React 19 useOptimistic Explained: Why the Git Rebase Mental Model Matters

React 19's useOptimistic hook is widely misunderstood as a general-purpose state management tool, but treating it that way leads to UI flickering and data sync bugs. The hook is better understood through a Git rebase analogy: optimistic updates act as temporary local commits layered on top of a server-sourced base state, which is always the source of truth. When a server action completes, React discards the local optimistic layer and re-renders from the fresh server response rather than merging changes. Developers must also ensure addOptimistic is called inside a startTransition or form action, as invoking it outside a transition boundary causes the optimistic state to disappear immediately. Additionally, useOptimistic offers no built-in error handling, so engineers must manually catch server errors and notify users, even though the UI automatically reverts when the base state goes unchanged.