There is a problem with one of the paths to register a contact for an event where if an error is returned from the payment processor (in this case IATS) the form just hangs and never returns. When the page it reloaded it displays the error.
This only happens when the form is an overlay/popup. If the user opens the link in a new tab instead of allowing for the overlay, the error is returned to the user (although in a less than pretty way). This does not happen in the same context for contributions, that form reacts well to all situations. The form does not hang if the response from the processor does not contain an error message.
I'm submitting this here to get feedback before submitting this to gitlab as an issue