User can select past dates even when constraint=no past and readonly=true is applied to a datebox

In our application, we have a requirement where the user should SELECT a date which is either current or future date.
So we have used the datebox component with the tag as:
<datebox id="testDt" format="dd-MMM-yyyy" constraint="no past" mold="rounded" readonly="true" />

Now when the user selects a date (i.e. future date), but uses the keyboard arrow keys to navigate through the dates, then they can select the past dates too.

The expected scenario is that the user should not be allowed to navigate (anyhow) to past dates. But in actual user can and even select them.