nouveau.noaccel=1 seems to fix the problem.

But your "modesetting" DDX comment made me dig into the Xorg config.
I was using the modesetting DDX and added the following config:

$ cat /etc/X11/xorg.conf.d/20-nouveau.conf 
Section "Device"
    Identifier "Nvidia card"
    Driver "nouveau"
EndSection

That also seems to fix the problem.

I'm saying "seems" because I have not done a lot of testing.

I will update this bug once I have made sure this is "user error" (no nouveau
config).

Oh and mesa is at 17.2.2-2.fc26.x86_64.