Fix updateEventEmitter clobbering deferred text styles and preserve EventEmitter key in traitCollectionDidChange

- updateEventEmitter: use _pendingDefaultTextAttributes as base when it
  exists, so deferred text-style changes from updateProps are not lost
- traitCollectionDidChange: preserve EventEmitter key in the non-composing
  branch (was already preserved in the composing branch)
- RCTUITextView/RCTUITextField: replace @"EventEmitter" string literal
  with local constant kRCTEventEmitterAttributeKey for consistency
- Remove duplicate test file from React/Tests/TextInput/ (canonical copy
  is in rn-tester/RNTesterUnitTests/)

Author: kdwkr

packages/react-native/Libraries/Text/TextInput/Multiline/RCTUITextView.mm

@@ -13,6 +13,10 @@
 #import <React/RCTBackedTextInputDelegateAdapter.h>
 #import <React/RCTTextAttributes.h>
 
+// Must match RCTAttributedStringEventEmitterKey in RCTAttributedTextUtils.h
+// (cannot import directly — Libraries/Text must not depend on ReactCommon).
+static NSString *const kRCTEventEmitterAttributeKey = @"EventEmitter";
+
 @implementation RCTUITextView {
   UILabel *_placeholderView;
   UITextView *_detachedTextView;
@@ -138,7 +142,7 @@ - (void)setDefaultTextAttributes:(NSDictionary<NSAttributedStringKey, id> *)defa
   // Strip attributes that interfere with UIKit's IME composition underline rendering.
   // Only remove no-op defaults; preserve user-specified values.
   NSMutableDictionary *typingAttrs = [defaultTextAttributes mutableCopy];
-  [typingAttrs removeObjectForKey:@"EventEmitter"];
+  [typingAttrs removeObjectForKey:kRCTEventEmitterAttributeKey];
   NSShadow *shadow = typingAttrs[NSShadowAttributeName];
   if (shadow && CGSizeEqualToSize(shadow.shadowOffset, CGSizeZero) && shadow.shadowBlurRadius == 0) {
     [typingAttrs removeObjectForKey:NSShadowAttributeName];

packages/react-native/Libraries/Text/TextInput/Singleline/RCTUITextField.mm

@@ -12,6 +12,10 @@
 #import <React/RCTUtils.h>
 #import <React/UIView+React.h>
 
+// Must match RCTAttributedStringEventEmitterKey in RCTAttributedTextUtils.h
+// (cannot import directly — Libraries/Text must not depend on ReactCommon).
+static NSString *const kRCTEventEmitterAttributeKey = @"EventEmitter";
+
 @implementation RCTUITextField {
   RCTBackedTextFieldDelegateAdapter *_textInputDelegateAdapter;
   NSDictionary<NSAttributedStringKey, id> *_defaultTextAttributes;
@@ -99,7 +103,7 @@ - (void)setDefaultTextAttributes:(NSDictionary<NSAttributedStringKey, id> *)defa
   // clear background) — preserve user-specified values.
   // EventEmitter is a React-internal attribute (NSData wrapping C++ weak_ptr).
   NSMutableDictionary *uikitAttrs = [defaultTextAttributes mutableCopy];
-  [uikitAttrs removeObjectForKey:@"EventEmitter"];
+  [uikitAttrs removeObjectForKey:kRCTEventEmitterAttributeKey];
   NSShadow *shadow = uikitAttrs[NSShadowAttributeName];
   if (shadow && CGSizeEqualToSize(shadow.shadowOffset, CGSizeZero) && shadow.shadowBlurRadius == 0) {
     [uikitAttrs removeObjectForKey:NSShadowAttributeName];

packages/react-native/React/Fabric/Mounting/ComponentViews/TextInput/RCTTextInputComponentView.mm

@@ -117,8 +117,12 @@ - (void)updateEventEmitter:(const EventEmitter::Shared &)eventEmitter
 {
   [super updateEventEmitter:eventEmitter];
 
+  // Use pending attributes as the base if they exist (e.g., from a prior updateProps
+  // that deferred during composition), to avoid clobbering deferred text-style changes.
   NSMutableDictionary<NSAttributedStringKey, id> *defaultAttributes =
-      [_backedTextInputView.defaultTextAttributes mutableCopy];
+      _pendingDefaultTextAttributes
+          ? [_pendingDefaultTextAttributes mutableCopy]
+          : [_backedTextInputView.defaultTextAttributes mutableCopy];
 
   defaultAttributes[RCTAttributedStringEventEmitterKey] = RCTWrapEventEmitter(_eventEmitter);
 
@@ -173,6 +177,11 @@ - (void)traitCollectionDidChange:(UITraitCollection *)previousTraitCollection
       _needsUpdateDefaultTextAttributes = YES;
       _pendingDefaultTextAttributes = [attributes copy];
     } else {
+      // Preserve the event emitter key since RCTNSTextAttributesFromTextAttributes does not include it.
+      id emitter = _backedTextInputView.defaultTextAttributes[RCTAttributedStringEventEmitterKey];
+      if (emitter) {
+        attributes[RCTAttributedStringEventEmitterKey] = emitter;
+      }
       _backedTextInputView.defaultTextAttributes = attributes;
     }
   }