Add instrumentation tests for FabricMountingManager view lifecycle

Summary:
Add Android instrumentation tests that validate the Java-side and C++ registry
behavior around view preallocation, destruction, and recreation — the core
scenario fixed by D98729251.

Two layers of tests:

1. **Java-side tests** (IntBufferBatchMountItem): Construct mount item int/obj
   buffers manually and execute against MountingManager. Validates CREATE→INSERT
   and Preallocate→Delete→CREATE→INSERT sequences through the Java mount system.

2. **Native registry tests** (FabricMountingManagerTestHelper): JNI hybrid class
   that creates a real C++ FabricMountingManager and exercises the
   allocatedViewRegistry_ lifecycle. Validates that after preallocate + destroy,
   the tag is removed from the registry (the D98729251 fix), so executeMount
   would correctly emit a CREATE instruction.

The native test helper is built as a separate library (libfabricjni_test_helper.so)
under jni/react/fabric/test/, with friend access to FabricMountingManager internals.

Changelog: [Internal]

Differential Revision: D98935855

Author: javache

packages/react-native/ReactAndroid/src/main/jni/react/fabric/FabricMountingManager.cpp

@@ -52,6 +52,15 @@ void FabricMountingManager::onSurfaceStop(SurfaceId surfaceId) {
   allocatedViewRegistry_.erase(surfaceId);
 }
 
+bool FabricMountingManager::isViewAllocated(SurfaceId surfaceId, Tag tag) {
+  std::lock_guard lock(allocatedViewsMutex_);
+  auto it = allocatedViewRegistry_.find(surfaceId);
+  if (it == allocatedViewRegistry_.end()) {
+    return false;
+  }
+  return it->second.count(tag) > 0;
+}
+
 namespace {
 
 #ifdef REACT_NATIVE_DEBUG

packages/react-native/ReactAndroid/src/main/jni/react/fabric/FabricMountingManager.h

@@ -40,6 +40,20 @@ class FabricMountingManager final {
    */
   void drainPreallocateViewsQueue();
 
+  /*
+   * Preallocates a view on the Java side and registers the tag in
+   * allocatedViewRegistry_ so that executeMount skips the redundant Create
+   * mount item for this tag.
+   */
+  void preallocateShadowView(const ShadowView &shadowView);
+
+  /*
+   * Returns true if the given tag is registered in allocatedViewRegistry_
+   * for the given surface. A registered tag means executeMount will skip
+   * the Create mount item (the view was already preallocated).
+   */
+  bool isViewAllocated(SurfaceId surfaceId, Tag tag);
+
   void executeMount(const MountingTransaction &transaction);
 
   void dispatchCommand(const ShadowView &shadowView, const std::string &commandName, const folly::dynamic &args);
@@ -75,12 +89,6 @@ class FabricMountingManager final {
 
   std::unordered_map<SurfaceId, std::unordered_set<Tag>> allocatedViewRegistry_{};
   std::recursive_mutex allocatedViewsMutex_;
-
-  /*
-   * Calls FabricUIManager.preallocateView() on the Java side if view needs to
-   * be preallocated.
-   */
-  void preallocateShadowView(const ShadowView &shadowView);
 };
 
 } // namespace facebook::react

packages/react-native/ReactAndroid/src/main/jni/react/fabric/test_helper/FabricMountingManagerTestHelper.cpp

