I have a 2015 MacBook Pro with Retina and a LG Ultrawide display (3440 x 1600). I had no issues with the native resolution until upgrading, now it will only display a default resolution of (2560 x 1080). It intermittently worked when I enabled DisplayPort 1.2 on the monitor but since removing the connection its back to the lower res display. I am using a DisplayPort cable for the connection to the monitor.