Xul：使用vbox动态填充hbox会导致元素垂直放置

<hbox style="overflow:auto;"  id="canvasContainer"> </hbox>

  this.canvasContainer = document.getElementById("canvasContainer");
  for(var i = 0;i<k;i++){
    let imgCanvas = document.createElementNS("http://www.w3.org/1999/xhtml",'html:canvas');
    imgCanvas.setAttribute("width",200);
    imgCanvas.setAttribute("height",150);
    imgCanvas.getContext("2d").fillRect(0,0,200,150);   
    let canvasVbox = document.createElementNS("http://www.w3.org/1999/xhtml",'vbox');
    this.canvasContainer.appendChild(canvasVbox);

    let canvasLabel = document.createElement("label");
    canvasLabel.setAttribute("value",i);
    canvasLabel.setAttribute("flex",1);
    canvasVbox.setAttribute("flex",1);
    canvasVbox.appendChild(imgCanvas);
    canvasVbox.appendChild(canvasLabel);

    this.canvasContainer.appendChild(canvasVbox);
  }