The following segfaults if run with "qml".   Since I've never tried using Binding before this might be incorrect code.  But presumably segfault indicates a bug somewhere.

import QtQuick
Rectangle {
  id: top; objectName: "top"
  visible:true; width:300; height:300  
  Text {
    id: trtext
    width: 30; height: 30
    text: "1234.56"
    font.bold: true    
    Binding {
      target: trtext.font; property: 'bold'
      value: false; when: width < 30
    }
  }
}

P.S. Something is whacked with pasting code into this bug system – newlines seem to be randomly removed and have to be put back with hand editing