Fix flushing suspense fallbacks at the end of Act when the scheduler is mocked

Author: rickhanlonii

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

@@ -3737,7 +3737,6 @@ function shouldForceFlushFallbacksInDEV() {
 }
 
 const flushMockScheduler = Scheduler.unstable_flushAllWithoutAsserting;
-const isSchedulerMocked = typeof flushMockScheduler === 'function';
 
 // Returns whether additional work was scheduled. Caller should keep flushing
 // until there's no work left.
@@ -3867,11 +3866,7 @@ export function act(callback: () => Thenable<mixed>): Thenable<void> {
         called = true;
         result.then(
           () => {
-            if (
-              actingUpdatesScopeDepth > 1 ||
-              (isSchedulerMocked === true &&
-                previousIsSomeRendererActing === true)
-            ) {
+            if (actingUpdatesScopeDepth > 1) {
               onDone();
               resolve();
               return;
@@ -3907,10 +3902,7 @@ export function act(callback: () => Thenable<mixed>): Thenable<void> {
 
     // flush effects until none remain, and cleanup
     try {
-      if (
-        actingUpdatesScopeDepth === 1 &&
-        (isSchedulerMocked === false || previousIsSomeRendererActing === false)
-      ) {
+      if (actingUpdatesScopeDepth === 1) {
         // we're about to exit the act() scope,
         // now's the time to flush effects
         flushActWork();

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

@@ -3593,7 +3593,6 @@ function shouldForceFlushFallbacksInDEV() {
 }
 
 const flushMockScheduler = Scheduler.unstable_flushAllWithoutAsserting;
-const isSchedulerMocked = typeof flushMockScheduler === 'function';
 
 // Returns whether additional work was scheduled. Caller should keep flushing
 // until there's no work left.
@@ -3723,11 +3722,7 @@ export function act(callback: () => Thenable<mixed>): Thenable<void> {
         called = true;
         result.then(
           () => {
-            if (
-              actingUpdatesScopeDepth > 1 ||
-              (isSchedulerMocked === true &&
-                previousIsSomeRendererActing === true)
-            ) {
+            if (actingUpdatesScopeDepth > 1) {
               onDone();
               resolve();
               return;
@@ -3763,10 +3758,7 @@ export function act(callback: () => Thenable<mixed>): Thenable<void> {
 
     // flush effects until none remain, and cleanup
     try {
-      if (
-        actingUpdatesScopeDepth === 1 &&
-        (isSchedulerMocked === false || previousIsSomeRendererActing === false)
-      ) {
+      if (actingUpdatesScopeDepth === 1) {
         // we're about to exit the act() scope,
         // now's the time to flush effects
         flushActWork();