One solution is to apply the same factory method used in `Comments` to memoize the selector for each key. Unfortunately, I'm not sure how to re-use the same factory for both `Comments` and `CommentReactions`, so we will be memoizing it twice. This feels a bit dumb.
Since `CommentReactions` is almost-always** a child of `Comments` and using the same comment key, just get the calculated value from the parent through an optional prop.
** notifications uses it too, but I guess we can be inefficient there since it's not a component that's constantly updating.
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