Canvas inside rectangle resizes incorrectly

import QtQuick 2.0

Rectangle {
    width: 400; height: 400
    Canvas {
        id: canvas
        // canvas size
        anchors.fill: parent
        // handler to override for drawing
        onPaint: {
            // get context
            var ctx = getContext("2d")
            // setup the stroke
            ctx.lineWidth = 8
            ctx.strokeStyle = "blue"
            // setup the fill
            ctx.fillStyle = "steelblue"
            // make a rectangle with a border
            ctx.fillRect(0, 0, canvas.width, canvas.height)
            ctx.strokeRect(0, 0, canvas.width, canvas.height)
        }
    }
}