Problem with selection state in editable combobox

I have an editable combo box, with a fixed list of items. Let's say:

foo

bar

If I select 'foo' from the drop down, the combobox text changes as expected. Now I edit the text and type in 'xxx'. Then I select 'foo' again from the drop down, but the text doesn't change. The combobox selection state is still 'foo', so the change event doesn't fire.

I've tried to clear the combobox selection state whenever the user types in the combobox, but that just clears the text.

Any idea how to get the second 'foo' selection to register, and change the combobox text?

Thanks,

Aaron