While I use a MacBook Pro for work, I use an M2 MacBook Air for personal projects around the house. I have used a Plugable Thunderbolt dock for the past few years to run the display for my gaming PC, work Mac, and any other system I may have sitting around. The monitor I use is a Samsung CRG9 49" "super" ultra wide. This size monitor is quite nice for a single monitor setup and allows me to have a good spread of windows throughout the day.
I was interested in using my M2 Air in clamshell mode to drive the CRG9 at its full resolution of 5120x1440, pretty big, but not quite 4K levels of pixels. The behavior I was witnessing was quite odd, with the M2 lid open with the laptop screen on, the monitor was able to drive the full resolution of the monitor just fine, but in clamshell mode the monitor was clearly getting a signal, but no video was displayed. The first troubleshooting item of course would be to verify that the system was not going to sleep with the lid closed, that would very much be an issue for clamshell mode, but this wasn't the cause. The M2 Airs have already had their fair share of criticism for their display support shortcomings, but as I only have one monitor and the resolution is with in the spec of the system, I was at a loss. I was curious if the Plugable dock might be a cause of the issues since it was in line from the M2 to the monitor, I contacted Plugable support and they quickly were able to discuss the issue with me via email, even though I had purchased my dock a few years ago.
Talking with Plugable support, we weren't sure what the issue might be, the M2 should be displaying the full resolution of the CRG9, their dock should handle that without an issue, and in fact does on a daily basis with my MacBook Pro. The CRG9 has a neat feature called picture by picture (PBP) mode, where it can take two different system inputs and display them side by side, the equivalent resolution of two 27" monitors next to each other. My MacBook Pro from 2018 and before the Monterey operating system would not display the full 5120x1440 resolution of the CRG9 but it would display to two monitors at 2660x1440, so I was able to have two virtual monitors on one physical system. I noticed that the M2 would do proper clamshell mode when one of the monitor inputs was set to the Plugable dock using PBP mode, that made me more inclined to believe that the issue was one of resolution. Plugable support suggested an update to the dock firmware that might help with the issue and since I didn't have a system that would be able to update the firmware, they offered to send me a dock with updated firmware to see if that was the issue.
After receiving the dock with updated firmware my testing showed that it did not actually fix my issue! I was a bit surprised by this and wanted to verify that the M2 actually did support clamshell mode at a 4K resolution. Hooking up the Plugable dock via HDMI to my TV clamshell mode worked just fine! So the computer supported it, but why not on my monitor? Well, here is where a few paragraphs could have just been a bolded line at the start of all of this. The CRG9 supports a maximum of 120Hz refresh and while this hasn't been an issue for my MacBook Pro nor my Windows gaming PC I did notice that even when the M2 supported the full resolution of the monitor with the lid open, I was having to set the display option in MacOS to be 60Hz. The CRG9 also has AMD's FreeSync built in. FreeSync is a variable refresh rate (VRR) technology that can match the refresh rate of the monitor to the actual frame rate the graphics card on a computer is outputting, so if your graphics card is being able to output 75 fps, the monitor will recognize that and sync that at a 75Hz refresh. The CRG9 also has the ability to fully turn off FreeSync and manually set the refresh rate of the monitor to be 60Hz, 75Hz, or 120Hz. Realizing this I set the output to 60Hz without VRR and the M2 in clamshell mode started working as expected!
So, why the post about this? When researching what the potential issue was I never came across any information about VRR and M2 Airs specifically, documenting some of my issues just might help someone else who doesn't have a question about how many monitors are supported on the M2 Air, or what the maximum resolution is. Ensuring the display is set to 60Hz is incredibly important when using an M2 Air in clamshell mode (at least in very late 2022 when this post was written running the Ventura operating system). It is also a great example of red herrings when troubleshooting technology issues. Remember when I tried PBP mode at a lower resolution and clamshell mode worked fine? Well, PBP mode sets each virtual display to 60Hz (without explicitly telling you). So it didn't work because it was a lower resolution, it worked because the monitor was 60Hz, which the M2 required!.