From f2d264192fe2636a0a0f464b0a540dfc189025b3 Mon Sep 17 00:00:00 2001
From: Pieter De Baets <pieterdb@meta.com>
Date: Thu, 9 Jul 2026 05:50:40 -0700
Subject: [PATCH] Move shadow-node ref update thread tag to Fabric install

Summary:
The runtime shadow-node reference update thread tag was being set on every `RuntimeExecutor` tick in `ReactInstance`. Because the underlying flag is `thread_local` and only meaningful on the JS runtime thread, it's enough to set it once during Fabric installation.

Move the `ShadowNode::setUseRuntimeShadowNodeReferenceUpdateOnThread` call into `UIManagerBinding::createAndInstallIfNeeded`, which runs on the JS thread and is guarded to execute once per runtime. This lets the `runtime` module drop its direct dependency on `renderer/core`, reducing coupling between the Bridgeless runtime host and the Fabric renderer.

Behavior is unchanged: the write was already unconditional; the read at `updateMountedFlag` is still gated by the existing feature flag.

Changelog: [Internal]

Reviewed By: lenaic, zeyap

Differential Revision: D111059967
---
 .../react/renderer/uimanager/UIManagerBinding.cpp          | 7 +++++++
 .../ReactCommon/react/runtime/ReactInstance.cpp            | 2 --
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/packages/react-native/ReactCommon/react/renderer/uimanager/UIManagerBinding.cpp b/packages/react-native/ReactCommon/react/renderer/uimanager/UIManagerBinding.cpp
index c79fc75df5b4..ed1c62d97394 100644
--- a/packages/react-native/ReactCommon/react/renderer/uimanager/UIManagerBinding.cpp
+++ b/packages/react-native/ReactCommon/react/renderer/uimanager/UIManagerBinding.cpp
@@ -13,6 +13,7 @@
 #include <react/debug/react_native_assert.h>
 #include <react/renderer/components/view/PointerEvent.h>
 #include <react/renderer/core/LayoutableShadowNode.h>
+#include <react/renderer/core/ShadowNode.h>
 #include <react/renderer/dom/DOM.h>
 #include <react/renderer/runtimescheduler/RuntimeSchedulerBinding.h>
 #include <react/renderer/uimanager/primitives.h>
@@ -35,6 +36,12 @@ void UIManagerBinding::createAndInstallIfNeeded(
     auto object = jsi::Object::createFromHostObject(runtime, uiManagerBinding);
     runtime.global().setProperty(
         runtime, uiManagerModuleName, std::move(object));
+
+    // Tag this thread (the JS runtime thread) as the one allowed to propagate
+    // runtime shadow-node reference updates during Fabric commit. The flag is
+    // thread_local, so it must be set on the JS thread; installing the binding
+    // is guaranteed to run there and to happen before any JS-driven commit.
+    ShadowNode::setUseRuntimeShadowNodeReferenceUpdateOnThread(true);
   }
 }
 
diff --git a/packages/react-native/ReactCommon/react/runtime/ReactInstance.cpp b/packages/react-native/ReactCommon/react/runtime/ReactInstance.cpp
index 4d12c90edfb3..e6b8f129d5f4 100644
--- a/packages/react-native/ReactCommon/react/runtime/ReactInstance.cpp
+++ b/packages/react-native/ReactCommon/react/runtime/ReactInstance.cpp
@@ -18,7 +18,6 @@
 #include <jsi/instrumentation.h>
 #include <jsinspector-modern/HostTarget.h>
 #include <react/featureflags/ReactNativeFeatureFlags.h>
-#include <react/renderer/core/ShadowNode.h>
 #include <react/renderer/runtimescheduler/RuntimeSchedulerBinding.h>
 #include <react/runtime/JSRuntimeBindings.h>
 #include <react/timing/primitives.h>
@@ -98,7 +97,6 @@ ReactInstance::ReactInstance(
             jsi::Runtime& jsiRuntime = runtime->getRuntime();
             TraceSection s("ReactInstance::_runtimeExecutor[Callback]");
             try {
-              ShadowNode::setUseRuntimeShadowNodeReferenceUpdateOnThread(true);
               callback(jsiRuntime);
             } catch (jsi::JSError& originalError) {
               jsErrorHandler->handleError(jsiRuntime, originalError, true);