Skip the iOS ScrollView clip walk when no view clips its subviews

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: hryhoriiK97

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

@@ -92,6 +92,9 @@ @interface RCTScrollViewComponentView () <
 
 @end
 
+// Defined in RCTViewComponentView.mm.
+NSUInteger RCTViewComponentViewClippingViewCount(void);
+
 @implementation RCTScrollViewComponentView {
   ScrollViewShadowNode::ConcreteState::Shared _state;
   CGSize _contentSize;
@@ -1007,6 +1010,11 @@ - (void)_remountChildren
     return;
   }
 
+  // The walk below only changes something at a view with `removeClippedSubviews` on.
+  if (RCTViewComponentViewClippingViewCount() == 0) {
+    return;
+  }
+
   [_scrollView updateClippedSubviewsWithClipRect:CGRectInset(_scrollView.bounds, -kClippingLeeway, -kClippingLeeway)
                                   relativeToView:_scrollView];
 }

packages/react-native/React/Fabric/Mounting/ComponentViews/View/RCTViewComponentView.mm

@@ -105,6 +105,25 @@ static BOOL RCTViewIsInteractiveAccessibilityElement(UIView *view, const ViewPro
 }
 #endif
 
+// The number of live views with `removeClippedSubviews` on. Only such views act on
+// `updateClippedSubviewsWithClipRect:relativeToView:`, so while it is 0 a ScrollView
+// can skip walking its subviews. Views are only created and changed on the main thread.
+static NSUInteger RCTClippingViewCount = 0;
+
+NSUInteger RCTViewComponentViewClippingViewCount(void)
+{
+  return RCTClippingViewCount;
+}
+
+static void RCTUpdateClippingViewCount(BOOL wasClipping, BOOL isClipping)
+{
+  if (isClipping && !wasClipping) {
+    RCTClippingViewCount++;
+  } else if (wasClipping && !isClipping) {
+    RCTClippingViewCount--;
+  }
+}
+
 @implementation RCTViewComponentView {
   UIColor *_backgroundColor;
   CALayer *_backgroundColorLayer;
@@ -366,6 +385,7 @@ - (void)updateProps:(const Props::Shared &)props oldProps:(const Props::Shared &
     // Compare against the current state, not `_props`: `prepareForRecycle` resets
     // `_removeClippedSubviews` but keeps `_props`.
     if (_removeClippedSubviews != newViewProps.removeClippedSubviews) {
+      RCTUpdateClippingViewCount(_removeClippedSubviews, newViewProps.removeClippedSubviews);
       _removeClippedSubviews = newViewProps.removeClippedSubviews;
       [self _updateRemoveClippedSubviewsState];
     }
@@ -850,6 +870,11 @@ - (void)finalizeUpdates:(RNComponentViewUpdateMask)updateMask
   [self invalidateLayer];
 }
 
+- (void)dealloc
+{
+  RCTUpdateClippingViewCount(_removeClippedSubviews, NO);
+}
+
 - (void)prepareForRecycle
 {
   [super prepareForRecycle];
@@ -887,6 +912,7 @@ - (void)prepareForRecycle
   _propKeysManagedByAnimated_DO_NOT_USE_THIS_IS_BROKEN = nil;
   _eventEmitter.reset();
   _isJSResponder = NO;
+  RCTUpdateClippingViewCount(_removeClippedSubviews, NO);
   _removeClippedSubviews = NO;
   _reactSubviews = [NSMutableArray new];
   _layoutMetrics = EmptyLayoutMetrics;

packages/react-native/React/Tests/Mounting/RCTViewComponentViewTests.mm

@@ -20,6 +20,9 @@
   return props;
 }
 
+// Defined in RCTViewComponentView.mm.
+NSUInteger RCTViewComponentViewClippingViewCount(void);
+
 @interface RCTViewComponentViewTests : XCTestCase
 @end
 
@@ -163,6 +166,48 @@ - (void)testRemoveClippedSubviewsIsKeptOnRecycledView
   XCTAssertEqual(reactSubviews.count, 1u);
 }
 
+#pragma mark - Clipping view count
+
+// The count is shared by all views, so each test compares it against its value at the start.
+
+- (void)testClippingViewCountFollowsRemoveClippedSubviews
+{
+  NSUInteger initialCount = RCTViewComponentViewClippingViewCount();
+  RCTViewComponentView *first = [RCTViewComponentView new];
+  RCTViewComponentView *second = [RCTViewComponentView new];
+
+  [first updateProps:makeViewProps(true) oldProps:ViewShadowNode::defaultSharedProps()];
+  [second updateProps:makeViewProps(true) oldProps:ViewShadowNode::defaultSharedProps()];
+  XCTAssertEqual(RCTViewComponentViewClippingViewCount(), initialCount + 2);
+
+  // Props that keep `removeClippedSubviews` on don't count the view again.
+  [first updateProps:makeViewProps(true) oldProps:nullptr];
+  XCTAssertEqual(RCTViewComponentViewClippingViewCount(), initialCount + 2);
+
+  [first updateProps:makeViewProps(false) oldProps:nullptr];
+  XCTAssertEqual(RCTViewComponentViewClippingViewCount(), initialCount + 1);
+  [second updateProps:makeViewProps(false) oldProps:nullptr];
+  XCTAssertEqual(RCTViewComponentViewClippingViewCount(), initialCount);
+}
+
+- (void)testClippingViewCountAfterRecycleAndDealloc
+{
+  NSUInteger initialCount = RCTViewComponentViewClippingViewCount();
+  @autoreleasepool {
+    RCTViewComponentView *view = [RCTViewComponentView new];
+    [view updateProps:makeViewProps(true) oldProps:ViewShadowNode::defaultSharedProps()];
+    XCTAssertEqual(RCTViewComponentViewClippingViewCount(), initialCount + 1);
+
+    [view prepareForRecycle];
+    XCTAssertEqual(RCTViewComponentViewClippingViewCount(), initialCount);
+
+    [view updateProps:makeViewProps(true) oldProps:nullptr];
+    XCTAssertEqual(RCTViewComponentViewClippingViewCount(), initialCount + 1);
+  }
+  // The view was deallocated with `removeClippedSubviews` on.
+  XCTAssertEqual(RCTViewComponentViewClippingViewCount(), initialCount);
+}
+
 #pragma mark - hitTest against non-invertible transforms (#50797)
 
 - (void)testHitTestReturnsNilForZeroScaleYView