Just a heads up for the macOS people: the 'Supplemental Update 2' recently released can cause the HDMI output to be garbled. This bug is not fixed in Catalina.
Workaround is to unplug/plug the HDMI connector.
See https://discussions.apple.com/thread/250516712 for more details
Workaround is to unplug/plug the HDMI connector.
See https://discussions.apple.com/thread/250516712 for more details