fix(ios): center TextInput text, placeholder, and caret when lineHeight > fontSize

On iOS, when a TextInput's lineHeight exceeds its font's line height, UIKit
anchors glyphs to the bottom of the attributed-string line box instead of
centering them within it. The same misalignment affects the placeholder.
On single-line UITextField the caret is also sized to the full line box.

This patch fixes all three surfaces. The approach varies by UIKit
rendering path:

UITextView (multi-line) typed text — honors NSBaselineOffsetAttributeName.
  Call RCTApplyBaselineOffset in RCTTextInputComponentView._setAttributedString:
  to inject the offset. Re-seed NSParagraphStyleAttributeName from
  defaultTextAttributes on ranges missing it (or carrying a zero-line-height
  stub), because UIKit's typingAttributes drops the paragraph style between
  keystrokes — without the re-seed the helper sees maximumLineHeight == 0 and
  bails for typed content.

UITextField (single-line) typed text — does NOT honor NSBaselineOffsetAttributeName.
  Per-range zero out paragraphStyle.minimumLineHeight / maximumLineHeight on
  the displayed attributedText so UITextField uses the font's natural line
  height; its built-in vertical centering then positions the glyph in the
  bounds, and the caret rect (sized from the same line box) shrinks to match.
  Other paragraph-style fields (alignment, indent) are preserved so nested
  <Text> styling survives. The shadow node measures from state, so undo the
  strip in _updateState before pushing — restore stripped line heights from
  defaultTextAttributes so the cell height stays at the configured lineHeight
  across edits.

Placeholder on both UITextField.attributedPlaceholder (UILabel draw) and
RCTUITextView._placeholderView — both honor NSBaselineOffsetAttributeName.
  Add the offset computation to _placeholderTextAttributes on both backing
  views so the placeholder is centered to match (single string, single set
  of attributes — a direct computation is equivalent to RCTApplyBaselineOffset).

Bug only manifests on Fabric. Paper has had a similar baseline-offset fix in
RCTUITextView.attributedText since 0.66 (see RCTUITextView setAttributedText:),
which never made it into the Fabric component view.

Author: janicduplessis

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

@@ -11,6 +11,7 @@
 #import <React/UIView+React.h>
 
 #import <React/RCTBackedTextInputDelegateAdapter.h>
+#import <React/RCTBackedTextInputViewLineHeightUtils.h>
 #import <React/RCTTextAttributes.h>
 
 @implementation RCTUITextView {
@@ -364,6 +365,7 @@ - (void)_updatePlaceholder
     [textAttributes setValue:defaultPlaceholderFont() forKey:NSFontAttributeName];
   }
 
+  RCTApplyPlaceholderBaselineOffset(textAttributes);
   return textAttributes;
 }
 

packages/react-native/Libraries/Text/TextInput/RCTBackedTextInputViewLineHeightUtils.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.
+ */
+
+#import <UIKit/UIKit.h>
+
+NS_ASSUME_NONNULL_BEGIN
+
+/*
+ * When the configured `lineHeight` (encoded in `paragraphStyle.maximumLineHeight`)
+ * exceeds the font's natural line height, UIKit's draw paths anchor glyphs to the
+ * bottom of the paragraph line box. These helpers contain the small set of
+ * transformations needed to re-center the glyph and the caret across the various
+ * UIKit rendering paths used by `<TextInput>`.
+ */
+
+/*
+ * Returns a copy of `defaultTextAttributes` with `paragraphStyle.{min,max}LineHeight`
+ * zeroed when greater than the font's line height. UITextField / UITextView feed
+ * `defaultTextAttributes` into `typingAttributes`, so handing them a stripped copy
+ * makes the typed-text path render at the font's natural metrics — vertical centering
+ * then positions the glyph in the bounds and the caret rect (sized from the same line
+ * box) shrinks to match. Returns the input dictionary unchanged if no strip is needed.
+ */
+NSDictionary<NSAttributedStringKey, id> *RCTStripDefaultTextAttributesLineHeight(
+    NSDictionary<NSAttributedStringKey, id> *defaultTextAttributes);
+
+/*
+ * Per-range zero of `paragraphStyle.{min,max}LineHeight` on `attributedString`,
+ * preserving any other paragraph-style fields (alignment, indent) the user may have
+ * set on nested <Text>. No-ops on ranges that already have a zero line-height stub.
+ */
+void RCTStripAttributedStringLineHeights(NSMutableAttributedString *attributedString);
+
+/*
+ * Adds `NSBaselineOffsetAttributeName` to `placeholderAttributes` when
+ * `paragraphStyle.maximumLineHeight > font.lineHeight`. The placeholder UILabel
+ * draw path used by both UITextField.attributedPlaceholder and
+ * RCTUITextView._placeholderView honors baseline offset, so a single uniform offset
+ * is all that's needed to re-center the placeholder glyph in the line box.
+ */
+void RCTApplyPlaceholderBaselineOffset(NSMutableDictionary<NSAttributedStringKey, id> *placeholderAttributes);
+
+NS_ASSUME_NONNULL_END

