Facebook's team is always testing out new features, and that even includes a spiffy new user interface, which is exactly what they are doing right now. According to Android Police, the Facebook Messenger team is apparently testing out a new UI for M. For those that are unaware, M is Facebook's own personal assistant that lives within Facebook Messenger. Now it's important to note that M itself is still being beta tested, so there will be very, very few people seeing this change in their Facebook Messenger app.
Looking at the screenshots below, it appears that the updated interface looks a bit more modern and spacious in some screens, it also reduces functionality a bit. So looking at the message screen, Facebook has moved all of the buttons from two rows, down into one. That means you have more space to read the messages in the conversation, but there is also less space to tap on the text field. Smaller touch targets can lead to some user experience or UX issues. But that's why this is still being tested. Now, those items hidden in the "+" have also been redesigned. Instead of getting a list, you now get a horizontal scrolling list of round buttons for each item. Making some things a bit harder to find than before.
Stickers, GIFs, and Emojis now have their own section, and have their own panels in that section. So you can tap on one button and find the sticker, GIF or emoji that you were looking for. This is actually probably the best part about this update, as it makes it much easier to find a GIF and send it to your friend or family member. There are a few other interface tweaks that have been made here, but nothing too crazy. It's a pretty interesting update for Facebook Messenger, but it'll probably receive a mixed reaction if it does make it to the stable build. Remember that this is just a test for the Facebook Messenger app, and it may not become the official UI, and there will likely be changes before it rolls out.