Commits on Jan 7, 2024

1. Fix: uDV initialValue suspends without switching to final …

   Fixes a bug in the experimental `initialValue` option for
   `useDeferredValue` (added in facebook#27500).
   
   If rendering the `initialValue` causes the tree to suspend, React should
   skip it and switch to rendering the final value instead. It should not
   wait for `initialValue` to resolve.
   
   This is not just an optimization, because in some cases the initial
   value may _never_ resolve — intentionally. For example, if the
   application does not provide an instant fallback state. This capability
   is, in fact, the primary motivation for the `initialValue` API.
   
   I mostly implemented this correctly in the original PR, but I missed
   some cases where it wasn't working:
   
   - If there's no Suspense boundary between the `useDeferredValue` hook
   and the component that suspends, and we're not in the shell of the
   transition (i.e. there's a parent Suspense boundary wrapping the
   `useDeferredValue` hook), the deferred task would get
   incorrectly dropped.
   - Similarly, if there's no Suspense boundary between the
   `useDeferredValue` hook and the component that suspends, and we're
   rendering a synchronous update, the deferred task would get incorrectly
   dropped.
   
   What these cases have in common is that it causes the `useDeferredValue`
   hook itself to be replaced by a Suspense fallback. The fix was the same
   for both. (It already worked in cases where there's no Suspense
   fallback at all, because those are handled differently, at the root.)
   
   The way I discovered this was when investigating a particular bug in
   Next.js that would happen during a 'popstate' transition (back/forward),
   but not during a regular navigation. That's because we render popstate
   transitions synchronously to preserve browser's scroll position — which
   in this case triggered the second scenario above.

   

   acdlite committed Jan 7, 2024
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