Apple doesn't support overdraw across apps, but there are still things Apple can learn here. Many system apps still use "insecure" dialogs that can (and are!) used to train users to do unsafe things. This[1] dialog can be drawn by an app (or injected using an advertisement!), and Apple ID failures are "common enough" that this works to collect people's Apple IDs.

[1]: https://i.imgur.com/85aeL4H.png