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`.
'Functions may close over mutable values and cause tearing.',
        );
      }
    }
    return snapshot;
  } else {
    
    
    
    
    
    
    
    
    
    markSourceAsDirty(source);
    
    
    
    
    
    
    throw new Error(
      'Cannot read from mutable source during the current render without tearing. This may be a bug in React. Please file an issue.',
    );
  }
}
function useMutableSource<Source, Snapshot>(
  hook: Hook,
  source: MutableSource<Source>,