packages/react-native/Libraries/Text/TextInput/RCTBackedTextInputViewLineHeightUtils.mm

@@ -0,0 +1,57 @@
+/*
+ * 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.
+ */
+
+#import "RCTBackedTextInputViewLineHeightUtils.h"
+
+static BOOL RCTNeedsLineHeightStrip(NSDictionary<NSAttributedStringKey, id> *attributes)
+{
+  UIFont *font = attributes[NSFontAttributeName];
+  NSParagraphStyle *paragraphStyle = attributes[NSParagraphStyleAttributeName];
+  return font && paragraphStyle && paragraphStyle.maximumLineHeight > font.lineHeight;
+}
+
+NSDictionary<NSAttributedStringKey, id> *RCTStripDefaultTextAttributesLineHeight(
+    NSDictionary<NSAttributedStringKey, id> *defaultTextAttributes)
+{
+  if (!RCTNeedsLineHeightStrip(defaultTextAttributes)) {
+    return defaultTextAttributes;
+  }
+  NSMutableDictionary<NSAttributedStringKey, id> *stripped = [defaultTextAttributes mutableCopy];
+  NSMutableParagraphStyle *strippedStyle = [defaultTextAttributes[NSParagraphStyleAttributeName] mutableCopy];
+  strippedStyle.minimumLineHeight = 0;
+  strippedStyle.maximumLineHeight = 0;
+  stripped[NSParagraphStyleAttributeName] = strippedStyle;
+  return stripped;
+}
+
+void RCTStripAttributedStringLineHeights(NSMutableAttributedString *attributedString)
+{
+  [attributedString
+      enumerateAttribute:NSParagraphStyleAttributeName
+                 inRange:NSMakeRange(0, attributedString.length)
+                 options:0
+              usingBlock:^(NSParagraphStyle *style, NSRange range, __unused BOOL *stop) {
+                if (!style || (style.maximumLineHeight == 0 && style.minimumLineHeight == 0)) {
+                  return;
+                }
+                NSMutableParagraphStyle *stripped = [style mutableCopy];
+                stripped.minimumLineHeight = 0;
+                stripped.maximumLineHeight = 0;
+                [attributedString addAttribute:NSParagraphStyleAttributeName value:stripped range:range];
+              }];
+}
+
+void RCTApplyPlaceholderBaselineOffset(NSMutableDictionary<NSAttributedStringKey, id> *placeholderAttributes)
+{
+  if (!RCTNeedsLineHeightStrip(placeholderAttributes)) {
+    return;
+  }
+  NSParagraphStyle *paragraphStyle = placeholderAttributes[NSParagraphStyleAttributeName];
+  UIFont *font = placeholderAttributes[NSFontAttributeName];
+  placeholderAttributes[NSBaselineOffsetAttributeName] =
+      @((paragraphStyle.maximumLineHeight - font.lineHeight) / 2.0);
+}

