dillon1000/react

Allow suspending outside a Suspense boundary (#23267)

(If the update is wrapped in startTransition)

Currently you're not allowed to suspend outside of a Suspense boundary.
We throw an error:

> A React component suspended while rendering, but no fallback UI
was specified

We treat this case like an error because discrete renders are expected
to finish synchronously to maintain consistency with external state.
However, during a concurrent transition (startTransition), what we can
do instead is treat this case like a refresh transition: suspend the
commit without showing a fallback.

The behavior is roughly as if there were a built-in Suspense boundary
at the root of the app with unstable_avoidThisFallback enabled.
Conceptually it's very similar because during hydration you're already
showing server-rendered UI; there's no need to replace that with
a fallback when something suspends.