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`.
markRootMutableRead(root, lane);
      }
      
      
      
      markRootEntangled(root, root.mutableReadLanes);
    }
  }, [getSnapshot, source, subscribe]);
  
  dispatcher.useEffect(() => {
    const handleChange = () => {
      const latestGetSnapshot = refs.getSnapshot;
      const latestSetSnapshot = refs.setSnapshot;
      try {
        latestSetSnapshot(latestGetSnapshot(source._source));
        
        const lane = requestUpdateLane(fiber);
        markRootMutableRead(root, lane);
      } catch (error) {
        
        
        
        
        latestSetSnapshot(
          (() => {
            throw error;
          }: any),
        );
      }
    };