();
        }
    }
}