Facebook Messenger's Chat Heads changed how we interact with notifications and messages, spawning several similar apps such as Brave Browser (né LinkBubble) or Floating Touch, among others. However, Chat Heads have always had one disadvantage in that you cannot paste things into the text entry field. Not anymore, as Facebook has now fixed this.
It might seem like a very simple thing, but that's kind of why it's so interesting: it's so simple that it seems unbelievable that up until now chat heads haven't had the ability to paste. Chat Heads have been around for about 3 years, so either it's not been a priority for the Messenger team, or it's just found how to enable it. The paste dialog is similar to the system Android context bar, but looks different (see below). That's pretty much the only change - I assume the non-standard Chat Head implementation (as in, it's not covered by any API) stopped Facebook from using the system context bar.
Left:Messenger Chat Head, compared to Right in the app. Note the difference in paste dialogs.
Again, it may seem small and simple, but it should make many users happy. If you've got Chat Heads turned off, you should be able to enable them in the settings tab. Otherwise, Facebook Messenger is available on Google Play, or APK Mirror if that's more your thing.