[Fabric] Fix reparenting in legacy Suspense mount

Fixes a weird case during legacy Suspense mount where the offscreen host
container of a tree that suspends during initial mount is recreated
instead of cloned, since there's no current fiber to clone from.

Fabric considers this a reparent even though the parent from the first
pass never committed.

Instead we can override the props from the first pass before the
container completes. It's a bit of a hack, but no more so than the rest
of the legacy root Suspense implementation — the hacks are designed
to make it usable by non-strict mode-compliant trees.

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

@@ -2212,21 +2212,6 @@ function mountSuspenseFallbackChildren(
     primaryChildFragment.childLanes = NoLanes;
     primaryChildFragment.pendingProps = primaryChildProps;
 
-    if (
-      supportsPersistence &&
-      (workInProgress.mode & ConcurrentMode) === NoMode
-    ) {
-      const offscreenContainer: Fiber = (primaryChildFragment.child: any);
-
-      const containerProps = getOffscreenContainerProps(
-        'hidden',
-        primaryChildren,
-      );
-      offscreenContainer.pendingProps = containerProps;
-      offscreenContainer.memoizedProps = containerProps;
-      completeSuspendedOffscreenHostContainer(null, offscreenContainer);
-    }
-
     if (enableProfilerTimer && workInProgress.mode & ProfileMode) {
       // Reset the durations from the first pass so they aren't included in the
       // final amounts. This seems counterintuitive, since we're intentionally

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

@@ -2212,21 +2212,6 @@ function mountSuspenseFallbackChildren(
     primaryChildFragment.childLanes = NoLanes;
     primaryChildFragment.pendingProps = primaryChildProps;
 
-    if (
-      supportsPersistence &&
-      (workInProgress.mode & ConcurrentMode) === NoMode
-    ) {
-      const offscreenContainer: Fiber = (primaryChildFragment.child: any);
-
-      const containerProps = getOffscreenContainerProps(
-        'hidden',
-        primaryChildren,
-      );
-      offscreenContainer.pendingProps = containerProps;
-      offscreenContainer.memoizedProps = containerProps;
-      completeSuspendedOffscreenHostContainer(null, offscreenContainer);
-    }
-
     if (enableProfilerTimer && workInProgress.mode & ProfileMode) {
       // Reset the durations from the first pass so they aren't included in the
       // final amounts. This seems counterintuitive, since we're intentionally

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

@@ -33,6 +33,10 @@ import {
   LifecycleEffectMask,
   ForceUpdateForLegacySuspense,
 } from './ReactFiberFlags';
+import {
+  supportsPersistence,
+  getOffscreenContainerProps,
+} from './ReactFiberHostConfig';
 import {shouldCaptureSuspense} from './ReactFiberSuspenseComponent.new';
 import {NoMode, ConcurrentMode, DebugTracingMode} from './ReactTypeOfMode';
 import {
@@ -313,6 +317,26 @@ function throwException(
           // all lifecycle effect tags.
           sourceFiber.flags &= ~(LifecycleEffectMask | Incomplete);
 
+          if (supportsPersistence) {
+            // Another legacy Suspense quirk. In persistent mode, if this is the
+            // initial mount, override the props of the host container to hide
+            // its contents.
+            const currentSuspenseBoundary = workInProgress.alternate;
+            if (currentSuspenseBoundary === null) {
+              const offscreenFiber: Fiber = (workInProgress.child: any);
+              const offscreenContainer = offscreenFiber.child;
+              if (offscreenContainer !== null) {
+                const children = offscreenContainer.memoizedProps.children;
+                const containerProps = getOffscreenContainerProps(
+                  'hidden',
+                  children,
+                );
+                offscreenContainer.pendingProps = containerProps;
+                offscreenContainer.memoizedProps = containerProps;
+              }
+            }
+          }
+
           if (sourceFiber.tag === ClassComponent) {
             const currentSourceFiber = sourceFiber.alternate;
             if (currentSourceFiber === null) {

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

@@ -33,6 +33,10 @@ import {
   LifecycleEffectMask,
   ForceUpdateForLegacySuspense,
 } from './ReactFiberFlags';
+import {
+  supportsPersistence,
+  getOffscreenContainerProps,
+} from './ReactFiberHostConfig';
 import {shouldCaptureSuspense} from './ReactFiberSuspenseComponent.old';
 import {NoMode, ConcurrentMode, DebugTracingMode} from './ReactTypeOfMode';
 import {
@@ -313,6 +317,26 @@ function throwException(
           // all lifecycle effect tags.
           sourceFiber.flags &= ~(LifecycleEffectMask | Incomplete);
 
+          if (supportsPersistence) {
+            // Another legacy Suspense quirk. In persistent mode, if this is the
+            // initial mount, override the props of the host container to hide
+            // its contents.
+            const currentSuspenseBoundary = workInProgress.alternate;
+            if (currentSuspenseBoundary === null) {
+              const offscreenFiber: Fiber = (workInProgress.child: any);
+              const offscreenContainer = offscreenFiber.child;
+              if (offscreenContainer !== null) {
+                const children = offscreenContainer.memoizedProps.children;
+                const containerProps = getOffscreenContainerProps(
+                  'hidden',
+                  children,
+                );
+                offscreenContainer.pendingProps = containerProps;
+                offscreenContainer.memoizedProps = containerProps;
+              }
+            }
+          }
+
           if (sourceFiber.tag === ClassComponent) {
             const currentSourceFiber = sourceFiber.alternate;
             if (currentSourceFiber === null) {