How to solve the failure about Flex VBOX's vertical scroll policy

We know the verticalScrollPolicy property can make VBOX control the appearance of the vertical scroll bar when the container’s contents exceed the dimensions of the container.

 

Sometimes failed to scroll, but we do this every time, so why it fail?

 

We see the following code:

<?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="1024" minHeight="768"><fx:Script><!--[CDATA[private function updateScrollPosition():void {vSP.text = vBox.verticalScrollPosition.toString();mVSP.text = vBox.maxVerticalScrollPosition.toString();}]]--></fx:Script><fx:Style>@namespace s "library://ns.adobe.com/flex/spark";@namespace mx "library://ns.adobe.com/flex/mx";mx|VBox {paddingLeft: 10;paddingRight: 10;paddingTop: 10;paddingBottom: 10;}</fx:Style><s:Panel id="myPanel" x="0" y="0" width="100%" height="100%"><s:layout><s:VerticalLayout horizontalAlign="center"/></s:layout><mx:ApplicationControlBar dock="true" width="100%"><mx:Form><mx:FormItem label="verticalScrollPolicy:"><mx:ComboBox id="comboBox"><mx:dataProvider><fx:Array><fx:Object label="auto" /><fx:Object label="on" /><fx:Object label="off" /></fx:Array></mx:dataProvider></mx:ComboBox></mx:FormItem><mx:FormItem label="height:"><mx:HSlider id="slider"minimum="500"maximum="1000"value="50"liveDragging="true"snapInterval="1"tickInterval="50" /></mx:FormItem><mx:FormItem label="verticalScrollPosition:"><mx:Label id="vSP" /></mx:FormItem><mx:FormItem label="maxVerticalScrollPosition:"><mx:Label id="mVSP" /></mx:FormItem></mx:Form></mx:ApplicationControlBar><mx:VBox id="vBox" verticalScrollPolicy="on" backgroundColor="haloSilver" width="100%" height="100%" minHeight="0" updateComplete="updateScrollPosition();" creationComplete="updateScrollPosition();"><mx:Box id="box"backgroundColor="haloBlue"width="50%"height="{slider.value}" /></mx:VBox></s:Panel></s:Application> 

 

You can not use mouse wheel to control the scroll bar, but did you find the error?

 

The mystical code is "backgroundColor="haloSilver""

 

You add this property to the VBOX, now try again!

 

It works, right?