I was playing around with the structure block (cloning TNT as shown below ) and I found out that in spectator mode, or gamemode 3, the border lines that show where you're going to place your blocks is invisible in spectator mode, I also found out 2 seconds ago that they are invisible in gamemode 0 and 2 as well... I'm thinking this isn't a bug but in case it is, I decided to post this. Thinking about it more... It should be invisible in gamemodes 0 and 2 but I'm thinking it shouldn't in gamemode 3 because it's just creative without the ability to place blocks. So I'm not sure if this is a bug or feature request... you decide...