But where does that state live? I imagine GP would like to live on the `Cart` component, but since the Add to Cart button isn't a child of the Cart component, y

But where does that state live? I imagine GP would like to live on the `Cart` component, but since the Add to Cart button isn't a child of the Cart component, you would have to "pass up" some connector. Alternatively you would just have the state live in a common ancestor of both (or at the Root) but I imagine that GP doesn't like that either (global state).