Facebook Messenger was the pioneer of floating notification and windows on Android, and now the app is one of the first to adopt Google’s new Bubbles notification API.
Available on handsets running Android 11, version 268.0.0.3.118 of the app now adopts the native Android API. Behaviour remains largely the same, with the chathead clinging to the side of the screen, and opening up a reply window when tapped.
The feature can be seen demonstrated in video below:

Android 11 is expected to roll out to handsets which are being offered upgrades in Q3 2020.
Via XDA-Dev