Hi! I just discovered this bug. When I check the “Hide Viewport UI” check box in the Quick Settings menu it doesn’t hide any viewport UI elements. The only thing that is disparaging is the yellow outline of the selected viewport, but all the buttons and labels stay toggled on. It’s working fine in 4.1.1, but it seems to be broken in 4.2.1
Thank you for your bug report. I was able to reproduce the issue with ease. I have also entered the issue into our bud database so that we may further investigate and fix the issue in future UE4 builds.
Thanks again and hope you have a great day!