useTransition should optionally also consider nested suspense boundaries

React version: 18.2.0 ## Background We are working on a web framework that can be used to build highly personalized and data-driven applications. A page that is rendered by the framework consists of a tree of composable, declarative and self-sufficient widgets. A widget can be seen as a mini-application that specifies its own data dependencies and behavior. Since the widgets are self-sufficient, it gives us lots of flexibility to choose which widgets should be shown on page and how they are arranged. It allows us to have personalized pages or layouts per user depending on certain criteria, e.g. their browsing history. The way widgets are chosen to be displayed on the page can be seen as a dynamic, nested routing mechanism. Here is an example of an outfit view that can be used in an ecommerce application:  We are currently migrating the framework to React 18 with the new Suspense SSR and streaming architecture. The main idea is that we wrap each widget in its own suspense boundary to handle the loading of their code and data as soon as the framework decides to display it on the page. For initial requests it works well, but we face a problem with client-side updates that lead to new sub-tree of widgets. ## Steps to Reproduce Here is a link to a code sandbox that contains a minimal code example to reproduce the problem with “new” suspense boundaries and useTransition: https://codesandbox.io/s/romantic-haslett-psc4f4?file=/src/App.js ## Current Behavior useTransition waits for the first Suspense boundary to be ready and then re-renders the DOM although nested Suspense boundaries are still in their fallback state. It is similar to the problem that has been reported in [this issue](https://github.com/facebook/react/issues/24759). In our code example, you can see a big loading overlay after clicking the button to fetc…