MacBook 12 Retina runs LG34UM95-P full resolution 3440x1440
For all who are asking: The MacBook 12 Retina can drive the 34UM95-P at full res./full 60Mhz NTSC with a Display1.2 Cable (USB-C to DisplayPort 3ft Cable) from Monoprice even when running on battery (one port taken).
while with the Apple HDMI Combo (USB/USB-C) Adapter it only runs at 50Mhz PAL/full res. of 3440 x 1440.
Both with the internal display active as well ...
All the BEST to ya`
sjh