Set up gating for position: relative as default (#41711)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/41711

We want the default position to be relative for a number of reasons. This should be fine for the most part but putting a killswitch around this change just in case things blow up.

Changelog: [Internal]

Reviewed By: NickGerleman

Differential Revision: D51643446

fbshipit-source-id: 4f7d1e498eb663801ef6d88ba9cd9b64c781d66b

packages/react-native/React/Fabric/RCTSurfacePresenter.mm

@@ -284,6 +284,10 @@ static BackgroundExecutor RCTGetBackgroundExecutor()
     CoreFeatures::enableClonelessStateProgression = true;
   }
 
+  if (reactNativeConfig && reactNativeConfig->getBool("react_fabric:position_relative_default")) {
+    CoreFeatures::positionRelativeDefault = true;
+  }
+
   auto componentRegistryFactory =
       [factory = wrapManagedObject(_mountingManager.componentViewRegistry.componentViewFactory)](
           const EventDispatcher::Weak &eventDispatcher, const ContextContainer::Shared &contextContainer) {

packages/react-native/ReactAndroid/src/main/java/com/facebook/react/config/ReactFeatureFlags.java

@@ -182,4 +182,7 @@ public class ReactFeatureFlags {
    *  when there is work to do.
    */
   public static boolean enableOnDemandReactChoreographer = false;
+
+  /** When enabled, the default value of the position style property is relative. */
+  public static boolean positionRelativeDefault = false;
 }

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

@@ -424,6 +424,8 @@ void Binding::installFabricUIManager(
       getFeatureFlagValue("enableClonelessStateProgression");
   CoreFeatures::excludeYogaFromRawProps =
       getFeatureFlagValue("excludeYogaFromRawProps");
+  CoreFeatures::positionRelativeDefault =
+      getFeatureFlagValue("positionRelativeDefault");
 
   // RemoveDelete mega-op
   ShadowViewMutation::PlatformSupportsRemoveDeleteTreeInstruction =

packages/react-native/ReactCommon/react/renderer/components/view/YogaStylableProps.cpp

@@ -38,7 +38,9 @@ YogaStylableProps::YogaStylableProps(
 /*static*/ const yoga::Style& YogaStylableProps::defaultStyle() {
   static const auto defaultStyle = []() {
     yoga::Style style;
-    style.setPositionType(yoga::PositionType::Static);
+    style.setPositionType(
+        CoreFeatures::positionRelativeDefault ? yoga::PositionType::Relative
+                                              : yoga::PositionType::Static);
     return style;
   }();
 

packages/react-native/ReactCommon/react/utils/CoreFeatures.cpp

@@ -23,5 +23,6 @@ bool CoreFeatures::enableClonelessStateProgression = false;
 bool CoreFeatures::excludeYogaFromRawProps = false;
 bool CoreFeatures::enableMicrotasks = false;
 bool CoreFeatures::enableReportEventPaintTime = false;
+bool CoreFeatures::positionRelativeDefault = false;
 
 } // namespace facebook::react

packages/react-native/ReactCommon/react/utils/CoreFeatures.h

@@ -67,6 +67,9 @@ class CoreFeatures {
   // Report paint time inside the Event Timing API implementation
   // (PerformanceObserver).
   static bool enableReportEventPaintTime;
+
+  // Sets the default position of nodes to be relative instead of static
+  static bool positionRelativeDefault;
 };
 
 } // namespace facebook::react