Lan Wangji and Wei Wuxian have been at peace with everything for the past couple of weeks. Yes weeks. They can't stay peaceful for that long.
The sword that Wei Wuxian held in his younger years beside Lan Wangji's Bichen. The name of that sword was Suibian yet the flute named Chenqing was never far from Wei Wuxian.
Wei Wuxian was up and out before sunrise and was practicing his flute calming songs. This made sure Lan Wangji wouldn't wake past Mao hour but he succeeded to make him stay awake past Hai hour. So this was a new thing for Wei Wuxian to prey out of Lan Wangji. More sleep. This excited Wei Wuxian more than anyone could know. The tunes flowed one into the other until Lan Wangji finally woke up.
Wei Wuxian went quiet so Lan Wangji couldn't catch his little deed. He slipped into the bathhouse without being seen but of course Jiang Cheng was there with Lan Xichen. Lan Xichen is Lan Wangji's twin brother.