Caprine is an unofficial Facebook Messenger app for Windows.
When closing the window, the app will continue running in the background, in the dock on macOS and the tray on Linux/Windows. Right-click the dock/tray icon and choose Quit to completely quit the app. On macOS, click the dock icon to show the window. On Linux, right-click the tray icon and choose Toggle to toggle the window. On Windows, click the tray icon to toggle the window.
You can toggle dark mode in the Caprine menu or with Cmd+D / Ctrl+D
Desktop notifications can be turned on in Preferences.