After moving to their Electron app, Facebook has been rapidly iterating and adding features to their Facebook Messenger app.
The latest improvement, discovered by ALumia, is the addition of Fluent Design to the menus in Facebook Messenger.
This is particularly welcome due to how garishly the current pop-up menu clashes with the design of the rest of the app. Adding Fluent Design should allow the old Windows-style menu to fit in better.
The update is not yet available in my version of the Beta app, but I assume it will not be too long before this is the case.
You can download and install Facebook’s Messenger Desktop(Beta) app from this link, or you can visit Microsoft Store and search for the app.