Flex, a mx:Tree component in Spark application, the scrollbar auto send to top when collapsed a tree node

<?xml version="1.0" encoding="utf-8"?>
<s:Application xmlns:fx="http://ns.adobe.com/mxml/2009" 
               xmlns:s="library://ns.adobe.com/flex/spark" 
               xmlns:mx="library://ns.adobe.com/flex/mx" minWidth="955" minHeight="600" 
               creationComplete="init()">
<fx:Script>
    <![CDATA[
        import mx.collections.XMLListCollection;

        [Bindable]
        private var treeData:XML =
            <node label='root'>
                <node label="Monkeys">
                    <node label="South America">
                        <node label="Coastal"/>
                        <node label="Inland"/>
                    </node>
                    <node label="Africa" isBranch="true">
                        <node label="Coastal"/>
                        <node label="Inland"/>
                    </node>
                    <node label="Asia" isBranch="true">
                        <node label="Coastal"/>
                        <node label="Inland"/>
                    </node>
                </node>
                <node label="Sharks">
                    <node label="South America" isBranch="true">
                        <node label="Coastal"/>
                        <node label="Inland"/>
                    </node>
                    <node label="Africa" isBranch="true">
                        <node label="Coastal"/>
                        <node label="Inland"/>
                    </node>
                    <node label="Asia" >
                        <node label="Coastal"/>
                        <node label="Inland"/>
                    </node>
                </node>
            </node>;

        private function init():void{
            myTree.expandChildrenOf(myTree.firstVisibleItem, true);
        }
    ]]>
<开发者_运维技巧;/fx:Script>
    <mx:Tree id="myTree" 
             y="50" 
             width="221" 
             height="207" 
             horizontalCenter="0"
             dataProvider="{treeData.node}"
             labelField="@label"/> 
</s:Application>