An external display can be a good way of adding more USB-C ports to a Mac, but a strange bug can cause a lot of trouble with this otherwise excellent solution.

As I wrote back in November last year, letting an external display sleep can precipitate a kernel panic or other problem when the display goes to sleep, or the Mac tries to wake it up.

Maxwell @mxswd has this week pointed out another more serious problem with external display sleep: if you use that external display as a USB-C hub, when the display goes to sleep, USB hub power is disabled, shutting down and forcibly ejecting all peripherals connected to that display. Although this may stem from a bug (or inappropriate behaviour) in macOS management of external displays and USB-C devices, it also begs the question as to why any device acting as a USB-C hub should shut down power to connected devices when the display merely goes to sleep: surely, its USB-C hub feature should only power its ports down when the display itself is shutting down, not merely when going to sleep.

The solution remains to use the command tool caffeinate, or an app like Amphetamine, Lungo or Caffeine, to prevent the external display from going to sleep in the first place.

Hopefully Apple will change this potentially damaging behaviour in an update to Catalina.