Please file a bug at http://bugs.adobe.com/flex and include a simple test case and somebody from Adobe will take a look.
Since you’re trying to launch a popup from a ComboBox change event, you may need to try manually closing the Spark ComboBox control using the close() method.

Peter

I have a problem that the spark combobox that i am using is not closing after selecting the item in dropdown.

here is the correct scenario, I have 3 fixed labels in dropdown of combobox, when i click on the first label a pop up should appear( which contains yes and no buttons).clicking YES will send a blazeds call.the dropdown is not closing even after that popup closed .for rest of the labels there will not be any popups clicking on those. in that case the dropdown is closing.

Thanks in advance.

It turns out you CAN set the openEasingFunction and closeEasingFunction in CSS, but it was a compiler feature added in the new Flex 4 SDK. For more information, and an example, see “Changing the opening and closing easing functions on a Halo ComboBox control in Flex 4″.

Peter

Unfortunately, I do not believe that syntax is supported by the Flex compiler.
But, I do have a few other workarounds you could try:

1) Use the StyleManager to globally override the easing functions for all ComboBox controls:

<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        initialize="init();">
 
    <mx:Script>
        <![CDATA[
            import mx.effects.easing.Bounce;
 
            private function init():void {
                var styObj:CSSStyleDeclaration = StyleManager.getStyleDeclaration("ComboBox");
                styObj.setStyle("openEasingFunction", Bounce.easeOut);
            }
        ]]>
    </mx:Script>
 
    <mx:ComboBox id="cb1"
            dataProvider="[The,Quick,Brown,Fox,Jumps,Over,The,Lazy,Dog]"
            openDuration="2000" />
 
    <mx:ComboBox id="cb2"
            dataProvider="[The,Quick,Brown,Fox,Jumps,Over,The,Lazy,Dog]"
            openDuration="2000" />
 
</mx:Application>

– or –

2) Extend the ComboBox control and specify the openEasingFunction style in the MXML. Then you can use the custom ComboBox control and still just change the openEasingFunction style in one place and have it change all ComboBox controls in your entire site.

<?xml version="1.0" encoding="utf-8"?>
<mx:Application xmlns:mx="http://www.adobe.com/2006/mxml"
        xmlns:comps="comps.*">
 
    <comps:CustomComboBox id="customCB"
            dataProvider="[The,Quick,Brown,Fox,Jumps,Over,The,Lazy,Dog]"
            openDuration="2000" />
 
</mx:Application>

And the custom ComboBox component, comps/CustomComboBox.mxml, is as follows:

<?xml version="1.0" encoding="utf-8"?>
<mx:ComboBox xmlns:mx="http://www.adobe.com/2006/mxml" openEasingFunction="Bounce.easeOut">
 
    <mx:Script>
        import mx.effects.easing.Bounce;
    </mx:Script>
 
</mx:ComboBox>

Hope that helps,
Peter

ComboBox
{
    openEasingFunction: ClassReference(fullNamespace.Bounce.Style);
}

does not work =(