Wild guess: The editor cannot decide between displaying scroll bars or not. It first decides that it needs a vertical scroll bar, shrinks the level area accordingly, then notices that it actually doesn't need the scroll bar and forgets to enlarge the level area again.

. This all stems directly from my own experience as a player: Such objects have been very rare, so when I first encountered it, I wasn't even aware that NeoLemmix supported triggered animations. So I tried for half an hour to find out, what this gadget actually does and never found anything! 