I did not work on this Optiplex 7780 model before so considering any attempts on helping also were a learning curve for myself.  Another step to take is removing the drive and complete the OOBE on a different machine, then installing it back to the Optiplex....  
So brand new out of the box, this AIO machine was dropped on the floor, damaging the LCD panel. Upon powering it up, most of the display is glitched, but enough is still intact that I can see the Windows 10 OOBE is running. I'm assuming that other than the display, the machine is functional. Since the basic warranty didn't cover this type of damage and the owner bought another one right away, the damaged machine was gifted to me. I can't afford to replace the panel at this time, but I'd like to be able to use it with an external monitor until I can repair it. The issue is trying to figure out if I can disable the primary display, and so far no luck. When I plug in an external HDMI monitor, it is immediately recognized as a 2nd monitor to the right of the primary screen (can move the mouse to it), but otherwise it is black. I don't know if it is possible to move the OOBE to the 2nd screen. I don't know if it is possible to disable the built-in screen in the BIOS, because I cannot see more than a sliver of the BIOS interface when I hit F2 on bootup. The BIOS does not come up duplicated on the 2nd HDMI-connected screen. I even opened up the back of the machine and (after following the service manual guides) found and disconnected the ribbon cable for the display from the motherboard. It made no difference at all in my progress. The BIOS is not visible on the HDMI-connected display, and when Windows boots it still sees the external display as the 2nd screen (black, but can move the mouse around).  I don't know if connecting a 2nd display through the DisplayPort would be any different, but I don't have a DP monitor to test regardless. Does anyone have any ideas on how I might get this machine functional with an external display?
Optiplex 7780 AIO - Brand new, but broken screen