@@ -0,0 +1,93 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#include "FabricMountingManagerTestHelper.h"
+
+#include <react/renderer/components/view/ViewComponentDescriptor.h>
+#include <react/renderer/components/view/ViewProps.h>
+#include <react/renderer/core/ComponentDescriptor.h>
+#include <react/renderer/mounting/ShadowView.h>
+
+namespace facebook::react {
+
+FabricMountingManagerTestHelper::FabricMountingManagerTestHelper(
+    jni::alias_ref<JFabricUIManager::javaobject> jFabricUIManager)
+    : javaUIManager_(jni::make_global(jFabricUIManager)) {
+  mountingManager_ = std::make_shared<FabricMountingManager>(javaUIManager_);
+
+  auto params = ComponentDescriptorParameters{
+      .eventDispatcher = EventDispatcher::Shared{},
+      .contextContainer = std::make_shared<ContextContainer>(),
+      .flavor = nullptr};
+  viewComponentDescriptor_ = std::make_unique<ViewComponentDescriptor>(params);
+}
+
+void FabricMountingManagerTestHelper::initHybrid(
+    jni::alias_ref<jhybridobject> jobj,
+    jni::alias_ref<JFabricUIManager::javaobject> jFabricUIManager) {
+  setCxxInstance(jobj, jFabricUIManager);
+}
+
+void FabricMountingManagerTestHelper::startSurface(jint surfaceId) {
+  mountingManager_->onSurfaceStart(surfaceId);
+}
+
+void FabricMountingManagerTestHelper::stopSurface(jint surfaceId) {
+  mountingManager_->onSurfaceStop(surfaceId);
+}
+
+void FabricMountingManagerTestHelper::preallocateView(
+    jint surfaceId,
+    jint tag) {
+  ShadowView sv{};
+  sv.componentName = "View";
+  sv.surfaceId = surfaceId;
+  sv.tag = tag;
+  sv.props = std::make_shared<const ViewProps>();
+  sv.layoutMetrics.frame.size = {100, 100};
+  mountingManager_->preallocateShadowView(sv);
+}
+
+void FabricMountingManagerTestHelper::destroyUnmountedView(
+    jint surfaceId,
+    jint tag) {
+  auto family = viewComponentDescriptor_->createFamily(
+      {.tag = tag, .surfaceId = surfaceId, .instanceHandle = nullptr});
+  mountingManager_->destroyUnmountedShadowNode(*family);
+}
+
+bool FabricMountingManagerTestHelper::wouldSkipCreate(
+    jint surfaceId,
+    jint tag) {
+  return mountingManager_->isViewAllocated(surfaceId, tag);
+}
+
+bool FabricMountingManagerTestHelper::isTagAllocated(jint surfaceId, jint tag) {
+  return mountingManager_->isViewAllocated(surfaceId, tag);
+}
+
+void FabricMountingManagerTestHelper::registerNatives() {
+  registerHybrid({
+      makeNativeMethod(
+          "initHybrid", FabricMountingManagerTestHelper::initHybrid),
+      makeNativeMethod(
+          "startSurface", FabricMountingManagerTestHelper::startSurface),
+      makeNativeMethod(
+          "stopSurface", FabricMountingManagerTestHelper::stopSurface),
+      makeNativeMethod(
+          "preallocateView", FabricMountingManagerTestHelper::preallocateView),
+      makeNativeMethod(
+          "destroyUnmountedView",
+          FabricMountingManagerTestHelper::destroyUnmountedView),
+      makeNativeMethod(
+          "wouldSkipCreate", FabricMountingManagerTestHelper::wouldSkipCreate),
+      makeNativeMethod(
+          "isTagAllocated", FabricMountingManagerTestHelper::isTagAllocated),
+  });
+}
+
+} // namespace facebook::react

packages/react-native/ReactAndroid/src/main/jni/react/fabric/test_helper/FabricMountingManagerTestHelper.h

@@ -0,0 +1,47 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#pragma once
+
+#include <fbjni/fbjni.h>
+#include <react/renderer/components/view/ViewComponentDescriptor.h>
+
+#include <react/fabric/FabricMountingManager.h>
+
+namespace facebook::react {
+
+/**
+ * JNI test helper that wraps a real FabricMountingManager
+ */
+class FabricMountingManagerTestHelper : public jni::HybridClass<FabricMountingManagerTestHelper> {
+ public:
+  static constexpr auto kJavaDescriptor = "Lcom/facebook/react/fabric/FabricMountingManagerTestHelper;";
+
+  void startSurface(jint surfaceId);
+  void stopSurface(jint surfaceId);
+  void preallocateView(jint surfaceId, jint tag);
+  void destroyUnmountedView(jint surfaceId, jint tag);
+  bool wouldSkipCreate(jint surfaceId, jint tag);
+  bool isTagAllocated(jint surfaceId, jint tag);
+
+  static void registerNatives();
+
+ private:
+  friend HybridBase;
+
+  explicit FabricMountingManagerTestHelper(jni::alias_ref<JFabricUIManager::javaobject> jFabricUIManager);
+
+  static void initHybrid(
+      jni::alias_ref<jhybridobject> jobj,
+      jni::alias_ref<JFabricUIManager::javaobject> jFabricUIManager);
+
+  jni::global_ref<JFabricUIManager::javaobject> javaUIManager_;
+  std::shared_ptr<FabricMountingManager> mountingManager_;
+  std::unique_ptr<ViewComponentDescriptor> viewComponentDescriptor_;
+};
+
+} // namespace facebook::react

packages/react-native/ReactAndroid/src/main/jni/react/fabric/test_helper/OnLoad.cpp

@@ -0,0 +1,16 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+#include <fbjni/fbjni.h>
+
+#include "FabricMountingManagerTestHelper.h"
+
+JNIEXPORT jint JNICALL JNI_OnLoad(JavaVM* vm, void*) {
+  return facebook::jni::initialize(vm, [] {
+    facebook::react::FabricMountingManagerTestHelper::registerNatives();
+  });
+}