* Extract `act` environment check into function
`act` checks the environment to determine whether to fire a warning.
We're changing how this check works in React 18. As a first step, this
refactors the logic into a single function. No behavior changes yet.
* Use IS_REACT_ACT_ENVIRONMENT to disable warnings
If `IS_REACT_ACT_ENVIRONMENT` is set to `false`, we will suppress
any `act` warnings. Otherwise, the behavior of `act` is the same as in
React 17: if `jest` is defined, it warns.
In concurrent mode, the plan is to remove the `jest` check and only warn
if `IS_REACT_ACT_ENVIRONMENT` is true. I have not implemented that
part yet.
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.
Esteban