Nexus app live streaming to YouTube and Facebook both returned videos with thick black borders as seen in the attached screenshot from Facebook:
The YouTube videos show the same borders. But when recording to the device storage ("INTERNAL MEMORY / DCIM / NEXUSSCREENRECORD" on Android) rather than live streaming, there are no such significant borders.
The recording device's display resolution is 3040x1440 pixels with aspect ratio 19:9. This wide aspect ratio could explain the horizontal black borders on top and below the streaming video (letterboxing) in 16:9 as in 4:3 ratio players. Though the black borders to the left and to the right of the video (pillarboxing) indicate that the Nexus app streams in 4:3 aspect ratio (looks like it in the screenshot too) und thus the 16:9 ratio YouTube and Facebook players add pillarboxing. Is there a possibility to remove these borders (change aspect ratio of Nexus app live stream to 16:9) in a future release of the Nexus app?