[macOS] Fix Paper ScrollView layout with legacy scrollbars

Apply paddingEnd on the ScrollView's shadow node to account for legacy
scrollbar width, matching the Fabric approach. A new RCTScrollViewShadowView
(defined inline in RCTScrollViewManager.m) reads +[NSScroller
preferredScrollerStyle] and +scrollerWidthForControlSize: directly
during layoutWithMetrics: — these are thread-safe class methods, so no
localData round-trip is needed.

Key changes:
- Add RCTScrollViewShadowView with paddingEnd for scrollbar width
- Revert RCTScrollContentShadowView to stock (only RTL fix remains)
- Delete RCTScrollContentLocalData files (no longer used)
- Update preferredScrollerStyleDidChange: in RCTScrollView to re-tile
  and trigger content size update on system preference changes

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: Saadnajmi

packages/react-native/React/Views/ScrollView/MacOS/RCTScrollContentLocalData.h

@@ -9,28 +9,10 @@
 
 #import <yoga/Yoga.h>
 
-#if TARGET_OS_OSX // [macOS
-#import "RCTScrollContentLocalData.h"
-#endif // macOS]
-
 #import "RCTUtils.h"
 
 @implementation RCTScrollContentShadowView
 
-#if TARGET_OS_OSX // [macOS
-- (void)setLocalData:(RCTScrollContentLocalData *)localData
-{
-  RCTAssert(
-      [localData isKindOfClass:[RCTScrollContentLocalData class]],
-      @"Local data object for `RCTScrollContentView` must be `RCTScrollContentLocalData` instance.");
-
-  super.marginEnd = (YGValue){localData.verticalScrollerWidth, YGUnitPoint};
-  super.marginBottom = (YGValue){localData.horizontalScrollerHeight, YGUnitPoint};
-
-  [self didSetProps:@[@"marginEnd", @"marginBottom"]];
-}
-#endif // macOS]
-
 - (void)layoutWithMetrics:(RCTLayoutMetrics)layoutMetrics layoutContext:(RCTLayoutContext)layoutContext
 {
   if (layoutMetrics.layoutDirection == UIUserInterfaceLayoutDirectionRightToLeft) {

packages/react-native/React/Views/ScrollView/MacOS/RCTScrollContentLocalData.m

@@ -10,11 +10,6 @@
 #import <React/RCTAssert.h>
 #import <React/UIView+React.h>
 
-#if TARGET_OS_OSX // [macOS
-#import <React/RCTUIManager.h>
-#import "RCTScrollContentLocalData.h"
-#endif // macOS]
-
 #import "RCTScrollView.h"
 
 @implementation RCTScrollContentView
@@ -44,22 +39,7 @@ - (void)reactSetFrame:(CGRect)frame
 
   [scrollView updateContentSizeIfNeeded];
 #if TARGET_OS_OSX // [macOS
-  // On macOS scroll indicators may float over the content view like they do in iOS
-  // or depending on system preferences they may be outside of the content view
-  // which means the clip view will be smaller than the scroll view itself.
-  // In such cases the content view layout must shrink accordingly otherwise
-  // the contents will overflow causing the scroll indicators to appear unnecessarily.
   NSScrollView *platformScrollView = [scrollView scrollView];
-  if ([platformScrollView scrollerStyle] == NSScrollerStyleLegacy) {
-    BOOL contentHasHeight = platformScrollView.contentSize.height > 0;
-    CGFloat horizontalScrollerHeight = ([platformScrollView hasHorizontalScroller] && contentHasHeight) ? NSHeight([[platformScrollView horizontalScroller] frame]) : 0;
-    CGFloat verticalScrollerWidth = [platformScrollView hasVerticalScroller] ? NSWidth([[platformScrollView verticalScroller] frame]) : 0;
-
-    RCTScrollContentLocalData *localData = [[RCTScrollContentLocalData alloc] initWithVerticalScrollerWidth:verticalScrollerWidth horizontalScrollerHeight:horizontalScrollerHeight];
-
-    [[[scrollView bridge] uiManager] setLocalData:localData forView:self];
-  }
-
   if ([platformScrollView accessibilityRole] == NSAccessibilityTableRole) {
       NSMutableArray *subViews = [[NSMutableArray alloc] initWithCapacity:[[self subviews] count]];
       for (NSView *view in [self subviews]) {

packages/react-native/React/Views/ScrollView/RCTScrollContentShadowView.m

@@ -1431,6 +1431,14 @@ - (void)keyUp:(NSEvent *)event {
 
 - (void)preferredScrollerStyleDidChange:(__unused NSNotification *)notification {
   RCT_SEND_SCROLL_EVENT(onPreferredScrollerStyleDidChange, (@{ @"preferredScrollerStyle": RCTStringForScrollerStyle([NSScroller preferredScrollerStyle])}));
+
+  // When the system scrollbar style changes, force the scroll view to adopt the
+  // new style, re-tile, and trigger a content size update. The ScrollView's
+  // shadow view (RCTScrollViewShadowView) will detect the new scroller style on
+  // the next layout pass and update its padding accordingly.
+  _scrollView.scrollerStyle = [NSScroller preferredScrollerStyle];
+  [_scrollView tile];
+  [self updateContentSizeIfNeeded];
 }
 #endif // macOS]
 

packages/react-native/React/Views/ScrollView/RCTScrollContentView.m

@@ -12,6 +12,42 @@
 #import "RCTShadowView.h"
 #import "RCTUIManager.h"
 
+#if TARGET_OS_OSX // [macOS
+// Custom shadow view for ScrollView on macOS. Applies paddingEnd to account for
+// legacy (always-visible) scrollbar width so Yoga lays out children within the
+// actual visible area (clip view), not the full ScrollView frame.
+@interface RCTScrollViewShadowView : RCTShadowView
+@end
+
+@implementation RCTScrollViewShadowView
+
+- (void)applyScrollbarPadding
+{
+  CGFloat verticalScrollerWidth = 0;
+  if ([NSScroller preferredScrollerStyle] == NSScrollerStyleLegacy) {
+    verticalScrollerWidth = [NSScroller scrollerWidthForControlSize:NSControlSizeRegular
+                                                      scrollerStyle:NSScrollerStyleLegacy];
+  }
+
+  YGValue currentPaddingEnd = super.paddingEnd;
+  BOOL needsUpdate =
+      (currentPaddingEnd.unit != YGUnitPoint || currentPaddingEnd.value != verticalScrollerWidth);
+
+  if (needsUpdate) {
+    super.paddingEnd = (YGValue){verticalScrollerWidth, YGUnitPoint};
+    [self didSetProps:@[@"paddingEnd"]];
+  }
+}
+
+- (void)layoutWithMetrics:(RCTLayoutMetrics)layoutMetrics layoutContext:(RCTLayoutContext)layoutContext
+{
+  [self applyScrollbarPadding];
+  [super layoutWithMetrics:layoutMetrics layoutContext:layoutContext];
+}
+
+@end
+#endif // macOS]
+
 #if !TARGET_OS_OSX // [macOS]
 @implementation RCTConvert (UIScrollView)
 
@@ -62,6 +98,13 @@ - (RCTPlatformView *)view // [macOS]
   return [[RCTScrollView alloc] initWithEventDispatcher:self.bridge.eventDispatcher];
 }
 
+#if TARGET_OS_OSX // [macOS
+- (RCTShadowView *)shadowView
+{
+  return [RCTScrollViewShadowView new];
+}
+#endif // macOS]
+
 RCT_EXPORT_VIEW_PROPERTY(alwaysBounceHorizontal, BOOL)
 RCT_EXPORT_VIEW_PROPERTY(alwaysBounceVertical, BOOL)
 RCT_EXPORT_NOT_OSX_VIEW_PROPERTY(bounces, BOOL) // [macOS]