This is a viewer of message on entire screen.
The viewer shows message text in major part of screen. Additionally, viewer shows other items:
Navigating to previous or next message
It is possible to swipe screen to left/right to go to previous or next message in a message list. While swiping, a message counter starts appearing at bottom of screen. When the counter becomes blue, and you release finger, message is switched.
The list of message depends on parent activity that was used before entering message viewer. Typically message viewer is opened from Message list, so the list being navigated is those of message list.
In case that message viewer was opened from other place, there may be no logical previous/next message, so message switching won’t be available.