Minor Description
If you throw a new RestartResponseAtInterceptPageException from inside onClick in an AjaxFallbackLink, Wicket does the right thing in redirecting you to a SignIn page, but upon completing the sign-in, the continueToOriginalDestination() call continues to the Ajax response (which isn't particularly surprising).
It might be nice to have magic added to make this work seamlessly and restart the onClick() somehow. It would be adequate to make the AjaxFallBackLink behave as though fallback had happened in this situation.
A fairly simple workaround is to check in the Panel/Page to see if sign-in has happened already and use a normal Link in the case that it hasn't.