“No themed login form” oh no! Anyway. (I really think that the “theming” on login and sign-up forms breaches which bits are done by the browser and which bits are not. We’ve had to kludge back support)
non-iOS browsers seem to handle HTTP basic auth gracefully.
I have to look into some protections to this. I recently did a project and used Basic Auth for expediency thinking that later I would replace with a "proper" auth form. My theory was that this was better because sending cookies is based on the protocol also not just the domain.
They're even deprecating auth via non-browser means in newer OAuth/GNAP standards. It's like they're designing around what things are now, rather than thinking ahead as to what would be best. Like rather than website - managed auth, it be fully browser controlled (not HTML forms). Not cookies, but something prescribed by the browser (or whatever other application you might use, rather than needing to embed a browser yet again in everything).
I had assumed the cause was something like this: Product management people want functionality which is fundamentally incompatible with security-- like complete control of the appearance and behavior of the login form (which can't be squared with impersonation attacks). Security people substantially control the https auth experience, and say NO. Product management people say fine, and just don't use the secure login functionality.
Everyone is happy: PMs get the security dumpster fire they asked for, Security people keep their perfect ice castle and can complain that all breaches are the fault of the PMs doing the wrong stuff. Except the users, of course, who get a needlessly insecure experience because there are many improvements to the http auth process that don't substantially compromise security.
[0] My favorite: opening DevTools triggers a second credential prompt because the DevTools session wants to download some mapfiles or something. No infrastructure exists to make that HTTP request in the context of the current tab, apparently.