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
    }
  }
}

The example from the linked bug should have either been rejected by binding, or it should have worked correctly. Considering that Binding documents that it takes an object, we might want to simply reject it (and  simply support the use case above with generalized grouped properties, which should already exist for Binding in 6.3).