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
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)