Glow effect cuts off the bottom and top

import QtQuick 2.6
import QtQuick.Window 2.2
import QtGraphicalEffects 1.0

Window {
    visible: true
    width: 640
    height: 480
    title: qsTr("Hello World")
    color: "black"
    Image {
        id: imageSource
        source: "file:/home/kapinter/tmp/qt-everywhere-src-5.12.0-libpng/qtgraphicaleffects/tests/manual/testbed/images/butterfly.png"
        anchors.centerIn: parent
    }

    Glow {
        id: effect
        anchors.fill: imageSource
        color: "white"
        spread: 1.0
        radius: 8
        samples: radius*2+1
        visible: true
        source: imageSource
    }
}