Back button works and there are two options to quit app. If you use Canvas+ you can use Exit app. If not you can use also Browser-Close function. Both of them works with the new Cocoon, however after exiting the app, it gives "app has stopped" error. First I was thinking this may be from my game (memory or not destroying some processes well), then I have tried it with empty project and get the same error. I am now very unhappy with the new Cocoon system and it will be activated at the end of this month with deprecating the previous CocoonJs platform. If these kind of problems not solved properly we will get many complaints from users.
I'm using Cocoon.io since the beginning and I never have had such issues.
I'm only using the official Cocoon Atomic plugins for Ads and so on, made a third-party plugin / behavior is causing the crash.
Use the developer app to see why your game had crashed.