Google introduced a new API called “Bubbles” which would make it easier for app developers to push more visible notifications directly on top of the user’s display, much like how Facebook Messenger does with its “Chat Heads” feature.
The folks over at XDA-Developers were able to manually enable the Android Message Bubbles and they’ve posted some screen shots of the feature in action. There’s honestly not much new to the feature compared to Facebook’s Chat Heads – as you receive a message, you’ll get a visual notification on top of your screen, so you don’t have to leave the app you’re using.
You can drag and place the floating bubble to other parts of the screen, or you can drag it to the bottom to dismiss it – again, much like Chat Heads. As someone who dislikes Facebook’s Messenger’s Chat Heads (frankly, I find it annoying and obtrusive), I’m glad there will still be an option to disable the feature.
Source: developers