Why are DropDown values Editable after being selected?

Hi,

When I select a choice from the DropDown menu, I'm then able to edit the value.

When I click the "Start" button, it fails if the values have been edited because it doesn't match one of the choice restrictions.

This also happens in the widget_demo.py DropDowns.

Is this expected behaviour? Is there a way to force it to be Read Only do that the user can't mess with the values after they select one?

Thanks

Neil