Strange behavior with childrenRect in which the onChildrenRectChanged signal doesn't always fire.

childrenRect_bug.qml

import QtQuick 2.5
import QtQuick.Controls 1.4

Item {
  width: 800
  height: 600
  property string src: "import QtQuick 2.5; Rectangle{ width: 50; height: 50; color: \"red\"}"

  Item {
    id: a_container
    x: 0
    y: 100
    width: 100
    height: 100
    property string name: "A"
    onChildrenRectChanged: {
      console.log(name)
    }
  }

  Item {
    id: b_container
    x: 300
    y: 100
    width: 100
    height: 100
    property string name: "B"
    onChildrenRectChanged: {
      console.log(name)
    }
    property real _x: childrenRect.x // <-- CRITICAL LINE
  }

  Button {
    x: 0
    text: "A"
    onClicked: {
      Qt.createQmlObject(src, a_container)
    }
  }

  Button {
    x: 300
    text: "B"
    onClicked: {
      Qt.createQmlObject(src, b_container)
    }
  }
  
}