Item.childrenRect broken when all children lie outside the first quadrant

import QtQuick 2.1

Item {
    width: 800
    height: 600

    Item {
        anchors.centerIn: parent

        Item {
            id: container
            Rectangle {
                x: -200
                y: -200
                width: 30
                height: 30
                color: "red"
                MouseArea {
                    anchors.fill: parent
                    drag.target : parent
                }
            }

            Rectangle {
                x: -60
                y: -60
                width: 30
                height: 30
                color: "red"
                MouseArea {
                    anchors.fill: parent
                    drag.target : parent
                }
            }
        }

        Rectangle {
            id: origin
            width: 5
            height: 5
            color: "black"
        }

        Rectangle {
            id: visualizeChildrenRect
            x: container.childrenRect.x
            y: container.childrenRect.y
            width: container.childrenRect.width
            height: container.childrenRect.height
            color: "red"
            opacity: 0.5
        }
    }
}