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?