Many thanks to Ian Choong, who has confirmed that the beautiful, green and graceful damselfly that I had thought might be the Vestalis species, is actually the Neurobasis chinensis, female, also of the family Calopterygidae. This species is widespread in mainland tropical Asia and Sumatra.
I had needed that expert eye to confirm this and when it comes to Odonata of Peninsular Malaysia, he really is the expert here, having spent many years observing and studying them.
Since he has confirmed the identification of this species, I now notice the white pseudo-pterostigma on the hindwings too. This applies to the females of this species. I do hope to come across the male of this species one of these days. From the Guide Book, the male looks like he'll really make heads turn. It's definitely a wow! So I am keeping my fingers crossed and now have renewed motivation to keep scouring Langkawi for it.
Previously, I had been looking at the thorax and appendages; literally everything but the markings on the wings! And sometimes when you are looking for outstanding details in too many pictures of similar things for too long, they all become a blur and look the same! Yet now, the markings on the wings seem so obvious. I had been so blind to it.
And, as I would always say, learning never stop.