Characters of semi-transparent Text overlap

import QtQuick 2.0
import QtQuick.Window 2.0
import QtQuick.Controls 1.3

Window {
    id: window
    visible: true
    width: 600
    height: 300

    Text {
        font.pixelSize: pixelSizeSlider.value
        font.letterSpacing: letterSpacingSlider.value
        opacity: 0.5
        text: !typeSwitch.checked ? "Hello" : "يتعلّقم"
        anchors.top: parent.top
        anchors.topMargin: 10
        anchors.horizontalCenter: parent.horizontalCenter
    }

    Column {
        anchors.horizontalCenter: parent.horizontalCenter
        anchors.bottom: parent.bottom

        Text {
            text: "Arabic"
        }

        Switch {
            id: typeSwitch
        }

        Text {
            text: "Pixel size"
        }

        Slider {
            id: pixelSizeSlider
            minimumValue: 30
            maximumValue: 200
        }

        Text {
            text: "Letter spacing"
        }

        Slider {
            id: letterSpacingSlider
            minimumValue: -30
            maximumValue: 30
        }
    }
}