Canvas is repainted when moved

import QtQuick 2.0

Rectangle {
    id: root
    width: 400
    height: 400

    Canvas {
        id: canvas
        width: 30
        height: 30

        onPaint: {
            print("painted");

            var ctx = getContext("2d");
            ctx.beginPath();
            ctx.fillStyle = "red";
            ctx.ellipse(0, 0, width, height);
            ctx.fill();
        }
    }

    Timer {
        running: true
        repeat: true
        interval: 100
        onTriggered: canvas.x += 0.1
    }
}