macOS deliberately stays awake in clamshell mode whenever an external display is connected and power is plugged in — lidwatch overrides exactly that one behavior, sending the Mac to sleep when the lid closes regardless. It runs as a brew services background daemon rather than a menu-bar app, by design: the author wanted something you enable once and forget, not another icon competing for space. It reacts to native macOS lid-state, power, and display-connection events directly instead of polling every few seconds, which is what caused a competing tool to occasionally sleep the Mac with the lid still open.