macOS High Sierra 10.13.4, the latest macOS version released on March 29, breaks support for many externally-connected displays, according to users of four popular app & driver providers.
Affected parties include DisplayLink, Duet Display, Air Display, and iDisplay. These companies provide apps and drivers that allow users to utilize multiple screen displays with their Mac devices, either via a USB or WiFi connection.
Only the external display's video feature was affected. External monitors with functionality such as Ethernet and audio were unaffected, only they didn't render anything on screen.
DisplayLink, Duet Display, and Air Display are now officially advising users not to update to the latest version of macOS if they wish to use their external displays.
"[macOS High Sierra 10.13.4] introduces severe WindowServer bugs that crash most inelegantly when you connect to a virtual monitor like Air Display (or its competitors)," said Avatron Software, maker of Air Display.
It is unclear if this is an actual macOS bug or a permanent change that Apple made to its OS without letting app developers know in advance.
If the latter, app makers will need to find a way around the issue and release app & driver updates that address the issue. In the meantime, countless of users who relied on this software to use two, three, or more screens with their main Mac device are advised to downgrade macOS to an older version.
Thanks to App Store policy, we can only change Air Display 3's app description, to warn that macOS 10.13.4 kernel panics when you connect to Air Display, when we submit a new version of the iOS app. But a new version won't be approved because the host crashes. Sigh.— Dave Howell (@DSHowell) March 31, 2018