The following problem just recently started happening after upgrading to gdm3 3.18.0-2. I have a Lenovo w540 laptop in a dock with two external monitors connected. I boot with the laptop lid closed. My two external monitors come up and the login screen is on the right side external screen. I select my user and enter the password and log in. Both of the external monitors go black and go into standby mode. I hear the fan on the laptop go off and it appears that the laptop has gone to sleep. I lift the laptop lid and the laptop wakes up and the monitors wake up and come on. I have been using this setup for a while and this did not use to happen.
I downgraded gdm3, gir1.2-gdm3, and libgdm1 to 3.14.2-2 and the problem went away. Gnome starts after logging in without putting the laptop to sleep.