Touch screen still active when connected to surface dock, and using external monitor. Why?

[Misterb]

I have the new Surface Dock with a SP4. I have an external monitor, and I have it set to only project the display on to the external monitor. The SP4 display is black, but I noticed I can touch the screen, and it responds. I don't like that, because it means I can't leave the cover on or the keyboard buttons press all over the screen and make it jump around, even though I'm not touching the device.

Any ideas?