How to get a Spark DropDownList skinClass and typicalItem to live happily together

How does one go about calling a skinClass on a DropDownList component without compromising the typicalItem functionality?

<s:DropDownList id="appList"
  skinClass="skins.IconComboBoxSkin"
  requireSelection="true"
  change="combobox_changeHandler(event)" 
  typicalItem="{appList.selectedItem}">

If I remove the skinClass it behaves correctly.

IconComboxSkin.mxml

<?xml version="1.0" encoding="utf-8"?>
<!-- http://blog.flexexamples.com/2009/04/14/displaying-images-in-a-spark-dropdownlist-control-in-flex-gumbo/ -->
<s:Skin name="CustomDropDownListSkin"
  xmlns:fx="http://ns.adobe.com/mxml/2009"
  xmlns:s="library://ns.adobe.com/flex/spark"
  alpha.disabled="0.5"> 
 <!-- states -->
 <s:states>
  <s:State name="normal" />
  <s:State name="open" />
  <s:State name="disabled" />
 </s:states>

 <!-- host component -->
 <fx:Metadata>
  <![CDATA[ 
  [HostComponent("spark.components.DropDownList")]
  ]]>
 </fx:Metadata> 

 <!--- The PopUpAnchor control that opens the drop-down list. -->
 <s:PopUpAnchor id="popUp"  displayPopUp.normal="false" displayPopUp.open="true" includeIn="open"
       left="0" right="0" top="0" bottom="0" itemDestructionPolicy="auto"
       popUpPosition="below" popUpWidthMatchesAnchorWidth="true">

  <!--- The drop down area of the skin. This includes borders, background colors, scrollers, and filters. -->
  <s:Group id="dropDown" maxHeight="134" minHeight="22" >

   <!-- drop shadow -->
   <s:RectangularDropShadow blurX="20" blurY="20" alpha="1.0" distance="5" 
          angle="90" color="#000000" left="0" top="0" right="0" bottom="0"/>

   <!-- border -->
   <s:Rect left="0" right="0" top="0" bottom="0">
    <s:stroke>
     <s:SolidColorStroke color="0x686868" weight="1"/>
    </s:stroke>
   </s:Rect>

   <!-- fill -->
   <!--- Defines the appearance of drop-down list's background fill. -->
   <s:Rect id="background" left="1" right="1" top="1" bottom="1" >
    <s:fill>
     <!--- The color of the drop down's background fill. The default color is 0xFFFFFF. -->
     <s:SolidColor id="bgFill" color="0xFFFFFF" />
    </s:fill>
   </s:Rect>

   <s:Scroller left="0" top="0" right="0" bottom="0" focusEnabled="false" minViewportInset="1">
    <!--- The container for the data items in the drop-down list. -->
    <s:DataGroup id="dataGroup" itemRenderer="spark.skins.spark.DefaultItemRenderer">
     <s:layout>
      &l开发者_Go百科t;s:VerticalLayout gap="0" horizontalAlign="contentJustify"/>
     </s:layout>
    </s:DataGroup>
   </s:Scroller>
  </s:Group>
 </s:PopUpAnchor>

 <!--- The anchor button used by the DropDownList. The default skin is DropDownListButtonSkin. -->
 <s:Button id="openButton" left="0" right="0" top="0" bottom="0" focusEnabled="false"
     skinClass="spark.skins.spark.DropDownListButtonSkin" />  

 <s:TextInput id="textInput" 
     left="0" right="18" top="0" bottom="0" 
     skinClass="spark.skins.spark.ComboBoxTextInputSkin"/>

</s:Skin>

rewrite measure function in spark.components.DropDownList class