Comment 8 for bug 1723025

I have a very similar issue in 17.04. The external display is treated as the second screen, and login screen's main area goes to laptop's inbuilt screen even though the lid is closed. This started in 17.04, was never an issue in 16.04.

External monitor connected through HDMI is still functional. It shows the background image with dots overlay at login screen. There are two workarounds:

1. If I open and close laptop's lid, the login screen switches over to external display.
2. If I move the mouse towards right, as it enters the second screen area, login screen comes to external monitor.

If either of these workarounds help in 17.10, probably it's a bug carried forward from 17.04