fix(iOS): main-thread deadlock in markdown measurement under locked ShadowTree commits

packages/react-native-enriched-markdown/ios/EnrichedMarkdown.h

@@ -13,6 +13,13 @@ NS_ASSUME_NONNULL_BEGIN
 - (void)renderMarkdownSynchronously:(NSString *)markdownString;
 - (BOOL)hasRenderedMarkdown:(NSString *)markdown;
 - (BOOL)hasRenderedWithStyleFingerprint:(size_t)fingerprint;
+
+/// Size last committed by Fabric via `updateLayoutMetrics:`, readable from
+/// any thread without blocking (issue #550: the shadow-node measure path must
+/// never wait on the main thread, and reading `bounds` off-main is unsafe).
+/// Backed by a single lock-free atomic, so the pair can't tear. Returns
+/// CGSizeZero until the first layout is committed.
+- (CGSize)lastCommittedLayoutSize;
 @end
 
 NS_ASSUME_NONNULL_END

packages/react-native-enriched-markdown/ios/EnrichedMarkdown.mm

@@ -2,6 +2,7 @@
 #import "ContextMenuUtils.h"
 #import "ENRMAccessibilityLabels.h"
 #import "ENRMAsyncRenderCoordinator.h"
+#import "ENRMAtomicSize.h"
 #import "ENRMImageAttachment.h"
 #import "ENRMMarkdownParser.h"
 #import "ENRMTailFadeInAnimator.h"
@@ -67,7 +68,6 @@ typedef NS_OPTIONS(NSUInteger, ENRMDirtyFlags) {
 
 static char kENRMSegmentFadeAnimatorKey;
 
-
 @interface EnrichedMarkdown () <RCTEnrichedMarkdownViewProtocol, UITextViewDelegate, ENRMImageLayoutObserver>
 + (ENRMMd4cFlags *)flagsFromProps:(const EnrichedMarkdownMd4cFlagsStruct &)props;
 - (void)emitLinkPress:(NSString *)url;
@@ -119,6 +119,8 @@ @implementation EnrichedMarkdown {
 
   ENRMWritingDirectionMode _writingDirectionMode;
   NSWritingDirection _resolvedLayoutDirection;
+
+  ENRMAtomicSize _lastCommittedSize;
 }
 
 + (ComponentDescriptorProvider)componentDescriptorProvider
@@ -367,6 +369,11 @@ - (CGSize)measureSize:(CGFloat)maxWidth
   return CGSizeMake(measuredWidth, measuredHeight);
 }
 
+- (CGSize)lastCommittedLayoutSize
+{
+  return _lastCommittedSize.load();
+}
+
 - (BOOL)hasRenderedMarkdown:(NSString *)markdown
 {
   return _renderedMarkdown != nil && [_renderedMarkdown isEqualToString:markdown];
@@ -410,6 +417,8 @@ - (void)updateLayoutMetrics:(const LayoutMetrics &)layoutMetrics
 {
   [super updateLayoutMetrics:layoutMetrics oldLayoutMetrics:oldLayoutMetrics];
 
+  _lastCommittedSize.store(CGSizeMake(layoutMetrics.frame.size.width, layoutMetrics.frame.size.height));
+
   NSWritingDirection resolved = _resolvedLayoutDirection;
   if (layoutMetrics.layoutDirection == LayoutDirection::RightToLeft) {
     resolved = NSWritingDirectionRightToLeft;

packages/react-native-enriched-markdown/ios/EnrichedMarkdownText.h

@@ -13,6 +13,13 @@ NS_ASSUME_NONNULL_BEGIN
 - (void)renderMarkdownSynchronously:(NSString *)markdownString;
 - (BOOL)hasRenderedMarkdown:(NSString *)markdown;
 - (BOOL)hasRenderedWithStyleFingerprint:(size_t)fingerprint;
+
+/// Size last committed by Fabric via `updateLayoutMetrics:`, readable from
+/// any thread without blocking (issue #550: the shadow-node measure path must
+/// never wait on the main thread, and reading `bounds` off-main is unsafe).
+/// Backed by a single lock-free atomic, so the pair can't tear. Returns
+/// CGSizeZero until the first layout is committed.
+- (CGSize)lastCommittedLayoutSize;
 @end
 
 NS_ASSUME_NONNULL_END

packages/react-native-enriched-markdown/ios/EnrichedMarkdownText.mm

@@ -3,6 +3,7 @@
 #import "ContextMenuUtils.h"
 #import "ENRMAccessibilityLabels.h"
 #import "ENRMAsyncRenderCoordinator.h"
+#import "ENRMAtomicSize.h"
 #import "ENRMContextMenuTextView+macOS.h"
 #import "ENRMImageAttachment.h"
 #import "ENRMMarkdownParser.h"
@@ -112,6 +113,8 @@ @implementation EnrichedMarkdownText {
   NSWritingDirection _resolvedLayoutDirection;
 
   ENRMDirtyFlags _dirtyFlags;
+
+  ENRMAtomicSize _lastCommittedSize;
 }
 
 + (ComponentDescriptorProvider)componentDescriptorProvider
@@ -142,6 +145,11 @@ - (CGSize)measureSize:(CGFloat)maxWidth
   return size;
 }
 
+- (CGSize)lastCommittedLayoutSize
+{
+  return _lastCommittedSize.load();
+}
+
 - (BOOL)hasRenderedMarkdown:(NSString *)markdown
 {
   return _renderedMarkdown != nil && [_renderedMarkdown isEqualToString:markdown];
@@ -170,6 +178,8 @@ - (void)updateLayoutMetrics:(const LayoutMetrics &)layoutMetrics
 {
   [super updateLayoutMetrics:layoutMetrics oldLayoutMetrics:oldLayoutMetrics];
 
+  _lastCommittedSize.store(CGSizeMake(layoutMetrics.frame.size.width, layoutMetrics.frame.size.height));
+
   NSWritingDirection resolved = _resolvedLayoutDirection;
   if (layoutMetrics.layoutDirection == LayoutDirection::RightToLeft) {
     resolved = NSWritingDirectionRightToLeft;