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`.
events: null,
      stores: null,
    };
  };
}
function use<T>(usable: Usable<T>): T {
  if (usable !== null && typeof usable === 'object') {
    
    if (typeof usable.then === 'function') {
      
      const thenable: Thenable<T> = (usable: any);
      
      const index = thenableIndexCounter;
      thenableIndexCounter += 1;
      if (thenableState === null) {
        thenableState = createThenableState();
      }
      const result = trackUsedThenable(thenableState, thenable, index);
      if (
        currentlyRenderingFiber.alternate === null &&
        (workInProgressHook === null
          ? currentlyRenderingFiber.memoizedState === null
          : workInProgressHook.next === null)
      ) {
        
        
        
        
        if (__DEV__) {
          ReactCurrentDispatcher.current = HooksDispatcherOnMountInDEV;