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**Do you want to request a *feature* or report a *bug*?**

Bug

**What is the current behavior?**

Referring to [this twitter conversation](https://twitter.com/milankinen/status/1161718618633248768?s=20), it seems that `use-subscription` can cause "UI tearing" in some random cases due to a (possible) race condition w.r.t. the combination of `subscribe` and `getCurrentValue` in internal usage.

Here is a minimalistic application demonstrating the behaviour: https://github.com/milankinen/use-subscription-tearing-demo

**What is the expected behavior?**

I'd expect counters in the example application to *always* render same number.

**Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?**

Tested React version: 16.9.0 (haven't tested with older ones)
OS: OSX 10.14.6
Hardware: MacBook Pro (Retina, 15-inch, Mid 2015) 2.8 GHz Intel Core i7 & 16GB RAM
Browser: Chrome Version 76.0.3809.100 (Official Build) (64-bit)



@bvaughn 
if (nextProps.observed !== prevState.observed) {
          return {
            observed: nextProps.observed,
          };
        }
        return null;
      }
      componentDidMount() {
        log.push('Parent.componentDidMount');
      }
      componentDidUpdate() {
        log.push('Parent.componentDidUpdate');
      }
      render() {
        return <Subscription source={this.state.observed} />;
      }
    }
    const observableA = createBehaviorSubject('a-0');
    const observableB = createBehaviorSubject('b-0');
    let renderer;
    act(() => {
      renderer = ReactTestRenderer.create(<Parent observed={observableA} />, {
        unstable_isConcurrent: true,
      });
    });
    expect(Scheduler).toHaveYielded(['Child: a-0', 'Grandchild: a-0']);
    expect(log).toEqual(['Parent.componentDidMount']);
    
    act(() => {