DatePicker fails to notify component update when OnChangeAjaxBehavior is used on the target input.
It works well with AjaxFormComponentUpdatingBehavior.
The root cause seems to be in the Wicket.ChangeHandler added by the OnChangeAjaxBehavior, which hides user handler while the DatePicker calls onchange handler directly (wicket-date.js:186):
if (field.onchange != null && typeof(field.onchange) != 'undefined') field.onchange();
Possible fix could be dispatching onchange Event instead of direct handler invocation.
Yet I don't know what is purpose of Wicket.ChangeHandler here...