Laptop screen not working when an external monitor connected

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Laptop screen not working when an external monitor connected

I have a MacBook Pro 16 running Catalina 10.15.5 and I got a weird behaviour all works pretty well if I don't connect an external monitor, but once, I connect an external monitor (with a connector VGA/USB-C), screen on my laptop gets as it is shown in the picture. Any idea or someone had a similar issue? I don't think that it is an issue with the connector because screen that it gets messed is the one on my laptop and the external one gets the proper input.

MacBook Pro with Touch Bar

Posted on Jul 1, 2020 1:44 AM

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Answer 1

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pataquer Author

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Jul 1, 2020 2:24 PM in response to Luis Sequeira1

Not sure what caused it (last update?) BUT going to settings > display and setting up displays as mirrors and setup them again as they were before seems that solve the issue. So weird... anyway, thanks Luis for trying, and leaving my solution here, if someone has the same issue in a future.

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Answer 2

Jul 1, 2020 3:36 AM in response to pataquer

Please do this test: with the external display connected, take a screenshot by pressing Command-Shift-3.

There should be two files on your desktop, one for each display.

If necessary, disconnect the external display; then check if the png file corresponding to the internal display shows the same corrupted image or if it looks correct.

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Answer 3

Jul 1, 2020 7:44 AM in response to pataquer

That means that the OS is internally processing the screens correctly, and the issue is just in displaying.

I don't suppose you have a different adapter available, or a different display?

I also have a 16" MBP, and connect to external display almost permanently (doing it now), but via HDMI.

I am not sure if there is something in the adapter. I may try hooking up a VGA display, but cannot test until later today or tomorrow.

My guess is on something weird in the adapter, but it's just that, a guess.

If you have access to a different adapter/display, it may help narrowing it down.

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Answer 4

pataquer Author

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Jul 1, 2020 12:12 PM in response to Luis Sequeira1

No, nothing to test to validate it. Seems that I will need to go to a retailer to check it with another screen.

I wonder how a failing adapter can mess with the laptop screen, sounds weird, because which data it is sent back to mess with it?

Anyway, thanks for your help.

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Answer 5

pataquer Author

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Jul 1, 2020 7:31 AM in response to Luis Sequeira1

Even that issue persists when I plug the external device (external screen looks good, and laptop screen corrupted), taking screenshot as you said, both screenshots looks good, as it should be.

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Answer 6

Jul 1, 2020 12:20 PM in response to pataquer

Yes, it is strange. It may not have to do with the adapter, maybe some error in the GPU display code, I really don't know.

Just trying to test ideas looking for a eureka moment...

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Answer 7

Jul 1, 2020 3:17 PM in response to pataquer

Glad that you got it working and thanks for posting the solution.

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Laptop screen not working when an external monitor connected

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