Xbox One - swapchain is not 1080p
As you can see the jags in the attached pictures, the videos in mkv (not tested with other containers) seems to be rendered in 720p and then upscaled in the Xbox One final buffer.
Video: MPEG4 Video (H264) 1920x816 23.976fps [V: Captain.America.Civil.War.2016.BluRay.1080p.AVC.DTS-HD.MA 7.1 x264-ETRG [eng] (h264 high L4.1, yuv420p, 1920x816) [default]] Audio: DTS 48000Hz 8ch 12288kbps [A: DTSMA -ETRG [eng] (dts-hd ma, 48000 Hz, 7.1) [default]]
Edited with mediainfo if needed: Captain.America.Civil.War.2016.BluRay.1080p.AVC.DTS-HD.MA_7.1_x264-ETRG.mkv.MediaInfo.txt