Stop Messenger App from opening at Login
Not so much a question as a solution to the problem which many people have. Despite checking my Login Items (Messenger wasn't there) and unchecking the Open at Login option on the Messenger app in the dock, it kept loading. To get around the problem, set the Preference in the Messenger app to not Open when you start your computer and also, uncheck the Reopen windows when logging back in option when you Shutdown or Restart your Mac. Add the items you want to open in the Login Items and this solves the problem.
iMac 27″, macOS 11.6