dillon1000/react

Improve error boundary handling for unmounted subtrees (#20645)

A passive effect's cleanup function may throw after an unmount. Prior to this commit, such an error would be ignored. (React would not notify any error boundaries.)

After this commit, React will skip any unmounted boundaries and look for a still-mounted boundary. If one is found, it will call getDerivedStateFromError and/or componentDidCatch (depending on the type of boundary). Unmounted boundaries will be ignored, but as they have been unmounted– this seems appropriate.