Use the disableLegacyMode where ever we check the ConcurrentMode mode

packages/react-reconciler/src/ReactFiber.js

@@ -37,6 +37,7 @@ import {
   enableDebugTracing,
   enableDO_NOT_USE_disableStrictPassiveEffect,
   enableRenderableContext,
+  disableLegacyMode,
 } from 'shared/ReactFeatureFlags';
 import {NoFlags, Placement, StaticMask} from './ReactFiberFlags';
 import {ConcurrentRoot} from './ReactRootTags';
@@ -439,7 +440,7 @@ export function createHostRootFiber(
   concurrentUpdatesByDefaultOverride: null | boolean,
 ): Fiber {
   let mode;
-  if (tag === ConcurrentRoot) {
+  if (disableLegacyMode || tag === ConcurrentRoot) {
     mode = ConcurrentMode;
     if (isStrictMode === true) {
       mode |= StrictLegacyMode | StrictEffectsMode;
@@ -517,7 +518,7 @@ export function createFiberFromTypeAndProps(
       case REACT_STRICT_MODE_TYPE:
         fiberTag = Mode;
         mode |= StrictLegacyMode;
-        if ((mode & ConcurrentMode) !== NoMode) {
+        if (disableLegacyMode || (mode & ConcurrentMode) !== NoMode) {
           // Strict effects should never run on legacy roots
           mode |= StrictEffectsMode;
           if (

packages/react-reconciler/src/ReactFiberBeginWork.js

@@ -108,6 +108,7 @@ import {
   enablePostpone,
   enableRenderableContext,
   enableRefAsProp,
+  disableLegacyMode,
 } from 'shared/ReactFeatureFlags';
 import isArray from 'shared/isArray';
 import shallowEqual from 'shared/shallowEqual';
@@ -700,7 +701,10 @@ function updateOffscreenComponent(
       );
     }
 
-    if ((workInProgress.mode & ConcurrentMode) === NoMode) {
+    if (
+      !disableLegacyMode &&
+      (workInProgress.mode & ConcurrentMode) === NoMode
+    ) {
       // In legacy sync mode, don't defer the subtree. Render it now.
       // TODO: Consider how Offscreen should work with transitions in the future
       const nextState: OffscreenState = {
@@ -2347,6 +2351,7 @@ function mountSuspenseFallbackChildren(
   let primaryChildFragment;
   let fallbackChildFragment;
   if (
+    !disableLegacyMode &&
     (mode & ConcurrentMode) === NoMode &&
     progressedPrimaryFragment !== null
   ) {
@@ -2430,7 +2435,7 @@ function updateSuspensePrimaryChildren(
       children: primaryChildren,
     },
   );
-  if ((workInProgress.mode & ConcurrentMode) === NoMode) {
+  if (!disableLegacyMode && (workInProgress.mode & ConcurrentMode) === NoMode) {
     primaryChildFragment.lanes = renderLanes;
   }
   primaryChildFragment.return = workInProgress;
@@ -2471,6 +2476,7 @@ function updateSuspenseFallbackChildren(
   if (
     // In legacy mode, we commit the primary tree as if it successfully
     // completed, even though it's in an inconsistent state.
+    !disableLegacyMode &&
     (mode & ConcurrentMode) === NoMode &&
     // Make sure we're on the second pass, i.e. the primary child fragment was
     // already cloned. In legacy mode, the only case where this isn't true is
@@ -2607,7 +2613,7 @@ function mountSuspenseFallbackAfterRetryWithoutHydrating(
   primaryChildFragment.sibling = fallbackChildFragment;
   workInProgress.child = primaryChildFragment;
 
-  if ((workInProgress.mode & ConcurrentMode) !== NoMode) {
+  if (disableLegacyMode || (workInProgress.mode & ConcurrentMode) !== NoMode) {
     // We will have dropped the effect list which contains the
     // deletion. We need to reconcile to delete the current child.
     reconcileChildFibers(workInProgress, current.child, null, renderLanes);
@@ -3195,7 +3201,7 @@ function updateSuspenseListComponent(
   }
   pushSuspenseListContext(workInProgress, suspenseContext);
 
