iMessage is no longer working properly after the recent Ventura 13.2 update. It's chosen a message to highlight, and when trying to select a different message, it will either pin that one to the top, or attempt to move it off the screen. Moving the arrows up and down doesn't help either.
iMac
Did you try entering safemode? This will check many system problems and fix them. Possibly repairing this Messenger problem.
Start up your Mac in safe mode - Apple Support
Did you try entering safemode? This will check many system problems and fix them. Possibly repairing this Messenger problem.
Start up your Mac in safe mode - Apple Support
iMessage no longer works after Mac OS 13.2 update
