When you pass an unmemoized selector to useSyncExternalStoreExtra, we
have to reevaluate it on every render because we don't know whether
it depends on new props.
However, after reevalutating it, we should still compare the result
to the previous selection with `isEqual` and reuse the old one if it
hasn't changed.
Originally I did not implement this, because if the selector changes due
to new props or state, the component is going to have to re-render
anyway. However, it's still useful to return a memoized selection
when possible, because it may be the input to a downstream memoization.
In the test I wrote, the example I chose is selecting a list of
items from the store, and passing the list as a prop to a memoized
component. If the list prop is memoized, we can bail out.
Andrew Clark
fd5e01c2e0