What is a Partitioned cookie?

The answer

A Partitioned cookie is stored separately for each top-level site on which an embed appears, as part of CHIPS, Cookies Having Independent Partitioned State. MDN describes it as double-keyed by the setting origin and the top-level page, and says the Secure attribute must be set. A chat widget can then remember a conversation on each host site without being able to recognise the same person across different sites. Browser support has rolled out gradually, so check it before relying on it.