[pull] main from facebook:main #262
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Fixes a bug in useDeferredValue's optional `initialValue` argument. In the regression case, if a new useDeferredValue hook is mounted while an earlier transition is suspended, the `initialValue` argument of the new hook was ignored. After the fix, the `initialValue` argument is correctly rendered during the initial mount, regardless of whether other transitions were suspended. The culprit was related to the mechanism we use to track whether a render is the result of a `useDeferredValue` hook: we assign the deferred lane a TransitionLane, then entangle that lane with the DeferredLane bit. During the subsequent render, we check for the presence of the DeferredLane bit to determine whether to switch to the final, canonical value. But because transition lanes can themselves become entangled with other transitions, the effect is that every entangled transition was being treated as if it were the result of a `useDeferredValue` hook, causing us to skip the initial value and go straight to the final one. The fix I've chosen is to reserve some subset of TransitionLanes to be used only for deferred work, instead of using entanglement. This is similar to how retries are already implemented. Originally I tried not to implement it this way because it means there are now slightly fewer lanes allocated for regular transitions, but I underestimated how similar deferred work is to retries; they end up having a lot of the same requirements. Eventually it may be possible to merge the two concepts.
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