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

@@ -8,6 +8,7 @@
 #import <React/RCTUITextField.h>
 
 #import <React/RCTBackedTextInputDelegateAdapter.h>
+#import <React/RCTBackedTextInputViewLineHeightUtils.h>
 #import <React/RCTTextAttributes.h>
 #import <React/RCTUtils.h>
 #import <React/UIView+React.h>
@@ -92,8 +93,13 @@ - (void)setDefaultTextAttributes:(NSDictionary<NSAttributedStringKey, id> *)defa
     return;
   }
 
+  // Keep the unstripped attributes available for `_placeholderTextAttributes`, which
+  // needs the configured `lineHeight` to compute the placeholder's baseline offset.
+  // Hand UIKit a copy with the paragraphStyle line-height zeroed so the typed-text
+  // path renders at the font's natural metrics — vertical centering then positions
+  // the glyph in the bounds and the caret rect shrinks to match.
   _defaultTextAttributes = defaultTextAttributes;
-  [super setDefaultTextAttributes:defaultTextAttributes];
+  [super setDefaultTextAttributes:RCTStripDefaultTextAttributesLineHeight(defaultTextAttributes)];
   [self _updatePlaceholder];
 }
 
@@ -169,6 +175,7 @@ - (void)setDisableKeyboardShortcuts:(BOOL)disableKeyboardShortcuts
     [textAttributes removeObjectForKey:NSForegroundColorAttributeName];
   }
 
+  RCTApplyPlaceholderBaselineOffset(textAttributes);
   return textAttributes;
 }
 

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

@@ -12,6 +12,7 @@
 #import <react/renderer/textlayoutmanager/RCTAttributedTextUtils.h>
 #import <react/renderer/textlayoutmanager/TextLayoutManager.h>
 
+#import <React/RCTBackedTextInputViewLineHeightUtils.h>
 #import <React/RCTBackedTextInputViewProtocol.h>
 #import <React/RCTScrollViewComponentView.h>
 #import <React/RCTUITextField.h>
@@ -47,6 +48,12 @@ @implementation RCTTextInputComponentView {
   UIView<RCTBackedTextInputViewProtocol> *_backedTextInputView;
   NSUInteger _mostRecentEventCount;
   NSAttributedString *_lastStringStateWasUpdatedWith;
+  // The most-recent attributedString JS pushed via `_setAttributedString:`, kept
+  // unstripped (with the configured `lineHeight`). Used as the source of truth
+  // for state pushes — the displayed `attributedText` may have its paragraph-style
+  // line-height zeroed for single-line rendering, but the shadow node measures
+  // from state and needs the real line-height to keep the cell stable.
+  NSAttributedString *_sourceAttributedString;
 