-  if ((workInProgress.mode & ConcurrentMode) === NoMode) {
+  if (!disableLegacyMode && (workInProgress.mode & ConcurrentMode) === NoMode) {
     // In legacy mode, SuspenseList doesn't work so we just
     // use make it a noop by treating it as the default revealOrder.
     workInProgress.memoizedState = null;
@@ -3443,7 +3449,7 @@ function resetSuspendedCurrentOnMountInLegacyMode(
   current: null | Fiber,
   workInProgress: Fiber,
 ) {
-  if ((workInProgress.mode & ConcurrentMode) === NoMode) {
+  if (!disableLegacyMode && (workInProgress.mode & ConcurrentMode) === NoMode) {
     if (current !== null) {
       // A lazy component only mounts if it suspended inside a non-
       // concurrent tree, in an inconsistent state. We want to treat it like

packages/react-reconciler/src/ReactFiberCommitWork.js

@@ -54,6 +54,7 @@ import {
   enableUseEffectEventHook,
   enableLegacyHidden,
   disableStringRefs,
+  disableLegacyMode,
 } from 'shared/ReactFeatureFlags';
 import {
   FunctionComponent,
@@ -1164,7 +1165,8 @@ function commitLayoutEffectOnFiber(
       break;
     }
     case OffscreenComponent: {
-      const isModernRoot = (finishedWork.mode & ConcurrentMode) !== NoMode;
+      const isModernRoot =
+        disableLegacyMode || (finishedWork.mode & ConcurrentMode) !== NoMode;
       if (isModernRoot) {
         const isHidden = finishedWork.memoizedState !== null;
         const newOffscreenSubtreeIsHidden =
@@ -2255,7 +2257,7 @@ function commitDeletionEffectsOnFiber(
     }
     case OffscreenComponent: {
       safelyDetachRef(deletedFiber, nearestMountedAncestor);
-      if (deletedFiber.mode & ConcurrentMode) {
+      if (disableLegacyMode || deletedFiber.mode & ConcurrentMode) {
         // If this offscreen component is hidden, we already unmounted it. Before
         // deleting the children, track that it's already unmounted so that we
         // don't attempt to unmount the effects again.
@@ -2932,7 +2934,7 @@ function commitMutationEffectsOnFiber(
       const isHidden = newState !== null;
       const wasHidden = current !== null && current.memoizedState !== null;
 
-      if (finishedWork.mode & ConcurrentMode) {
+      if (disableLegacyMode || finishedWork.mode & ConcurrentMode) {
         // Before committing the children, track on the stack whether this
         // offscreen subtree was already hidden, so that we don't unmount the
         // effects again.
@@ -2978,7 +2980,10 @@ function commitMutationEffectsOnFiber(
           //   - This Offscreen was not hidden before.
           //   - Ancestor Offscreen was not hidden in previous commit.
           if (isUpdate && !wasHidden && !wasHiddenByAncestorOffscreen) {
-            if ((finishedWork.mode & ConcurrentMode) !== NoMode) {
+            if (
+              disableLegacyMode ||
+              (finishedWork.mode & ConcurrentMode) !== NoMode
+            ) {
               // Disappear the layout effects of all the children
               recursivelyTraverseDisappearLayoutEffects(finishedWork);
             }
@@ -3676,7 +3681,7 @@ function commitPassiveMountOnFiber(
             committedTransitions,
           );
         } else {
-          if (finishedWork.mode & ConcurrentMode) {
+          if (disableLegacyMode || finishedWork.mode & ConcurrentMode) {
             // The effects are currently disconnected. Since the tree is hidden,
             // don't connect them. This also applies to the initial render.
             if (enableCache || enableTransitionTracing) {
@@ -3874,7 +3879,7 @@ export function reconnectPassiveEffects(
             includeWorkInProgressEffects,
           );
         } else {
-          if (finishedWork.mode & ConcurrentMode) {
+          if (disableLegacyMode || finishedWork.mode & ConcurrentMode) {
             // The effects are currently disconnected. Since the tree is hidden,
             // don't connect them. This also applies to the initial render.
             if (enableCache || enableTransitionTracing) {

