With previous version of user plugin, there was a redirect option in the component after login, allowing to call the onLogin method in an ajax way (and so, with ajax field validation instead of flash message).
With the new version, that option disappeared, which make ajax login impossible.
Thank you for the suggestion. I could fix it by defining a redirect property in my form_ajax call but it expose the logged in url to the curious, it would be better to get it as a component property.