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`.
let hasBeenInitialized = initialValue != null;
      let lazyInitGetterStack = null;
      let didCheckForLazyInit = false;
      
      let didWarnAboutRead = false;
      let didWarnAboutWrite = false;
      let current = initialValue;
      const ref = {
        get current() {