[flexcoders] ViewStack historyManagementEnabled fails when nested in spark component

[method_air]

Any ideas why historyManagementEnabled seems to be failing here:

<s:VGroup>
<mx:ViewStack historyManagementEnabled="true">
        <s:NavigatorContent >
                <tunnel:ProductTunnelView/>
        </s:NavigatorContent>
        <s:NavigatorContent>
                <editorHostViews:EditorAppView />
        </s:NavigatorContent>   
        <s:NavigatorContent >
                <s:Panel>
                        <gallery:GalleryView />
                </s:Panel>
        </s:NavigatorContent>
        </mx:ViewStack>
</s:VGroup>

The MX viewstack is nested in a spark component. History management worked when 
nested in an mx component.

Thanks,

Philip