During an urgent update, useDeferredValue should reuse the previous
value. The regression test I added shows that it was reverting to
the initial value instead.
The cause of the bug was trivial: the update path doesn't update the
hook's `memoizedState` field. Only the mount path.
None of the existing tests happened to catch this because to trigger the
bug, you have to do an urgent update that isn't the first update after
initial render. In all of the existing tests that included an urgent
update, it was the first update, so the "previous" value and the initial
value happened to be the same thing.
Andrew Clark
bd08137666