fix bug in reparenting where wrong parentTag is used in differentiator (#50884)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/50884

changelog: [internal]

Fix incorrect parentTag coming from differentiator when reparenting. The fix is hidden behind existing feature flag that is already fixing similar issue: D73428312

The problem occurs under very specific circumstances that are covered by an integration test. The *parentTag* is also [only used on Android](https://github.com/facebook/react-native/blob/main/packages/react-native/ReactAndroid/src/main/jni/react/fabric/FabricMountingManager.cpp#L566) and only if layout changes as part of the update.

This diff introduces a new test specifically triggering the incorrect behaviour: `Differentiator-itest.js`. Without this fix, the test fails on following assert:           [react_native_assert(hasTag(mutation.parentTag))](https://github.com/facebook/react-native/blob/main/packages/react-native/ReactCommon/react/renderer/mounting/stubs/StubViewTree.cpp#L245).

Reviewed By: mdvacca

Differential Revision: D73545165

fbshipit-source-id: 43344eebfa4cc0119e5b42170f6b9097dacd704d

packages/react-native/ReactCommon/react/renderer/mounting/Differentiator.cpp

@@ -711,6 +711,11 @@ static void calculateShadowViewMutationsFlattener(
 
           // Unflatten parent, flatten child
           if (childReparentMode == ReparentMode::Flatten) {
+            auto parentTagForUpdateWhenUnflattened =
+                ReactNativeFeatureFlags::
+                    enableFixForParentTagDuringReparenting()
+                ? newTreeNodePair.shadowView.tag
+                : parentTag;
             // Flatten old tree into new list
             // At the end of this loop we still want to know which of these
             // children are visited, so we reuse the `newRemainingPairs` map.
@@ -727,7 +732,7 @@ static void calculateShadowViewMutationsFlattener(
                 oldTreeNodePair,
                 (reparentMode == ReparentMode::Flatten
                      ? oldTreeNodePair.shadowView.tag
-                     : parentTag),
+                     : parentTagForUpdateWhenUnflattened),
                 subVisitedNewMap,
                 subVisitedOldMap,
                 adjustedOldCullingContext,

packages/react-native/src/private/renderer/mounting/__tests__/Mounting-itest.js

@@ -295,6 +295,62 @@ test('parent-child switching from unflattened-flattened to flattened-unflattened
   ]);
 });
 
+test('parent-child switching from flattened-unflattened to unflattened-flattened', () => {
+  const root = Fantom.createRoot();
+
+  Fantom.runTask(() => {
+    root.render(
+      <View
+        style={{
+          marginTop: 100,
+        }}>
+        <View
+          style={{
+            marginTop: 50,
+            opacity: 0,
+          }}>
+          <View nativeID={'child'} style={{height: 10, width: 10}} />
+        </View>
+      </View>,
+    );
+  });
+
+  expect(root.takeMountingManagerLogs()).toEqual([
+    'Update {type: "RootView", nativeID: (root)}',
+    'Create {type: "View", nativeID: (N/A)}',
+    'Create {type: "View", nativeID: "child"}',
+    'Insert {type: "View", parentNativeID: (N/A), index: 0, nativeID: "child"}',
+    'Insert {type: "View", parentNativeID: (root), index: 0, nativeID: (N/A)}',
+  ]);
+
+  // force view to be flattened.
+  Fantom.runTask(() => {
+    root.render(
+      <View
+        style={{
+          marginTop: 100,
+          opacity: 0,
+        }}>
+        <View
+          style={{
+            marginTop: 50,
+          }}>
+          <View nativeID={'child'} style={{height: 10, width: 10}} />
+        </View>
+      </View>,
+    );
+  });
+  expect(root.takeMountingManagerLogs()).toEqual([
+    'Update {type: "View", nativeID: "child"}',
+    'Remove {type: "View", parentNativeID: (root), index: 0, nativeID: (N/A)}',
+    'Remove {type: "View", parentNativeID: (N/A), index: 0, nativeID: "child"}',
+    'Delete {type: "View", nativeID: (N/A)}',
+    'Create {type: "View", nativeID: (N/A)}',
+    'Insert {type: "View", parentNativeID: (root), index: 0, nativeID: (N/A)}',
+    'Insert {type: "View", parentNativeID: (N/A), index: 0, nativeID: "child"}',
+  ]);
+});
+
 describe('reconciliation of setNativeProps and React commit', () => {
   it('props set by setNativeProps must not be overriden by React commit', () => {
     const root = Fantom.createRoot();