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`.
create: () => (() => void) | void,
  deps: Array<mixed> | void | null,
): void {
  return updateEffectImpl(UpdateEffect, HookInsertion, create, deps);
}
function mountLayoutEffect(
  create: () => (() => void) | void,
  deps: Array<mixed> | void | null,
): void {
  let fiberFlags: Flags = UpdateEffect | LayoutStaticEffect;
  if (
    __DEV__ &&
    (currentlyRenderingFiber.mode & StrictEffectsMode) !== NoMode
  ) {
    fiberFlags |= MountLayoutDevEffect;
  }
  return mountEffectImpl(fiberFlags, HookLayout, create, deps);
}
function updateLayoutEffect(
  create: () => (() => void) | void,
  deps: Array<mixed> | void | null,
): void {
  return updateEffectImpl(UpdateEffect, HookLayout, create, deps);
}
function imperativeHandleEffect<T>(
  create: () => T,
  ref: {current: T | null} | ((inst: T | null) => mixed) | null | void,
): void | (() => void) {
  if (typeof ref === 'function') {
    const refCallback = ref;