There seems to be some software issue in macOS that is affecting the use of an external display on Apple’s new M1 Mac computers. It appears to be intermittent for many, making troubleshooting more difficult. Apple has been made aware of the problem and is working on a solution that will come in a future macOS update. Knowing details about the configurations most likely to trigger the display trouble could help M1 Mac owners.
The M1 MacBook Air and MacBook Pro allow the use of one external monitor with up to 6K resolution through the USB-C port. The M1 Mac mini supports two monitors, one at up to 6K using a USB-C port, and one up to 4K using the HDMI 2.0 port. Those are the official recommendations, though more monitors can be connected by using a DisplayPort adapter and monitors which allows daisy-chaining multiple displays together, one connected to the next. However, this will limit performance, which is likely why Apple recommends at most two monitors for the Mac mini and one external display for the MacBook laptops.
The issue with external monitors that seem to be affecting some M1 Mac users, seems to be connected to the use of an ultra-wide or super-wide external display, according to Apple. This leads to intermittent flickering, flashing, black screens. A few Apple Discussions posts claimed to have login problems, as well. Due to the unrepeatable nature of the problem, the login issue may not be related to the display bug. According to a few posts on MacRumors Forums, an app called SwitchResX may solve the problem without waiting for Apple’s fix to arrive.
Troublesome Displays For Apple M1
The monitors that seem to have issues have wider than normal aspect ratios, which is to say, these displays are not simply larger, but wider in relation to the width. For example, displays with a 5120 by 1440 pixel resolution is more likely to have the problem, while a 3440 by 1440 pixel screen is unaffected. Ultra-wide monitors that have a software split-screen function do not seem to work either. Strangely some users with 5120 by 1440 resolution do not experience this issue and some seem to think it only happens when running certain apps. It is clearly puzzling for owners of super-wide monitors and M1 Mac computers. To find out the maximum resolution possible with a monitor, according to macOS, open the System Information app and check under ‘Graphics/Displays.’ Checking ‘Thunderbolt’ may provide some useful information also, since the M1 Mac uses that technology for higher resolution external displays.
Apple is working on a solution and will have more resources than users to analyze this and narrow down exactly what is causing this display issue. As the M1 Mac is supposed to support up to 6K resolution, it isn’t a matter of overloading the graphics processor and more likely was some assumption made about the aspect ratio of external monitors somewhere along the way in macOS. As soon as Apple tracks down the errant bit of code, it will be resolved and a new macOS update will give trouble-free use of ultra-wide and super-wide monitors.
Source: Apple, Apple Discussions, MacRumors Forums, SwitchResX