[macOS] Fix Paper ScrollView scrollbar inset handling without localData

Replace the buggy RCTScrollContentLocalData mechanism with direct
scrollbar detection in the shadow node. RCTScrollContentShadowView now
calls +[NSScroller preferredScrollerStyle] and +scrollerWidthForControlSize:
directly in layoutWithMetrics: to detect legacy scrollbar dimensions and
apply them as trailing margin. These are class methods that read cached
system state, so they are safe to call from the shadow thread.

Key changes:
- Use marginEnd (not paddingEnd) on the content shadow view so the
  documentView frame itself shrinks to match the clip view width.
  Padding only shrinks children inside the content view while leaving
  the documentView full-width, causing horizontal scrollbar overflow.
- Eliminate the localData round-trip through the UIManager bridge
- Observe NSPreferredScrollerStyleDidChangeNotification in RCTScrollView
  to re-layout when the system scrollbar preference changes

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

Author: Saadnajmi

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

@@ -9,25 +9,43 @@
 
 #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
+- (void)applyScrollbarPadding
 {
-  RCTAssert(
-      [localData isKindOfClass:[RCTScrollContentLocalData class]],
-      @"Local data object for `RCTScrollContentView` must be `RCTScrollContentLocalData` instance.");
+  // On macOS, legacy (always-visible) scrollbars sit inside the NSScrollView
+  // frame and reduce the NSClipView's visible area. Detect the scrollbar
+  // dimensions and apply them as padding so Yoga lays out children within
+  // the actual visible area.
+  //
+  // +preferredScrollerStyle and +scrollerWidthForControlSize: are class methods
+  // that read cached system state, so they are safe to call from any thread.
+  // Only account for the vertical scrollbar width. The horizontal scrollbar
+  // should not need padding — once the vertical scrollbar width is correct,
+  // content fits horizontally and no horizontal scrollbar appears.
+  CGFloat verticalScrollerWidth = 0;
+
+  if ([NSScroller preferredScrollerStyle] == NSScrollerStyleLegacy) {
+    verticalScrollerWidth = [NSScroller scrollerWidthForControlSize:NSControlSizeRegular
+                                                      scrollerStyle:NSScrollerStyleLegacy];
+  }
 
-  super.marginEnd = (YGValue){localData.verticalScrollerWidth, YGUnitPoint};
-  super.marginBottom = (YGValue){localData.horizontalScrollerHeight, YGUnitPoint};
+  // Use marginEnd (not paddingEnd) so the content view's own frame shrinks to
+  // match the clip view width. Padding only affects the content view's children,
+  // while the content view (documentView) frame stays full-width — causing the
+  // NSScrollView to show a horizontal scrollbar when legacy scrollbars are present.
+  YGValue currentMarginEnd = super.marginEnd;
 
-  [self didSetProps:@[@"marginEnd", @"marginBottom"]];
+  BOOL needsUpdate =
+      (currentMarginEnd.unit != YGUnitPoint || currentMarginEnd.value != verticalScrollerWidth);
+
+  if (needsUpdate) {
+    super.marginEnd = (YGValue){verticalScrollerWidth, YGUnitPoint};
+    [self didSetProps:@[@"marginEnd"]];
+  }
 }
 #endif // macOS]
 
@@ -43,6 +61,13 @@ - (void)layoutWithMetrics:(RCTLayoutMetrics)layoutMetrics layoutContext:(RCTLayo
     layoutMetrics.frame.origin.x = 0;
   }
 
+#if TARGET_OS_OSX // [macOS
+  // Check scrollbar padding on every layout pass. If the system scroller style
+  // changed, this will mark the node dirty and Yoga will recalculate on the
+  // next pass.
+  [self applyScrollbarPadding];
+#endif // macOS]
+
   [super layoutWithMetrics:layoutMetrics layoutContext:layoutContext];
 }
 

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 shadow view
+  // (RCTScrollContentShadowView) 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]