Oh, my dog Charlie does something very similar! Every time I walk into the room, he’ll stretch out like he’s just woken up from a nap, even if he’s been sitting there for a while. It always makes me laugh! I’ve read that it’s often a way for dogs to greet us and express how comfortable they feel in our presence.
It could also be that Luna’s just doing a little “I’m ready for attention” stretch, which is so sweet. Charlie definitely does it to get some extra love, so I wouldn’t be surprised if Luna’s doing the same thing. It’s one of those little quirks that makes me smile every time, even if it’s totally normal behavior!