Combobox has null value initially. Pressing down arrow key fires an onSelect event, but not onChange event. While servicing the onSelect event, the server-side will perform constraint validation on the current combobox's value. Since the value is null, it is coerced to an empty string, and violates the "no empty" constraint.
1. Set focus on the first combobox's text input box
2. Press "down arrow" key to change its selection
-> error message shows the violation of "no empty" constraint
3. Press "tab" to set focus on the second combobox
-> model, dependent on the first combobox's value, is not set
Apply the following script