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Apple's recent macOS 10.13.4 update is causing some havoc among people using screen extenders like Duet Display and Air Display, as well as USB DisplayLink-connected monitors.
"Installing macOS release Version 10.13.4 will cause Duet Display stop functioning after the OS upgrade," the app's developer said in a support article. "We have alerted Apple to this issue and are working hard to find a resolution. These features continue to work as expected in 10.13.3."
Duet Display and Avotron's Air Display lets Mac owners turn an iPad into a secondary display. Items viewed on the iPad support touch interactions, and users can even simulate the MacBook Pro's Touch Bar for extra control.
"We are actively looking into workarounds so that Duet is not affected by the macOS bug, and will keep you informed," the Duet Display support article continues.
Avatron, the makers of Air Display report similar problems.
Air Display users: Please don't update your host Mac to macOS 10.13.4. It introduces severe WindowServer bugs that crash most inelegantly when you connect to a virtual monitor like Air Display (or its competitors). 10.13.3 still works fine. Hopefully Apple will fix 10.13.4 soon.— Avatron Software (@Avatron) March 30, 2018
Issues are also serious with DisplayLink peripherals like some docks and USB monitors, as installing macOS 10.13.4 may cause them to go black. Secondary features such as sound and Ethernet connections should continue to work, as they are based on USB features baked into the protocol.
"We have alerted Apple to this issue and are working hard to find a resolution. These features continue to work as expected in 10.13.3," DisplayLink said, echoing Duet's statements.
As a workaround, downloading DisplayLink 4.3 drivers should at least enable using screens on clone mode, though mirror and extended modes are still broken. "Cloning" simply duplicates the main screen resolution without any display controls, though it will scale or letterbox as needed.
Both display extenders using an iPad and the DisplayLink products relied on unsupported workarounds to function in macOS. Apple performed a great deal of work on the graphics in macOS 10.13.4 to implement eGPU technologies, so it stands to reason the problems are related to that effort.
Similar problems, related to Apple's Metal platform, were encountered with DisplayLink in macOS 10.12 Sierra. These included window server crashes, and corrupted or missing graphics in some apps and interface elements. Those issues were ultimately resolved with the release of macOS 10.13.