Here is the solution that worked for me on Alienware 15 R3 with NVIDIA 1070. You need to tell the computer use high power graphic card when you use steam.
You can see how to do that on this video beginning from 1:11: https://www.youtube.com/watch?v=xZvlxzeHph4
Settings, Display, go bottom, Graphic Settings, Graphic performance preference, (Find steam directory and address the steam) add it, click options, and choose your NVIDIA card.
That way windows is not going to shift it based on its mind and it will always use NVIDIA.
