the hidden source is the watchful heart (слэш)

Wei Wuxian smiles at him. Lan Wangji smiles back, helpless not to.
‘Don’t do that,’ Wei Wuxian says.
‘Do—?’
‘Smile like that,’ says Wei Wuxian, in a very how dare you!!!! sort of fashion. ‘How am I supposed to think when I know you can smile like that!’
‘Don’t think,’ Lan Wangji advises.
.
Wei Wuxian comes back to the Cloud Recesses for the winter months. He and Lan Wangji learn that emotional intimacy and physical intimacy are not (yet) quite synonymous.
They have time to figure it out.