For example,

<combobox onSelect='alert("onSelect");self.invalidate()'>
<comboitem label="Add 1 item" />
<comboitem label="Add 2 item" />
<comboitem label="Add 3 item" />
</combobox>

1. Please select any one of the comboitems, and it will fire an onSelect event.
2. After close the dialog and reopen the combobox (don't select anything) and then click out side the combobox, it should not appear a constrain error or resend the onSelect event again.