Disable scroll anchoring if the user engages in a drag motion

third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp

 /*
  * Copyright (C) 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  *
  * Portions are Copyright (C) 1998 Netscape Communications Corporation.
  *
  * Other contributors:
  *   Robert O'Callahan <roc+@cs.cmu.edu>
  *   David Baron <dbaron@fas.harvard.edu>
  *   Christian Biesinger <cbiesinger@gmail.com>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 752 matching lines @@
 
     setNeedsScrollPositionClamp(false);
     resetScrollOriginChanged();
     m_scrollbarManager.destroyDetachedScrollbars();
 }
 
 bool PaintLayerScrollableArea::shouldPerformScrollAnchoring() const
 {
     return RuntimeEnabledFeatures::scrollAnchoringEnabled()
         && m_scrollAnchor.hasScroller()
-        && layoutBox()->style()->overflowAnchor() != AnchorNone;
+        && layoutBox()->style()->overflowAnchor() != AnchorNone
+        && box().frame()->view()->userMayHaveDragged();
 }
 
 ScrollBehavior PaintLayerScrollableArea::scrollBehaviorStyle() const
 {
     return box().style()->getScrollBehavior();
 }
 
 bool PaintLayerScrollableArea::hasHorizontalOverflow() const
 {
     // TODO(szager): Make the algorithm for adding/subtracting overflow:auto
@@ skipping 974 matching lines @@
 
 void PaintLayerScrollableArea::DelayScrollPositionClampScope::clampScrollableAreas()
 {
     for (auto& scrollableArea : *s_needsClamp)
         scrollableArea->clampScrollPositionsAfterLayout();
     delete s_needsClamp;
     s_needsClamp = nullptr;
 }
 
 } // namespace blink