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

Bug

**What is the current behavior?**

`isPending` is never set to true when calling `startTransition` within `useEffect`, but it *does* work properly when within a `useLayoutEffect`.

https://codesandbox.io/s/usetransition-useeffect-issues-p1j9s

Here's the correct behavior (accomplished via `useLayoutEffect`):

![good](https://user-images.githubusercontent.com/1500684/68164575-662d7b00-ff1a-11e9-9d02-71d7a22fd5cf.gif)

Here's the incorrect behavior (via `useEffect`):

![bad](https://user-images.githubusercontent.com/1500684/68164540-4c8c3380-ff1a-11e9-9191-d9c87ddecbed.gif)

Note the difference is that the opacity never changes to 0.4 (which is determined based on the `isPending` state).

**What is the expected behavior?**

I expect them to both behave the same (at least as far as the user can observe).

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

- react@0.0.0-experimental-f6b8d31a7
- react-dom@0.0.0-experimental-f6b8d31a7
setValue(value);
        } finally {
          ReactCurrentBatchConfig.suspense = previousConfig;
        }
      });
    },
    [value, config],
  );
  return prevValue;
}
function mountTransition(
  config: SuspenseConfig | void | null,
): [(() => void) => void, boolean] {
  const [isPending, setPending] = mountState(false);
  const startTransition = mountCallback(
    callback => {
      setPending(true);
      Scheduler.unstable_next(() => {
        const previousConfig = ReactCurrentBatchConfig.suspense;
        ReactCurrentBatchConfig.suspense = config === undefined ? null : config;
        try {
          setPending(false);
          callback();
        } finally {
          ReactCurrentBatchConfig.suspense = previousConfig;
        }
      });
    },
    [config, isPending],
  );