Dear Client, you asked whether the lap of a seated human constitutes common property. In a word: it does not. In more words, as is the custom of this profession, it constitutes a revocable licence, granted by the human and exercisable by the cat at any time of the cat’s choosing.
Where a lap is vacated while occupied, the cat may claim the displaced warmth under the Doctrine of Residual Heat. Where the human stands up suddenly, the cat should object, vocally. We recommend this opinion be read aloud, in a low voice, while standing on the relevant knees.