External display turning off in clamshell mode macOS Tahoe 26.5

Did you upgrade directly from Sequoia?

macOS has become progressively less tolerant of video data packet loss due to sub-par cables. There have been several reports of setups that worked in Sequoia not working in Tahoe, with nothing changed but macOS. 

Best recommendation at this point is to try a new cable. Be sure to use high quality cables – HDMI should be labeled Ultra or Premium, USB-C cables should be rated for high-speed data transfer, etc. Also, cables should be as short as possible (≤1 m for most cables, <0.5 m for Thunderbolt 5 cables). That random cable you found in a drawer might or might not work. 

FWIW, I have two 16” MacBook Pros (M1 Pro and M4 Pro), both work fine in clamshell mode (which is how I typically use them) via a Stone Pro TB3 dock connected to an LG 5K:2K display with TB3. Both Macs are running 26.5.

Hi,

I have just run into the same issue, but thankfully I have a solution that works (at least for me, aka the classic works on my machine).

Here is my setup:

Laptop: Apple MacBook Pro 13", M1 Processor, 2020 edition

External monitor: Apple Studio Display, purchased back in 2022 (I think, or 2023).

OS: Mac OS Tahoe 26.5.1 (25F80)

To resolve the issue, I had to run this command in the terminal:

sudo pmset -a disablesleep 1

After running that, I was able to close the lid on my laptop and get it to work in clamshell mode.

I do not use clamshell mode habitually, but as I had a keyboard, mouse and display, I tried this (14" M4Pro MBP), and it also did not work. There may have been some change in 26.5. You should add your report to feedback:

https://www.apple.com/feedback