One minute you’re working in GZDoom Builder’s Visual Mode, you fumble some keys accidentally, and now skies, dynamic lights, slopes, and 3d floors are all gone!!! What the heck!!! I would do this every once in a while and not know how to turn it back on. After some digging, I found that the TAB key toggles these features being rendered or not in Visual Mode.
Hopefully this info helps you out!