...but can be "revived" by slight click-n-drag. Not a show stopper, but had me confused initially, and will definitely discourage the less persistent/enthusiastic newbie users. Same behavior on two of my scenes \, [2\].
COMPUTER: MacBook Pro
OS: OS X 10.9.5 (Mavericks)
BROWSER: Chrome, Version 45.0.2454.85 (64-bit)