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`.
) {
      console.error(
        'Internal React error: Expected static flag was missing. Please ' +
          'notify the React team.',
      );
    }
  }
  didScheduleRenderPhaseUpdate = false;
  
  
  thenableIndexCounter = 0;
  thenableState = null;
  if (didRenderTooFewHooks) {
    throw new Error(
      'Rendered fewer hooks than expected. This may be caused by an accidental ' +
        'early return statement.',
    );
  }
  if (enableLazyContextPropagation) {
    if (current !== null) {
      if (!checkIfWorkInProgressReceivedUpdate()) {
        
        
        
        
        
        
        
        const currentDependencies = current.dependencies;
        if (