Messenger Demo Viewer

Messenger Demo Viewer is a fairly atypical application that's specifically aimed at users who want a cleaner look, so to speak, while using Facebook's Messenger since it automatically hides the Conversations sidebar.

Cross-platform Messenger wrapper that hides the sidebar by default

This is basically an Electron-based wrapper which allows you to access the full functionality of the official Facebook Messenger web app directly from the comforts of your computer or your Mac's desktop.
It might turn out to be very useful in various situations, for example, when you're in a fairly professional environment like a meeting or when you're recording a tutorial or a video, and you don't want the whole world to see your conversation previews.

Extremely easy to deploy and just so easy to live with

Apart from making sure that your computer is connected to the Internet and you have a valid Facebook account, there are no extra requirements, although you will have to install the app before hand which is, by all accounts, a very straightforward process.
Of course, regardless of the situation, you will need the Conversation sidebar at some point. That said, you can enable or disable it from the app's typical menu bar, or you can use the CTRL+B keyboard shortcut for the same effect.

A run-of-the-mill Messenger client that allows you to instantly enable or disable the Conversations sidebar

Taking everything into account, Messenger Demo Viewer is not exactly what you call an impressive application, and we're fairly sure it won't break all the popularity charts either. However, it will definitely come in handy for a few users who want to use a Messenger client with a cleaner and more  professional look and without having to use a web browser in the process of doing so.