act: Bypass microtask for "default sync" updates

When wrapping an update in act, instead of scheduling a microtask,
we can add the task to our internal queue.

The benefit is that the user doesn't have to await the act call. We can
flush the work synchronously. This doesn't account for microtasks that
are scheduled in userspace, of course, but it at least covers
React's usage.

Author: acdlite

packages/react-reconciler/src/ReactFiberWorkLoop.new.js

@@ -692,7 +692,14 @@ function ensureRootIsScheduled(root: FiberRoot, currentTime: number) {
     }
     if (supportsMicrotasks) {
       // Flush the queue in a microtask.
-      scheduleMicrotask(flushSyncCallbacks);
+      if (__DEV__ && ReactCurrentActQueue.current !== null) {
+        // Inside `act`, use our internal `act` queue so that these get flushed
+        // at the end of the current scope even when using the sync version
+        // of `act`.
+        ReactCurrentActQueue.current.push(flushSyncCallbacks);
+      } else {
+        scheduleMicrotask(flushSyncCallbacks);
+      }
     } else {
       // Flush the queue in an Immediate task.
       scheduleCallback(ImmediateSchedulerPriority, flushSyncCallbacks);

packages/react-reconciler/src/ReactFiberWorkLoop.old.js

@@ -692,7 +692,14 @@ function ensureRootIsScheduled(root: FiberRoot, currentTime: number) {
     }
     if (supportsMicrotasks) {
       // Flush the queue in a microtask.
-      scheduleMicrotask(flushSyncCallbacks);
+      if (__DEV__ && ReactCurrentActQueue.current !== null) {
+        // Inside `act`, use our internal `act` queue so that these get flushed
+        // at the end of the current scope even when using the sync version
+        // of `act`.
+        ReactCurrentActQueue.current.push(flushSyncCallbacks);
+      } else {
+        scheduleMicrotask(flushSyncCallbacks);
+      }
     } else {
       // Flush the queue in an Immediate task.
       scheduleCallback(ImmediateSchedulerPriority, flushSyncCallbacks);