Flex 4 spark Panel has an ugly gray top part

I have a Flex 4 spark Panel I'm popping up through the PopUpManager, but it has a gray portion at the top I can't get rid of. What is that and how can I remove it?

UPDATE: An example Panel is below. I simply call PopUpManager.addPopUp(new TestPanel(), background, true); on it and receive that solid gray bar above the button.

<s:Panel xml开发者_如何转开发ns:s="library://ns.adobe.com/flex/spark"
         xmlns:fx="http://ns.adobe.com/mxml/2009"
         dropShadowVisible="false"
         backgroundAlpha="0"
         controlBarVisible="false"
         borderVisible="false">
    <s:VGroup>
        <s:Button label="A button" width="150" height="55"/>
    </s:VGroup>
</s:Panel>

So, this is how I did it. I made a custom skin: HeaderlessPanelSkin.as

public class HeaderlessPanelSkin extends PanelSkin {
   public function HeaderlessPanelSkin() {
        super();

        topGroup.includeInLayout = false;
    }
}

Then, in the panel, I just reference the new skin: skinClass="HeaderlessPanelSkin"

That should do it :)

Create new skin, and in the panel declaration use it... like so

File->New MXML Skin, Host Component is panel.

Edit the Skin properties to change it how you like, in this case the gradient colors on the header.

Sounds like the Panel TitleBar

Create a custom skin and style the title bar how you want it to appear.