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`.
renderLanes = nextRenderLanes;
  currentlyRenderingFiber = workInProgress;
  if (__DEV__) {
    hookTypesDev =
      current !== null
        ? ((current._debugHookTypes: any): Array<HookType>)
        : null;
    hookTypesUpdateIndexDev = -1;
    
    ignorePreviousDependencies =
      current !== null && current.type !== workInProgress.type;
  }
  workInProgress.memoizedState = null;
  workInProgress.updateQueue = null;
  workInProgress.lanes = NoLanes;
  
  
  
  
  
  
  
  
  
  
  
  
  
  if (__DEV__) {
    if (current !== null && current.memoizedState !== null) {