// Do two React components need a global store?
// Teaching excerpts; see the explanation and boundaries on the episode page.

// One value. Two readers.
const [tab, setTab] =
  useState("details");
return <>
  <Toolbar tab={tab}
    onChange={setTab} />
  <Panel tab={tab} />
</>;

// Share farther when necessary.
<TabContext value={tab}>
  <PageContent />
</TabContext>