I just updated to 17.10 (Artful Ardvark), thereby making the switch to Gnome Shell. I found that when I logged in, it had set up Gnome Shell's "Display Scale" for the screen on my Lenovo ThinkPad Carbon X1 to 200% by default (leading to some weird desktop interface artifacts). I think this is because it thinks anything with ultra high res (hidpi?) should render this way. No big deal. I was able to change it to 100%, which is how I like it (crisp, small fonts). However, after that, when I tried to connect to another monitor (specifically an Apple 27' monitor), the laptop screen went back to 200%. Strangely though, when I go to try to open the Settings app to change this, the keyboard and/or screen locks up until I unplug the monitor. How do I get my monitor to run at the 100% scale when connected to an external monitor?