There has been a lot of work upstream in the graphics stack around MST, but not all of it has been merged yet. Would you be able to test with drm-next to see if the problem is resolved there? http://kernel.ubuntu.com/~kernel-ppa/mainline/drm-next/current/
If it's resolved there, there will need to some effort to bisect the particular commit that fixed it and backport it to an Ubuntu kernel.
There has been a lot of work upstream in the graphics stack around MST, but not all of it has been merged yet. Would you be able to test with drm-next to see if the problem is resolved there? kernel. ubuntu. com/~kernel- ppa/mainline/ drm-next/ current/
http://
If it's resolved there, there will need to some effort to bisect the particular commit that fixed it and backport it to an Ubuntu kernel.