Right now when a modal screen appears, the user can not able to go back by tapping anywhere on the screen accept the modal menu (For example: when you tap for the menu on an Instagram post (located at top right corner with 3 dot icon) you can able to go back by tapping anywhere when the screen appears).
You can already do this by creating a new group on the modal screen (a transparent group, brought to front) and making the action for that group to navigate ‘back’.