Matt Raible found this problem with the 4.1.3-SNAPSHOT release and reported it on the mailing list, but I didn't see a JIRA issue for it: http://www.nabble.com/Re%3A-Validation-and-CSS-with-4.1.3-SNAPSHOT-p12397222.html. The behavior is still there in this build.
The client-side validation code produces an empty popup when you submit a form if the form contains a PropertySelection component with validators. Here's the example from Matt's message:
<component id="countryField" type="PropertySelection">
<binding name="model" value="countries"/>
<binding name="value" value="user.address.country"/>
<binding name="validators" value="validators:required"/>
<binding name="displayName" value="message:user.address.country"/>
You get the blank popup even when the validation should pass (i.e., the field has a selected option). As a test I created an alternate required validator that decorates the markup but doesn't generate any validation on the client, and that one doesn't exhibit this behavior. The problem appears to lie either in changes to Dojo's "required" validation in this release or in how Tapestry is using it.
Matt questioned whether Dojo client-side validation works with this component. It has worked in previous versions.