During server rendering, a visible Offscreen subtree acts exactly like a
fragment: a pure indirection.
A hidden Offscreen subtree is not server rendered at all. It's ignored
during hydration, too. Prerendering happens only on the client. We
considered prerendering hidden trees on the server, too, but our
conclusion is that it's a waste of bytes and server computation. We
can't think of any compelling cases where it's the right trade off. (If
we ever change our mind, though, the way we'll likely model it is to
treat it as if it's a Suspense boundary with an empty fallback.)
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