ScrollAnimatorMac should post contentAreaScrolled only for explicit scrolls.

third_party/WebKit/Source/platform/mac/ScrollAnimatorMac.mm

 /*
  * Copyright (C) 2010, 2011 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 778 matching lines @@
 void ScrollAnimatorMac::ImmediateScrollTo(const ScrollOffset& new_offset) {
   ScrollOffset adjusted_offset = AdjustScrollOffsetIfNecessary(new_offset);
 
   bool offset_changed = adjusted_offset != current_offset_;
   if (!offset_changed && !GetScrollableArea()->ScrollOriginChanged())
     return;
 
   ScrollOffset delta = adjusted_offset - current_offset_;
 
   current_offset_ = adjusted_offset;
-  NotifyContentAreaScrolled(delta);
+  NotifyContentAreaScrolled(delta, kUserScroll);
   NotifyOffsetChanged();
 }
 
 void ScrollAnimatorMac::ImmediateScrollToOffsetForScrollAnimation(
     const ScrollOffset& new_offset) {
   ASSERT(scroll_animation_helper_);
   ImmediateScrollTo(new_offset);
 }
 
 void ScrollAnimatorMac::ContentAreaWillPaint() const {
@@ skipping 107 matching lines @@
     return;
 
   ASSERT(horizontal_scrollbar_painter_delegate_);
   [horizontal_scrollbar_painter_delegate_.Get() invalidate];
   horizontal_scrollbar_painter_delegate_ = nullptr;
 
   [painter setDelegate:nil];
   [scrollbar_painter_controller_.Get() setHorizontalScrollerImp:nil];
 }
 
-void ScrollAnimatorMac::NotifyContentAreaScrolled(const ScrollOffset& delta) {
+void ScrollAnimatorMac::NotifyContentAreaScrolled(const ScrollOffset& delta,
+                                                  ScrollType scrollType) {
   // This function is called when a page is going into the page cache, but the
   // page
   // isn't really scrolling in that case. We should only pass the message on to
   // the
   // ScrollbarPainterController when we're really scrolling on an active page.
-  if (GetScrollableArea()->ScrollbarsCanBeActive())
+  if (IsExplicitScrollType(scrollType) &&
+      GetScrollableArea()->ScrollbarsCanBeActive())
     SendContentAreaScrolledSoon(delta);
 }
 
 bool ScrollAnimatorMac::SetScrollbarsVisibleForTesting(bool show) {
   if (show)
     [scrollbar_painter_controller_.Get() flashScrollers];
   else
     [scrollbar_painter_controller_.Get() hideOverlayScrollers];
 
   [vertical_scrollbar_painter_delegate_.Get() updateVisibilityImmediately:show];
@@ skipping 137 matching lines @@
     rect_in_view_coordinates =
         vertical_scrollbar->ConvertToContainingFrameViewBase(scroller_thumb);
 
   if (rect_in_view_coordinates == visible_scroller_thumb_rect_)
     return;
 
   visible_scroller_thumb_rect_ = rect_in_view_coordinates;
 }
 
 }  // namespace blink