Must a lid for a pot used for steaming be vented?
If a lid doesn't have a vent and the pot does not have any sort of locking mechanism like a pressure cooker, then the pressure of the steam inside will lift the lid off the pot and steam will escape. That was the "dancing" of the lid that you experienced.  Venting holes in a lid just let some steam escape so the lid doesn't bang around, having a vent hole or not wouldn't make your idli's sticky.