This morning I had some time spent trying to get my Dell XPS 13 9550 to output to three displays.  Using the thunderbolt/USB-C Dell 3100 dock, and HDMI cable/monitor and a DP to mini-DP/monitor.  But my question is, during this fight, I attempts to run the laptop with the lid shut.  Outputting to the two external displays, powered up by power button on the dock.  Long story short it did not work.. but what's weird is that I raised the lid ever so slightly and could tell that it was on.
Now if the system senses the lid when it shuts to suspend.. why not in this case?  Could this have anything to do with the hybrid graphics?