react/no-implicit-ref-callback-return

React 19 interprets a value returned from a ref callback as cleanup. A concise assignment callback therefore has different semantics from earlier React versions:

<div ref={(node) => (instance = node)} />

Write a block body instead:

<div ref={(node) => { instance = node; }} />

The rule automatically fixes assignment and update expressions. It leaves a call expression such as node => register(node) alone because the called function may intentionally return a cleanup function.