dillon1000/react

Batch async actions even if useTransition is unmounted (#28078)

If there are multiple updates inside an async action, they should all be
rendered in the same batch, even if they are separate by an async
operation (`await`). We currently implement this by suspending in the
`useTransition` hook to block the update from committing until all
possible updates have been scheduled by the action. The reason we did it
this way is so you can "cancel" an action by navigating away from the UI
that triggered it.

The problem with that approach, though, is that even if you navigate
away from the `useTransition` hook, the action may have updated shared
parts of the UI that are still in the tree. So we may need to continue
suspending even after the `useTransition` hook is deleted.

In other words, the lifetime of an async action scope is longer than the
lifetime of a particular `useTransition` hook.

The solution is to suspend whenever _any_ update that is part of the
async action scope is unwrapped during render. So, inside useState and
useReducer.

This fixes a related issue where an optimistic update is reverted before
the async action has finished, because we were relying on the
`useTransition` hook to prevent the optimistic update from finishing.

This also prepares us to support async actions being passed to the
non-hook form of `startTransition` (though this isn't implemented yet).