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  • open MacBook Pro + connected external display -> no power cable required
  • closed MacBook Pro + connected external display -> power cable required

Why? Why not allow or require it in both cases? Why require the power cable in the case, in which the battery will last longer?

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Why? Why not allow or require it in both cases?

The default behavior when you close the lid on your MacBook is to put it to sleep. According to Apple:

To further expand on this point, let's look at the default behavior when the lid is open(ed) - the MacBook is awake. This makes sense as this is the generally accepted method of using a notebook. The external display, and USB ports are also, by the inherent nature of the computer being awake, functional.

You can save energy by putting your Mac to sleep when you're not using it. When your Mac is sleeping, it's on but consumes less power. And waking your Mac from sleep is quicker than starting it up after it's been shut down. To put your Mac to sleep immediately, do one of the following:

  • Choose Apple () menu > Sleep.
  • If you use a Mac notebook computer, close its built-in display.
  • Press Command (⌘)–Media Eject(⏏).
  • Tap the power button on your computer.

(Emphasis mine)

Having the AC adapter connected is just one of the components needed to override the default behavior; you need to have the AC power, keyboard and mouse connected, as well as a monitor. See Use your Mac notebook computer in closed-display mode with an external display for more details.

Why did Apple do it this way? Ultimately, only they can give you the official answer, but a little deduction tells us that if closing the lid puts the machine to sleep, how would it know to stay awake if the user wanted to operate in closed-clamshell mode? Answer: attach a monitor and the AC adapter.

Can you override the default behavior

Yes. Using pmset

To disable wake/sleep on lid state:

sudo pmset -a lidwake 0

To undo the last setting

sudo pmset -a lidwake 1

Though you can do this, I don't recommend it, because disabling this feature means when you close the lid on your laptop it remains awake; even when you pack it in your laptop bag and put it in the overhead compartment. It (the battery) will most likely be DOA, plus it tends to get quite hot in that scenario, shortening the life of the battery.

  • Thank you @Allan. I think you give an answer to what, but not to the why. You explain what needs to be done in order to use the mac in closed-clamshell mode, but not why they decided that. I mean, it should be possible to override the default behavior by having a keyboard and mouse only. – AlexBG Aug 25 '17 at 13:20
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    The why is explained in my very first sentence - it's the default behavior. And yes, you can override the default behavior. – Allan Aug 25 '17 at 13:28
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    Claiming someone is being emotional is just a way to attack the other person. Whether I am emotional or not has no bearing on whether I am correct. This logical fallacy is commonly known as tone policing, and actually is an emotional argument, not a logical one. – trlkly Aug 25 '17 at 19:52
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    The OP did not change the question. He pointed out that you did not fully answer it by pointing out what was missing. You did not actually answer the question "why." The word "why" means "What is the logic behind the decision?" You did not answer that question. You simply say "it's the default." If you asked me "why is this word offensive" and I just said "It's the default," you would not have answered my question either. – trlkly Aug 25 '17 at 19:54
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    And you are doing something that should not be done on a Q&A site. You are arguing that your answer is correct, rather than trying to fix it. That is using the meta to try and get points rather than making sure your answer is the best it can be. It is a downside of gamification, so I choose to downvote this to try and stop this anti-pattern that makes the site less useful. I can't tell if your answers are really the best, or you just argued your way into getting people to think it was. – trlkly Aug 25 '17 at 19:57
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The why only Apple can answer. There's no technical reason why it cannot be done.

However, I would guess that the reason is that if you have an open MacBook Pro, you know that you're using it and draining energy.

If you have a closed MacBook Pro, the average home user might think that the laptop is "turned off" and not consuming energy. I.e. you could create a situation where the user runs low on battery even though they thought the system was passive.

  • Interesting perspective... – AlexBG Aug 25 '17 at 12:17

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