Very common on and around the many ponds I visit, at Kadoorie, Lions Nature reserve and at Tai Po Kau, where I found this mating couple
Yes, these are Demoiselles, and also yes, technically they are not mating at this point, as their bodies are not touching. This male dragonfly remains grasping the female's neck to avoid her mating with another male, until she lays her eggs, as seen below.
Here are a few close ups:
