I need help identifying a feature I have been using an app called Foodpanda that is able to show a button on top of other apps and, on the home screen. 

It makes this little button which I have marked in the screenshots below with an arrow. Observations: 


The app does not have any special permissions 
The button appears only when a support chat is active on the app
The button takes me into that view and disappears when that view is active.
The button cannot be hidden if there is a chat active in the background.



  What is this feature/functionality called? Is there a way to disallow it?
I found out that the feature is Draw over other apps. Funnily, this is not a permission that needs to be granted. 

I've confirmed this. When I disable that permission, the app cannot draw that button.

Also, it is a long way down from the other permissions.