packages/react-reconciler/src/ReactFiberCompleteWork.js

@@ -40,6 +40,7 @@ import {
   enableTransitionTracing,
   enableRenderableContext,
   passChildrenWhenCloningPersistedNodes,
+  disableLegacyMode,
 } from 'shared/ReactFeatureFlags';
 
 import {now} from './Scheduler';
@@ -1740,7 +1741,11 @@ function completeWork(
         }
       }
 
-      if (!nextIsHidden || (workInProgress.mode & ConcurrentMode) === NoMode) {
+      if (
+        !nextIsHidden ||
+        (!disableLegacyMode &&
+          (workInProgress.mode & ConcurrentMode) === NoMode)
+      ) {
         bubbleProperties(workInProgress);
       } else {
         // Don't bubble properties for hidden children unless we're rendering

packages/react-reconciler/src/ReactFiberHooks.js

@@ -42,6 +42,7 @@ import {
   debugRenderPhaseSideEffectsForStrictMode,
   enableAsyncActions,
   enableUseDeferredValueInitialArg,
+  disableLegacyMode,
 } from 'shared/ReactFeatureFlags';
 import {
   REACT_CONTEXT_TYPE,
@@ -662,7 +663,7 @@ function finishRenderingHooks<Props, SecondArg>(
       // need to mark fibers that commit in an incomplete state, somehow. For
       // now I'll disable the warning that most of the bugs that would trigger
       // it are either exclusive to concurrent mode or exist in both.
-      (current.mode & ConcurrentMode) !== NoMode
+      (disableLegacyMode || (current.mode & ConcurrentMode) !== NoMode)
     ) {
       console.error(
         'Internal React error: Expected static flag was missing. Please ' +

packages/react-reconciler/src/ReactFiberThrow.js

@@ -43,6 +43,7 @@ import {
   enableLazyContextPropagation,
   enableUpdaterTracking,
   enablePostpone,
+  disableLegacyMode,
 } from 'shared/ReactFeatureFlags';
 import {createCapturedValueAtFiber} from './ReactCapturedValue';
 import {
@@ -189,6 +190,7 @@ function resetSuspendedComponent(sourceFiber: Fiber, rootRenderLanes: Lanes) {
   // A legacy mode Suspense quirk, only relevant to hook components.
   const tag = sourceFiber.tag;
   if (
+    !disableLegacyMode &&
     (sourceFiber.mode & ConcurrentMode) === NoMode &&
     (tag === FunctionComponent ||
       tag === ForwardRef ||
@@ -215,7 +217,10 @@ function markSuspenseBoundaryShouldCapture(
 ): Fiber | null {
   // This marks a Suspense boundary so that when we're unwinding the stack,
   // it captures the suspended "exception" and does a second (fallback) pass.
-  if ((suspenseBoundary.mode & ConcurrentMode) === NoMode) {
+  if (
+    !disableLegacyMode &&
+    (suspenseBoundary.mode & ConcurrentMode) === NoMode
+  ) {
     // Legacy Mode Suspense
     //
     // If the boundary is in legacy mode, we should *not*
@@ -354,7 +359,10 @@ function throwException(
       resetSuspendedComponent(sourceFiber, rootRenderLanes);
 
