if and unless tags are discussed here, but it doesn’t mention all of the things they can and can’t do. There are a few examples of unless="display" on these forums, but they’re in totally unrelated threads, so there’s no way you could have found them. (And apparently both were posted by me. Huh.)
The easiest way to figure this out for yourself would have been to look at OpenFL’s include.xml file, which uses unless="display" a couple times. (This is how I figured it out those previous two times.)
This time, I forgot about the easy answer and instead dug through the source code. Don’t try this at home unless you already know your way around Lime, or you have lots of time to kill.