dillon1000/react

Allow suspending in the shell during hydration (#23304)

* Allow suspending in the shell during hydration

Builds on behavior added in #23267.

Initial hydration should be allowed to suspend in the shell. In
practice, this happens because the code for the outer shell hasn't
loaded yet.

Currently if you try to do this, it errors because it expects there to
be a parent Suspense boundary, because without a fallback we can't
produce a consistent tree. However, for non-sync updates, we don't need
to produce a consistent tree immediately — we can delay the commit
until the data resolves.

In #23267, I added support for suspending without a parent boundary if
the update was wrapped with `startTransition`. Here, I've expanded this
to include hydration, too.

I wonder if we should expand this even further to include all non-sync/
discrete updates.

* Allow suspending in shell for all non-sync updates

Instead of erroring, we can delay the commit.

The only time we'll continue to error when there's no parent Suspense
boundary is during sync/discrete updates, because those are expected to
produce a complete tree synchronously to maintain consistency with
external state.