If there are any suspended fallbacks at the end of the `act` scope,
force them to display by running the pending timers (i.e. `setTimeout`).
The public implementation of `act` achieves the same behavior with an
extra check in the work loop (`shouldForceFlushFallbacks`). Since our
internal `act` needs to work in both development and production, without
additional runtime checks, we instead rely on Jest's mock timers.
This doesn't not affect refresh transitions, which are meant to delay
indefinitely, because in that case we exit the work loop without
posting a timer.
This updates the experimental Scheduler postTask build to call postTask
directly, instead of managing our own custom queue and work loop.
We still use a deadline 5ms mechanism to implement `shouldYield`.
The main thing that postTask is currently missing is the continuation
feature — when yielding to the main thread, the yielding task is sent
to the back of the queue, instead of maintaining its position.
While this would be nice to have, even without it, postTask may be good
enough to replace our userspace implementation.
We'll run some tests to see.
Andrew Clark