I have same ping/purple screen issue as the Optiplex 7480 AIO as described here, with a different hardware (i5-7600 with HD 630). I found the Dell Motherboard layout in the support manual. If I run High Sierra with Clover I get normal display but external hdmi causes both screens to go black.
sonicman66
From Linux boot ubuntu 18.04.xxx I'm pretty sure. Hdmi-2 internal display.
xrandr
Screen 0: minimum 320 x 200, current 3286 x 1080, maximum 16384 x 16384
DP-1 disconnected primary (normal left inverted right x axis y axis)
HDMI-1 disconnected (normal left inverted right x axis y axis)
HDMI-2 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 510mm x 287mm
1920x1080 60.00*+ 60.00 50.00 50.00 59.94
1680x1050 59.88
1280x1024 60.02
1440x900 59.90
1360x768 60.02
1280x720 60.00 50.00 59.94
1024x768 60.00
800x600 60.32
720x576 50.00
720x480 60.00 59.94
640x480 60.00 59.94
HDMI-3 connected 1366x768+1920+0 (normal left inverted right x axis y axis) 521mm x 293mm
1366x768 59.79*+
1920x1080 60.00 59.94 24.00 23.98
1920x1080i 60.00 59.94
1280x1024 60.02
1152x864 75.00
1280x768 59.87
1280x720 60.00 59.94
1024x768 85.00 75.03 70.07 60.00
832x624 74.55
800x600 85.06 75.00 60.32
720x480 60.00 59.94
720x480i 60.00 59.94
640x480 75.00 66.67 60.00 59.94
640x400 70.01
This is the layout with just the internal Display.
xrandr
Screen 0: minimum 320 x 200, current 1920 x 1080, maximum 16384 x 16384
DP-1 disconnected primary (normal left inverted right x axis y axis)
HDMI-1 disconnected (normal left inverted right x axis y axis)
HDMI-2 connected 1920x1080+0+0 (normal left inverted right x axis y axis) 510mm x 287mm
1920x1080 60.00*+ 60.00 50.00 50.00 59.94
1680x1050 59.88
1280x1024 60.02
1440x900 59.90
1360x768 60.02
1280x720 60.00 50.00 59.94
1024x768 60.00
800x600 60.32
720x576 50.00
720x480 60.00 59.94
640x480 60.00 59.94
HDMI-3 disconnected (normal left inverted right x axis y axis