dillon1000/react

useDeferredValue has higher priority than partial hydration (#27550)

By default, partial hydration is given the lowest possible priority,
because until a tree is updated, the server-rendered HTML is assumed to
match the final resolved HTML.

However, this isn't completely true because a component may choose to
"upgrade" itself upon hydration. The simplest example is a component
that calls setState in a useEffect to switch to a richer implementation
of the UI. Another example is a component that doesn't have a server-
rendered implementation, so it intentionally suspends to force a client-
only render.

useDeferredValue is an example, too: the server only renders the first
pass (the initialValue) argument, and relies on the client to upgrade to
the final value.

What we should really do in these cases is emit some information into
the Fizz stream so that Fiber knows to prioritize the hydration of
certain trees. We plan to add a mechanism for this in the future.

In the meantime, though, we can at least ensure that the priority of the
upgrade task is correct once it's "discovered" during hydration. In this
case, the priority of the task spawned by useDeferredValue should have
Transition priority, not Offscreen priority.