flex tree 自动显示横向滚动条

mxml
<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml" xmlns:cmp="com.render.*"
    height="100%" width="100%" paddingTop="0" paddingBottom="0" paddingLeft="0" paddingRight="0">
    <cmp:AutoSizeTree
            id="checkTree"
            itemRenderer="com.render.CheckTreeRenderer"
            change="updataDataGrid()"
            labelField="@label"
             width="100%" height="100%" 
             horizontalScrollPolicy="auto" >
     </cmp:AutoSizeTree>
</mx:Application>
actionscript
package com.render
{
    import flash.events.Event;
   
    import mx.controls.Alert;
    import mx.controls.Tree;
    import mx.core.ScrollPolicy;
    import mx.core.mx_internal;
    public class AutoSizeTree extends Tree
    {
        public function AutoSizeTree()
        {
            super();
            horizontalScrollPolicy = ScrollPolicy.AUTO;
        }
    
        override public function get maxHorizontalScrollPosition():Number
        {
             if (isNaN(mx_internal::_maxHorizontalScrollPo