Mail has a bug when using the application in full screen view. The blur/tint layer over the message list and message is extending too far down and covering the top portion. The columns display correctly when taking it out of full screen view and then returns when back in full screen. Screenshots of both below, Full screen first.
I didn't have this issue with 27 public beta 2, just started occurring with public beta 3.
I didn't have this issue with 27 public beta 2, just started occurring with public beta 3.