This is easily fixable by using the display options and setting the external monitor again as primary. It's mighty annoying and drops the quality perception of the OS to the floor. Nvidia drivers.

inxi -G
Graphics: Card-1: Intel Haswell Integrated Graphics Controller

Card-2: NVIDIA Device 11e1
Display Server: X.Org 1.18.4 driver: nvidia
Resolution: 2560x1440@59.95hz, 1920x1080@60.02hz
GLX Renderer: GeForce GTX 765M/PCIe/SSE2
GLX Version: 4.5.0 NVIDIA 375.26