When applying a custom constraint (implementing the Constraint and CustomConstraint) interfaces) to a datebox the client side behaves unexpected in some scenarios:

run the attachment datebox-custom-constraint.zul and follow the steps:

1. open the calendar drop down
2. click a date in the past
=> onChange with a null value is sent to the server resulting in an unexpected validation error
3. click the same date again
=> date is selected correctly
4. open the calendar drop down
5. select a future date
=> date is selected correctly and error message is displayed as expected
6. open the calendar drop down
7. select a date in the past
=> onChange with the old (future date)value is sent to the server resulting in an unexpected validation error flashing up
=> another onChange is sent to the server with the correct new value clearing the error message again

NOTE: the example uses a validation delay of 1000ms to make the effects more apparent, the same unexpected onChange events are sent to the server without the delay