fix(fabric,macos): force overlay scrollbar style in ScrollView

Force NSScrollerStyleOverlay on Fabric ScrollViews to avoid layout
issues with legacy (always-visible) scrollbars. Legacy scrollbars sit
inside the NSScrollView frame and reduce the clip view's visible area,
which would require compensating padding in the Yoga shadow tree.
Overlay scrollers float above content, so no layout compensation is
needed.

Additional fixes:
- Remove autoresizingMask from documentView to prevent AppKit frame
  corruption during tile/resize
- Remove the layoutSubviews workaround (no longer needed without
  autoresizingMask)
- Add [_scrollView tile] after content size updates to re-evaluate
  scroller visibility
- Fix hit testing: check NSScroller before content subviews so
  scrollbar clicks aren't swallowed by full-width content views

Fixes #2857

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

Author: Saadnajmi

packages/react-native/React/Fabric/Mounting/ComponentViews/ScrollView/RCTScrollViewComponentView.mm

@@ -157,7 +157,16 @@ - (instancetype)initWithFrame:(CGRect)frame
 #if !TARGET_OS_OSX // [macOS]
     [_scrollView addSubview:_containerView];
 #else // [macOS
-    _containerView.autoresizingMask = UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight;
+    // Force overlay scrollbar style. Overlay scrollers float above content and
+    // don't reduce the clip view's visible area, so no layout compensation is
+    // needed. Legacy (always-visible) scrollbars sit inside the frame and would
+    // require padding adjustments in the Yoga shadow tree — avoiding that
+    // complexity is the main motivation for forcing overlay style.
+    _scrollView.scrollerStyle = NSScrollerStyleOverlay;
+    // Do NOT set autoresizingMask on the documentView. AppKit's autoresizing
+    // corrupts the documentView frame during tile/resize (adding the clip view's
+    // size delta to the container, inflating it beyond the actual content size).
+    // React manages the documentView frame directly via updateState:.
     [_scrollView setDocumentView:_containerView];
 #endif // macOS]
 
@@ -185,26 +194,12 @@ - (void)dealloc
 }
 
 #if TARGET_OS_OSX // [macOS
-- (void)layoutSubviews
-{
-  [super layoutSubviews];
-
-  // On macOS, the _containerView is the NSScrollView's documentView and has autoresizingMask set so
-  // it fills the visible area before React's first layout pass. However, AppKit's autoresizing can
-  // corrupt the documentView's frame by adding the NSClipView's size delta to the container's
-  // dimensions (e.g., during initial tile or window resize), inflating it well beyond the correct
-  // content size. This produces massive horizontal and vertical overflow on first render.
-  //
-  // After React has set the content size via updateState:, we reset the documentView frame here to
-  // undo any autoresizing corruption. This runs after AppKit's layout (which triggers autoresizing),
-  // so it reliably corrects the frame.
-  if (!CGSizeEqualToSize(_contentSize, CGSizeZero)) {
-    CGRect containerFrame = _containerView.frame;
-    if (!CGSizeEqualToSize(containerFrame.size, _contentSize)) {
-      containerFrame.size = _contentSize;
-      _containerView.frame = containerFrame;
-    }
-  }
+- (void)_preferredScrollerStyleDidChange:(NSNotification *)notification
+{
+  // The user changed the system "Show scroll bars" preference. Re-force
+  // overlay style so legacy scrollbars don't appear and cause layout issues.
+  _scrollView.scrollerStyle = NSScrollerStyleOverlay;
+  [_scrollView tile];
 }
 #endif // macOS]
 
@@ -558,6 +553,11 @@ - (void)updateState:(const State::Shared &)state oldState:(const State::Shared &
   [self _preserveContentOffsetIfNeededWithBlock:^{
     self->_scrollView.contentSize = contentSize;
   }];
+
+#if TARGET_OS_OSX // [macOS
+  // Force the scroll view to re-evaluate which scrollers should be visible.
+  [_scrollView tile];
+#endif // macOS]
 }
 
 - (RCTPlatformView *)betterHitTest:(CGPoint)point withEvent:(UIEvent *)event // [macOS]
@@ -585,6 +585,20 @@ - (RCTPlatformView *)betterHitTest:(CGPoint)point withEvent:(UIEvent *)event //
     return nil;
   }
 
+#if TARGET_OS_OSX // [macOS
+  // Check if the hit lands on a scrollbar (NSScroller) BEFORE checking content
+  // subviews. Scrollers are subviews of the NSScrollView, not the documentView
+  // (_containerView). They must be checked first because content views typically
+  // fill the entire visible area and would otherwise swallow scroller clicks.
+  if (isPointInside) {
+    NSPoint scrollViewPoint = [_scrollView convertPoint:point fromView:self];
+    NSView *scrollViewHit = [_scrollView hitTest:scrollViewPoint];
+    if ([scrollViewHit isKindOfClass:[NSScroller class]]) {
+      return (RCTPlatformView *)scrollViewHit;
+    }
+  }
+#endif // macOS]
+
   for (RCTPlatformView *subview in [_containerView.subviews reverseObjectEnumerator]) { // [macOS]
     RCTPlatformView *hitView = RCTUIViewHitTestWithEvent(subview, point, self, event); // [macOS]
     if (hitView) {
@@ -889,12 +903,20 @@ - (void)viewDidMoveToWindow // [macOS]
     [defaultCenter removeObserver:self
                              name:NSViewBoundsDidChangeNotification
                            object:_scrollView.contentView];
+    [defaultCenter removeObserver:self
+                             name:NSPreferredScrollerStyleDidChangeNotification
+                           object:nil];
   } else {
     // Register for scrollview's clipview bounds change notifications so we can track scrolling
     [defaultCenter addObserver:self
                       selector:@selector(scrollViewDocumentViewBoundsDidChange:)
                           name:NSViewBoundsDidChangeNotification
                         object:_scrollView.contentView]; // NSClipView
+    // Re-force overlay style if the user changes the system "Show scroll bars" preference
+    [defaultCenter addObserver:self
+                      selector:@selector(_preferredScrollerStyleDidChange:)
+                          name:NSPreferredScrollerStyleDidChangeNotification
+                        object:nil];
   }
 #endif // macOS]