dillon1000/react

[Bugfix] Infinite uDV loop in popstate event (#32821)

Found a bug that occurs during a specific combination of very subtle
implementation details.

It occurs sometimes (not always) when 1) a transition is scheduled
during a popstate event, and 2) as a result, a new value is passed to an
already-mounted useDeferredValue hook.

The fix is relatively straightforward, and I found it almost
immediately; it took a bit longer to figure out exactly how the scenario
occurred in production and create a test case to simulate it.

Rather than couple the test to the implementation details, I've chosen
to keep it as high-level as possible so that it doesn't break if the
details change. In the future, it might not be trigger the exact set of
internal circumstances anymore, but it could be useful for catching
similar bugs because it represents a realistic real world situation —
namely, switching tabs repeatedly in an app that uses useDeferredValue.