Fabric implementation of react/react-native#38359

Author: fabOnReact

ios/RCTMarkdownUtils.mm

@@ -136,6 +136,8 @@ - (NSAttributedString *)parseMarkdown:(nullable NSAttributedString *)input withA
             }
         }];
 
+        RCTApplyBaselineOffset(attributedString);
+
         [attributedString endEditing];
 
         _prevInputString = inputString;
@@ -148,4 +150,47 @@ - (NSAttributedString *)parseMarkdown:(nullable NSAttributedString *)input withA
     }
 }
 
+static void RCTApplyBaselineOffset(NSMutableAttributedString *attributedText)
+{
+  __block CGFloat maximumLineHeight = 0;
+
+  [attributedText enumerateAttribute:NSParagraphStyleAttributeName
+                             inRange:NSMakeRange(0, attributedText.length)
+                             options:NSAttributedStringEnumerationLongestEffectiveRangeNotRequired
+                          usingBlock:^(NSParagraphStyle *paragraphStyle, __unused NSRange range, __unused BOOL *stop) {
+    if (!paragraphStyle) {
+      return;
+    }
+
+    maximumLineHeight = MAX(paragraphStyle.maximumLineHeight, maximumLineHeight);
+  }];
+
+  if (maximumLineHeight == 0) {
+    // `lineHeight` was not specified, nothing to do.
+    return;
+  }
+
+  __block CGFloat maximumFontLineHeight = 0;
+
+  [attributedText enumerateAttribute:NSFontAttributeName
+                             inRange:NSMakeRange(0, attributedText.length)
+                             options:NSAttributedStringEnumerationLongestEffectiveRangeNotRequired
+                          usingBlock:^(UIFont *font, NSRange range, __unused BOOL *stop) {
+    if (!font) {
+      return;
+    }
+
+    maximumFontLineHeight = MAX(font.lineHeight, maximumFontLineHeight);
+  }];
+
+  if (maximumLineHeight < maximumFontLineHeight) {
+    return;
+  }
+
+  CGFloat baseLineOffset = (maximumLineHeight - maximumFontLineHeight) / 2.0;
+  [attributedText addAttribute:NSBaselineOffsetAttributeName
+                         value:@(baseLineOffset)
+                         range:NSMakeRange(0, attributedText.length)];
+}
+
 @end