Bug: non-Hook startTransition waits for stale work to finish

Not sure if this is a bug, but it breaks my expectations. Here's the same example with `useTransition` and `startTransition`. - `useTransition`: https://codesandbox.io/s/kgs8wi?file=/App.js - `startTransition`: https://codesandbox.io/s/delicate-mountain-fr04u3?file=/App.js Repro steps: 1. Click "Posts" 2. Immediately click "Contact" With `useTransition`, the state change is instant because rendering slow Posts component is abandoned. With `startTransition`, the state change is slow because it waits for the Posts component to finish rendering anyway.