This is a fairly common UI pattern used in most modern web apps. However, it does not seem possible to fully implement in Caspio.

As it stands right now, you can have authentication in a modal window, assuming a user puts in valid credentials, and have it work. Where it breaks is when a user inputs no credentials or bad credentials. A user must click on the login button/link again to reopen the modal, and see the error message. This is poor UX. There is no way to set the redirect on fail back to the modal window, and have an error message displayed.

A possible solution would be allowing an ajax call to validate the form, and refreshing to that page with modal open and error message displayed.