Nested `startTransition` call "takes over" and makes parent `startTransition` unable to track `isPending`.

Seb says it's a bug.

Repro:

https://codesandbox.io/s/pensive-breeze-rg70wn?file=/IndexPage.js:192-288

1. Click the button
2. `isPending` in `IndexPage.js` is `true`

However, `isPending` in `App.js` is `false`.

Expected: `isPending` in `App.js` is also `true`.
if (checkIfSnapshotChanged(inst)) {
    
    forceStoreRerender(fiber);
  }
}
function subscribeToStore<T>(
  fiber: Fiber,
  inst: StoreInstance<T>,
  subscribe: (() => void) => () => void,
): any {
  const handleStoreChange = () => {
    
    
    if (checkIfSnapshotChanged(inst)) {
      
      forceStoreRerender(fiber);
    }
  };
  
  return subscribe(handleStoreChange);
}
function checkIfSnapshotChanged<T>(inst: StoreInstance<T>): boolean {
  const latestGetSnapshot = inst.getSnapshot;
  const prevValue = inst.value;
  try {
    const nextValue = latestGetSnapshot();
    return !is(prevValue, nextValue);
  } catch (error) {