Facebook released the trial version of Facebook messenger for windows which is a new, trial application that lets you use Facebook without being on www.facebook.com. Facebook Messenger for Windows works only on computers running on Windows 7 Operating System.
To dock and undock the Facebook Messenger for Windows, Click the icon at the bottom-right corner of the window to undock the app. Click to dock it again.
By default, the app is undocked so that you can easily move it around your computer screen (just click the top of the sidebar and drag it with your mouse). When it’s undocked you can also close the sidebar by clicking the ‘x’ and still get notifications and chats.
When you dock the window, it stays on the right side of your screen. [source]