Im having similar issue with my HS setup, dell latitute 3480, i5-7200U
Once the hdmi plugged in, the laptop display went blank, only could see the external tv
But, when i put my laptop to sleep & wake up again, both screen got display (hdmi still connected)
Even the hdmi could be plugged off & in (after sleep), the laptop display is fine
What could be the culprit?
Dell’s MacBook Pro IORegistry.zip
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Can I propose a new subforum be created for the new OpenCorePkg OpenCore front end being created by vit9696 and others, it is a fantastic piece of work:
Even at version 0.1 it runs my Mojave 10.14.4 setup very nearly flawlessly. It consists of a 10KB bootstrap BootX64.efi and a 200KB OpenCore.efi OS loader. All configuration is done using a very well documented config.plist