combobox doesn't open its popup when pressing down arrow key

Steps to Reproduce

    <combobox autodrop="true" >
        <comboitem label="Simple and Rich"/>
        <comboitem label="Cool!"/>
        <comboitem label="Ajax and RIA"/>
    </combobox>

1. press down arrow key

Current Result

the popup doesn't open

Expected Result

the popup opens

Debug Information

• with autodrop="true", a user types a character (starts to select an item), the combobox opens its popup. The same rule applies. Pressing the down arrow key also means to select an item. The component should open the popup, too.