ScrollAnimatorMac should post contentAreaScrolled only for explicit scrolls.

third_party/WebKit/Source/core/frame/FrameView.cpp

 /*
  * Copyright (C) 1998, 1999 Torben Weis <weis@kde.org>
  *                     1999 Lars Knoll <knoll@kde.org>
  *                     1999 Antti Koivisto <koivisto@kde.org>
  *                     2000 Dirk Mueller <mueller@kde.org>
  * Copyright (C) 2004, 2005, 2006, 2007, 2008 Apple Inc. All rights reserved.
  *           (C) 2006 Graham Dennis (graham.dennis@gmail.com)
  *           (C) 2006 Alexey Proskuryakov (ap@nypop.com)
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
@@ skipping 1958 matching lines @@
 void FrameView::DidUpdateElasticOverscroll() {
   Page* page = GetFrame().GetPage();
   if (!page)
     return;
   FloatSize elastic_overscroll = page->GetChromeClient().ElasticOverscroll();
   if (HorizontalScrollbar()) {
     float delta =
         elastic_overscroll.Width() - HorizontalScrollbar()->ElasticOverscroll();
     if (delta != 0) {
       HorizontalScrollbar()->SetElasticOverscroll(elastic_overscroll.Width());
-      GetScrollAnimator().NotifyContentAreaScrolled(FloatSize(delta, 0));
+      GetScrollAnimator().NotifyContentAreaScrolled(FloatSize(delta, 0),
+                                                    kCompositorScroll);
       SetScrollbarNeedsPaintInvalidation(kHorizontalScrollbar);
     }
   }
   if (VerticalScrollbar()) {
     float delta =
         elastic_overscroll.Height() - VerticalScrollbar()->ElasticOverscroll();
     if (delta != 0) {
       VerticalScrollbar()->SetElasticOverscroll(elastic_overscroll.Height());
-      GetScrollAnimator().NotifyContentAreaScrolled(FloatSize(0, delta));
+      GetScrollAnimator().NotifyContentAreaScrolled(FloatSize(0, delta),
+                                                    kCompositorScroll);
       SetScrollbarNeedsPaintInvalidation(kVerticalScrollbar);
     }
   }
 }
 
 IntSize FrameView::GetLayoutSize(
     IncludeScrollbarsInRect scrollbar_inclusion) const {
   return scrollbar_inclusion == kExcludeScrollbars
              ? ExcludeScrollbars(layout_size_)
              : layout_size_;
@@ skipping 3375 matching lines @@
 void FrameView::SetAnimationHost(
     std::unique_ptr<CompositorAnimationHost> host) {
   animation_host_ = std::move(host);
 }
 
 LayoutUnit FrameView::CaretWidth() const {
   return LayoutUnit(GetChromeClient()->WindowToViewportScalar(1));
 }
 
 }  // namespace blink