I just updated macos tahoe yesterday, and now the clamshell is not working

<< reducing the refresh rate from 60 Hz to 50 Hz in the monitor settings. >>

that suggests that the increasingly demanding newer versions of macOS may be finding fault with the connection to the display. If it makes any transmit errors, the display sometime drops out.

what make&model external display(s)?

what cables, adapters and docks are being used to make the connection? how long are the cables?

mircea89 wrote:

I have the same issue with Macbook Air M1. After update, clamshell mode doesn't work anymore.

I lowered refresh rate to 50Hz, even lowered resolution and refresh rate to 30Hz.

The external monitor's screen goes black after closing the lid. It's power light stays green (meaning it receives video signal), but it doesn't show anything, it is just back-illuminated.

Figured a solution:

When entering clamshell mode, it changed the resolution/refresh rate to one not supported by my external monitor, even though I selected a supported one beforehand.

Solution: acces Mac through Teamviewer while in clamshell mode and change resolution/refresh rate. Stupid it displays/changes resolution or refresh rate like this, to not supported ones and not being able to retain selected ones.

Serendipity_ wrote:

I changed my refresh rate setting and it worked.

It’s hard to tell whether this is an issue with the new operating system or just a one-off glitch.

Good computing Serendipity_ ! Appreciate your follow up to the issue.

Ok, then -- cables seems to be under control.

Is the macBook Pro on its power adapter when you close it?

switching from 60 to 50 Hz worked for me!!! Thank you

Serendipity_ wrote:

I usually use my Mac in clamshell mode, but after updating macOS yesterday, I came home from work today and suddenly clamshell mode isn’t working. Why is this happening? any solution?

So this is a new issue with Tahoe—

To enable MacBook clamshell mode, you must connect your MacBook Pro to an external display, a power adapter, and an external keyboard and mouse.

Did you try all variations, different cable connection, different port, etc...(?)

same here - just updated to Tahoe 26.1, MacBookPro M2, stopped working in clamshell mode with Dell monitor and a good expensive USB-C cable. This very same setup worked like charm for years, with older MacOS'es.

Lowering display refresh rate to 30Hz didn't help.

It's obviously a bug in Tahoe.

I bet they were trying to fix another bug, where MacBook would not fall asleep in clamshell mode sometimes, and would burn through the battery. But must have overdone it :)

I have the same issue with Macbook Air M1. After update, clamshell mode doesn't work anymore.

I lowered refresh rate to 50Hz, even lowered resolution and refresh rate to 30Hz.

The external monitor's screen goes black after closing the lid. It's power light stays green (meaning it receives video signal), but it doesn't show anything, it is just back-illuminated.

I am running a Macbook Pro 2021 M1 and have been using it in clamshell mode for years on a Dell thunderbolt dock attached to a Sony OLED TV. No issues at all, then updated to macOS 26 and clamshell mode stopped working. I did a full wipe and set up as new. same issue is happening. I can plug into dock and get the screen mirroring with clamshell open but if I close it, it's as if the mac will not wake from sleep. I have plugged into hardwired keyboard and mouse and still cannot get it to show a display. Tried the refresh rate from 60 to 50 people are talking about and that did not work.

<< Dell thunderbolt dock attached to a Sony OLED TV. >>

My Sony, TV, 4K capable, used to drop out from time to time when using it with a Sony BlueRay player. I could occasionally get it to work by reversing the cable end-for-end, but that was not reliable enough.

Then I started looking into the rats nest that is HDMI cable specifications. When I replaced the stock cable with a PREMIUM cable, there has not been a single drop-out since. This is the essence of what I learned:

HDMI cables you want for HDMI-only Displays (higher resolutions than 720p TV sets) are marked as Certified with an anti-counterfeiting tag and are labeled:

"PREMIUM High Speed HDMI cable" or that + "with Ethernet" (up to 4K at 30Hz) --OR--

“ULTRA High Speed HDMI cable" or that + "48G" (supports higher resolutions and backward-compatible)

Cables with No Certification tags are good for your standard 720p TV set, and not much more.

HDMI was invented for HD TV sets. it works great at its original resolution of 720i or 720p. At higher resolutions, it quickly develops issues that are complex to solve, and the cables and adapters required to solve are NOT intuitive.

I do agree with you that the HDMI cable is very important, I have had my share of issues inthe past with game consoles etc. But the cable I have is HDMI Cable 2.1 and supports up to 8K. Also I have been using this same cable with no issue for last year and half. In fact I am on my TV now with using my macbook pro mirroring to the TV but with the clamshell open and it working flawlessly. If I close clamshell it will not function.

FYI, clamshell mode is working normally on my M4 Pro MacBook Pro with Tahoe.

Any chance you’re relying on DisplayLink to drive your display(s)?