       if (__DEV__) {
-        if (getIsHydrating() && sourceFiber.mode & ConcurrentMode) {
+        if (
+          getIsHydrating() &&
+          (disableLegacyMode || sourceFiber.mode & ConcurrentMode)
+        ) {
           markDidThrowWhileHydratingDEV();
         }
       }
@@ -383,7 +391,7 @@ function throwException(
             // we don't have to recompute it on demand. This would also allow us
             // to unify with `use` which needs to perform this logic even sooner,
             // before `throwException` is called.
-            if (sourceFiber.mode & ConcurrentMode) {
+            if (disableLegacyMode || sourceFiber.mode & ConcurrentMode) {
               if (getShellBoundary() === null) {
                 // Suspended in the "shell" of the app. This is an undesirable
                 // loading state. We should avoid committing this tree.
@@ -451,14 +459,14 @@ function throwException(
               // We only attach ping listeners in concurrent mode. Legacy
               // Suspense always commits fallbacks synchronously, so there are
               // no pings.
-              if (suspenseBoundary.mode & ConcurrentMode) {
+              if (disableLegacyMode || suspenseBoundary.mode & ConcurrentMode) {
                 attachPingListener(root, wakeable, rootRenderLanes);
               }
             }
             return false;
           }
           case OffscreenComponent: {
-            if (suspenseBoundary.mode & ConcurrentMode) {
+            if (disableLegacyMode || suspenseBoundary.mode & ConcurrentMode) {
               suspenseBoundary.flags |= ShouldCapture;
               const isSuspenseyResource =
                 wakeable === noopSuspenseyCommitThenable;
@@ -522,7 +530,10 @@ function throwException(
   }
 
   // This is a regular error, not a Suspense wakeable.
-  if (getIsHydrating() && sourceFiber.mode & ConcurrentMode) {
+  if (
+    getIsHydrating() &&
+    (disableLegacyMode || sourceFiber.mode & ConcurrentMode)
+  ) {
     markDidThrowWhileHydratingDEV();
     const suspenseBoundary = getSuspenseHandler();
     // If the error was thrown during hydration, we may be able to recover by

packages/react-reconciler/src/ReactFiberWorkLoop.js

@@ -602,7 +602,7 @@ export function getCurrentTime(): number {
 export function requestUpdateLane(fiber: Fiber): Lane {
   // Special cases
   const mode = fiber.mode;
-  if ((mode & ConcurrentMode) === NoMode) {
+  if (!disableLegacyMode && (mode & ConcurrentMode) === NoMode) {
     return (SyncLane: Lane);
   } else if (
     (executionContext & RenderContext) !== NoContext &&
@@ -669,7 +669,7 @@ function requestRetryLane(fiber: Fiber) {
 
   // Special cases
   const mode = fiber.mode;
-  if ((mode & ConcurrentMode) === NoMode) {
+  if (!disableLegacyMode && (mode & ConcurrentMode) === NoMode) {
     return (SyncLane: Lane);
   }
 
@@ -824,6 +824,7 @@ export function scheduleUpdateOnFiber(
     if (
       lane === SyncLane &&
       executionContext === NoContext &&
+      !disableLegacyMode &&
       (fiber.mode & ConcurrentMode) === NoMode
     ) {
       if (__DEV__ && ReactCurrentActQueue.isBatchingLegacy) {
@@ -3794,7 +3795,7 @@ export function warnAboutUpdateOnNotYetMountedFiberInDEV(fiber: Fiber) {
       return;
     }
 
-    if (!(fiber.mode & ConcurrentMode)) {
+    if (!disableLegacyMode && !(fiber.mode & ConcurrentMode)) {
       return;
     }
 
@@ -3933,7 +3934,7 @@ function shouldForceFlushFallbacksInDEV() {
 
 function warnIfUpdatesNotWrappedWithActDEV(fiber: Fiber): void {
   if (__DEV__) {
-    if (fiber.mode & ConcurrentMode) {
+    if (disableLegacyMode || fiber.mode & ConcurrentMode) {
       if (!isConcurrentActEnvironment()) {
         // Not in an act environment. No need to warn.
         return;