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From React 18 to 19.3: Concurrent Rendering, Actions, use(), ref as Prop, Activity, useEffectEvent, ViewTransition and Fragment Refs

React 18 laid the base for concurrent rendering (createRoot, batching, transitions), 19 brought Actions, use() and ref as a prop, 19.2 Activity and useEffectEvent, and 19.3 stable ViewTransition and Fragment refs. We add ViewTransition and a Fragment ref to Bookstore, apply React Router 8.5's deprecation, and upgrade a small React 18 project to 19.3 with the 18.3 warnings, the TypeScript codemod and manual fixes.