   /*
    * UIKit uses either UITextField or UITextView as its UIKit element for <TextInput>. UITextField is for single line
@@ -712,10 +719,10 @@ - (void)handleInputAccessoryDoneButton
   return {
       .text = RCTStringFromNSString(_backedTextInputView.attributedText.string),
       .selectionRange = [self _selectionRange],
-      .eventCount = static_cast<int>(_mostRecentEventCount),
+      .contentSize = RCTSizeFromCGSize(_backedTextInputView.contentSize),
       .contentOffset = RCTPointFromCGPoint(_backedTextInputView.contentOffset),
       .contentInset = RCTEdgeInsetsFromUIEdgeInsets(_backedTextInputView.contentInset),
-      .contentSize = RCTSizeFromCGSize(_backedTextInputView.contentSize),
+      .eventCount = static_cast<int>(_mostRecentEventCount),
       .layoutMeasurement = RCTSizeFromCGSize(_backedTextInputView.bounds.size),
       .zoomScale = _backedTextInputView.zoomScale,
   };
@@ -729,7 +736,21 @@ - (void)_updateState
   NSAttributedString *attributedString = _backedTextInputView.attributedText;
   auto data = _state->getData();
   _lastStringStateWasUpdatedWith = attributedString;
-  data.attributedStringBox = RCTAttributedStringBoxFromNSAttributedString(attributedString);
+
+  // Single-line rendering may have stripped the paragraph-style line-height from
+  // the displayed `attributedText`, but the shadow node measures from state and
+  // needs the configured `lineHeight`. Use the saved unstripped source string
+  // when its content still matches the field; if the user has typed since the
+  // last push, fall back to building a fresh string from the field's text and
+  // the (unstripped) `defaultTextAttributes`.
+  NSAttributedString *sourceString = _sourceAttributedString;
+  if (![sourceString.string isEqualToString:attributedString.string]) {
+    sourceString = [[NSAttributedString alloc] initWithString:attributedString.string
+                                                   attributes:_backedTextInputView.defaultTextAttributes];
+    _sourceAttributedString = sourceString;
+  }
+
+  data.attributedStringBox = RCTAttributedStringBoxFromNSAttributedString(sourceString);
   _mostRecentEventCount += _comingFromJS ? 0 : 1;
   data.mostRecentEventCount = _mostRecentEventCount;
   _state->updateState(std::move(data));
@@ -768,6 +789,46 @@ - (void)_restoreTextSelectionAndIgnoreCaretChange:(BOOL)ignore
 
 - (void)_setAttributedString:(NSAttributedString *)attributedString
 {
+  // Save the unstripped source for layout / state push before any display
+  // transformation runs. `_updateState` reads this back so the shadow node
+  // measures from the configured `lineHeight`, not the render-time strip.
+  _sourceAttributedString = [attributedString copy];
+
+  // When `lineHeight > font.lineHeight`, UIKit's draw paths anchor glyphs to the
+  // bottom of the paragraph line box. UITextView honors NSBaselineOffsetAttributeName
+  // to re-center; UITextField does not, so for single-line we zero the paragraph-style
+  // line-height (UITextField then renders at the font's natural metrics, and its
+  // built-in vertical centering positions the glyph in the bounds).
+  NSDictionary<NSAttributedStringKey, id> *defaults = _backedTextInputView.defaultTextAttributes;
+  NSParagraphStyle *defaultParagraphStyle = defaults[NSParagraphStyleAttributeName];
+  UIFont *defaultFont = defaults[NSFontAttributeName];
+  if (attributedString.length > 0 && defaultParagraphStyle && defaultFont &&
+      defaultParagraphStyle.maximumLineHeight > defaultFont.lineHeight) {
+    NSMutableAttributedString *mutableString = [attributedString mutableCopy];
+    if ([_backedTextInputView isKindOfClass:[RCTUITextView class]]) {
+      // UIKit's typingAttributes drop NSParagraphStyle on the round-trip, so chars typed
+      // since the last state push arrive without a paragraph style. Re-seed those ranges
+      // (and ranges with a zero-line-height stub) from the default so UITextView resolves a
+      // consistent line-box height across the whole string. RCTApplyBaselineOffset then
+      // computes the centering offset using the max font.lineHeight present on each line —
+      // correct for mixed-font input from nested <Text> children.
+      [mutableString enumerateAttribute:NSParagraphStyleAttributeName
+                                inRange:NSMakeRange(0, mutableString.length)
+                                options:0
+                             usingBlock:^(NSParagraphStyle *style, NSRange range, __unused BOOL *stop) {
+                               if (!style || style.maximumLineHeight == 0) {
+                                 [mutableString addAttribute:NSParagraphStyleAttributeName
+                                                       value:defaultParagraphStyle
+                                                       range:range];
+                               }
+                             }];
+      RCTApplyBaselineOffset(mutableString);
+    } else {
+      RCTStripAttributedStringLineHeights(mutableString);
+    }
+    attributedString = mutableString;
+  }
+
   if ([self _textOf:attributedString equals:_backedTextInputView.attributedText]) {
     return;
   }