dillon1000/react

Don't flush synchronous work if we're in the middle of a ViewTransition async sequence (#32760)

Starting a View Transition is an async sequence. Since React can get a
sync update in the middle of sequence we sometimes interrupt that
sequence.

Currently, we don't actually cancel the View Transition so it can just
run as a partial. This ensures that we fully skip it when that happens,
as well as warn.

However, it's very easy to trigger this with just a setState in
useLayoutEffect right now. Therefore if we're inside the preparing
sequence of a startViewTransition, this delays work that would've
normally flushed in a microtask. ~Maybe we want to do the same for
Default work already scheduled through a scheduler Task.~ Edit: This was
already done.

`flushSync` currently will still lead to an interrupted View Transition
(with a warning). There's a tradeoff here whether we want to try our
best to preserve the guarantees of `flushSync` or favor the animation.
It's already possible to suspend at the root with `flushSync` which
means it's not always 100% guaranteed to commit anyway. We could treat
it as suspended. But let's see how much this is a problem in practice.