This app turns off the MacBook display when it connects to an external monitor
manualdousuario.net
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I think the assumption on macOS is that if you are working with a monitor on a single display, you’ll have an external keyboard and mouse. The solution in this case is to close the laptop and wake it up with the keyboard to bring back the single external monitor, controlled with the external keyboard and mouse.
Where it sounds like this utility would come in handy, is using the laptop’s keyboard and trackpad with an external display, without the laptop display enabled.
Yes, you're right. I use MacBook's keyboard and trackpad while it's connected to an external display. As a “one display only” guy, having two turned on is not ideal.