I connect a second monitor to my machine via mini Display Port. Most of the
time there is no issue doing this, but around 1 in 4 times I connect the
additional monitor it stays in power saving mode.

I have to bring up the Displays panel in gnome settings, change that screen
(which does show up) to be secondary, then press left and enter to cancel the
changes, at which point the screen wakes up.

Alternatively I can unplug and replug the mini display port cable until the
screen wakes up, but this is unreliable.

Hardware, laptop: Lenovo T550, monitor: Dell U2913WM

I've had this issue on both Gnome 3.16 from main repos and 3.18 from gnome
factory