Problem about combobox on DropDownList

[zhangsir199]

Hi, everyone
I have a problem about Ext.net Dropdownlist(ddl). My ddl has a dropdown panel which is a gridpanel, I filter the gridpanel by a combobox. But when I selecting the combobox, the ddl collapsed too, may be it's a bug?

<ext:DropDownField ID="ddl" runat="server" TriggerIcon="SimpleArrowDown">
    <Component>
        <ext:GridPanel ID="gridRecipient" runat="server" Height="350" Frame="true" 
            StoreID="store1">
            <ColumnModel ID="ColumnModel2">
                <Columns>
                    <ext:Column Header="username" DataIndex="username" />
                    <ext:Column Header="orgName" DataIndex="orgName" />
                    <ext:Column Header="roleName" DataIndex="roleName" />
                </Columns>
            </ColumnModel>
            <TopBar>
                <ext:Toolbar ID="Toolbar2" runat="server">
                    <Items>
                        <ext:ComboBox runat="server" ID="cmb1" Width="150" 
                            StoreID="store2" DisplayField="name" ValueField="val">
                            <Listeners>
                                <Select Handler="filterUsers();" />
                            </Listeners>
                        </ext:ComboBox>
                    </Items>
                </ext:Toolbar>
            </TopBar>
        </ext:GridPanel>
    </Component>
</ext:DropDownField>