Glossary

Cookie jar

The store a browser keeps cookies in. Everything sharing one jar shares one identity, which is why isolation is a property of the jar and not of the tab.

Also called: cookie store.

The cookie jar is where a browser keeps the cookies a site has set, keyed by domain, path and the flags that came with them. It is not the only thing a site can write — local storage, IndexedDB, the HTTP cache and service-worker registrations all persist too — but it is the one people think of, and its behaviour is the model for the rest: everything that shares a jar shares an identity. Two tabs share it. So does a second window. An incognito window gets a fresh one, and then shares that fresh one with every other incognito window.

The operational consequence is that separation has to happen at the level of the store, not the tab. In Argus the store belongs to the profile, which is backed by its own user-data directory rather than a partition inside a shared one, so a site reading the identifier it wrote for one account finds nothing at all in the next. A jar can also be filled deliberately: a saved session assigned to a profile is seeded at launch, which is what cookie sets are.