Interesting... I've had a similar issue, except the other monitor stays black but I do see the mouse moving around.
I gave up trying and have just been manually plugging in the HDMI cable
![Frown](images/smilies/frown.gif)
hoping I'll see some improvement in the next update (evdi seems to have been updated, so hopefully there'll be a driver update soon too).
Btw, i'm on a t460s and fedora 25 (with both GDM and the session running on X)