If you open iMessage on your iPhone under iOS 11, you will see the App-Bar where you can use different functions. If you don't like this app bar in iOS 11, you can disable or hide it. How exactly this works, we explain to you here:
Hide App Bar in iOS 11
Open a conversation in iMessage and you will see the app bar at the bottom of the display.
Next to the line for the text input you will see the app-symbol on the left side - type it and the app-bar will be hidden.
The app bar is now hidden until you tap the app icon again.
The useful thing is that this setting is also used in iMessage for the other conversations. Now you know the quick and easy way to hide the app bar.