Android O will now alert you when an app is displaying on top of another app. The particular action of drawing on top of other apps is nothing new as apps have had permissions to draw on top of other apps well before now, and users have the power to toggle these app permissions on and off from a granular level on a per app basis. What is new though is a persistent notification sitting in the notification tray that tells you an app is executing this permission, which, might just be the best thing ever if you like knowing these kinds of things, or it could be the word thing ever if you hate persistent notifications. Either way, it's a feature that Android O has incorporated so if you're running the first developer preview then you might see this pop up from time to time.
The nice thing about Android O allowing this notification is that it gives users the capability to turn off the permission from the notification itself, meaning you won't have to navigate to the apps menu and find the app and enter its permissions before you can actually disable it. This is the latest move from Google to make the notification system and in turn the notification tray a more powerful and useful part of the software.
This is bound to be a handy feature that many users will end up appreciating just for the ease of use factor, but it also serves as another level of security as many users may not be aware that apps have the "draw over other apps" permission enabled. Since Android Nougat and before don't have notifications for this action, users can install apps and never be aware that the permission is automatically enabled, which could cause problems for battery life and a number of other things in regards to how the device behaves. Now with Android O that won't be an issue, of course this is always something that could change or be removed completely in the end, as this is the first developer preview and there are three more to go as well as the final build, so Google still has time to modify things.