Added test to improve support of top-down update behaviour

[arhelmus]

Hey, in recent issue we found test case which violates principle of top-down state update behaviour when state change propagates through mapStateToProps: #1397.

This PR is adding reproducible test-case to highlight this issue, I believe this is important due to statement in this part of codebase:

if (lastThrownError) {
    // It's possible that we caught an error due to a bad mapState function, but the
    // parent re-rendered without this component and we're about to unmount.
    // This shouldn't happen as long as we do top-down subscriptions correctly, but
    // if we ever do those wrong, this throw will surface the error in our tests.
    // In that case, throw the error from here so it doesn't get lost.
    throw lastThrownError
}

I would be grateful if you will help me to find how to fix this test case in react-redux or at least merge this test to make sure that issue will be fixed eventually.

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[RrNn]

hope this gets merged, so we can get a fix. I've been in a little hell because of this #1397 (comment) 👈 (first part) in an earlier version.

[markerikson]

If this test case is truly showing a problem with our implementation, then I would really like someone to take some time to investigate the actual cause of the problem, and add that to this PR. The failing test case is helpful, but kind of useless on its own.

I unfortunately don't have time to investigate this issue myself right now.

[timdorr]

I'm actually not sure what the problem is here. Why do we care if mSTP is called on a removed component? It should be a pure function, so there are no side effects to calling it an extra time. The actual render never happens, which is the important part.

[ykagan]

I'm actually not sure what the problem is here. Why do we care if mSTP is called on a removed component? It should be a pure function, so there are no side effects to calling it an extra time. The actual render never happens, which is the important part.

It becomes a problem when mSTP is written under the assumption that the props passed in are valid props for this component. For example its common to do something like

render () {
   ....
   state.items.map((item, i) => <WrappedComponent index={i} />)
}

in the parent component. and

const item = state.items[props.i];

in the child mSTP. This will cause an exception when item i is removed from items but mSTP is called anyway.