Smooth scroll animation should not override scroll anchoring update (MT)

third_party/WebKit/Source/core/layout/ScrollAnchor.cpp

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "core/layout/ScrollAnchor.h"
 
 #include "core/frame/FrameView.h"
 #include "core/frame/UseCounter.h"
 #include "core/layout/LayoutView.h"
 #include "core/layout/line/InlineTextBox.h"
@@ skipping 154 matching lines @@
     m_savedRelativeOffset = computeRelativeOffset(m_anchorObject, m_scroller, m_corner);
 }
 
 void ScrollAnchor::restore()
 {
     if (!m_anchorObject)
         return;
 
     LayoutSize adjustment = computeRelativeOffset(m_anchorObject, m_scroller, m_corner) - m_savedRelativeOffset;
     if (!adjustment.isZero()) {
-        m_scroller->setScrollPosition(
-            m_scroller->scrollPositionDouble() + DoubleSize(adjustment),
-            AnchoringScroll);
+        ScrollAnimatorBase* animator = m_scroller->existingScrollAnimator();
+        if (!animator || !animator->hasRunningAnimation()) {
+            m_scroller->setScrollPosition(
+                m_scroller->scrollPositionDouble() + DoubleSize(adjustment),
+                AnchoringScroll);
+        } else {
+            // If in the middle of a scroll animation, stop the animation, make
+            // the adjustment, and continue the animation on the pending delta.
+            FloatSize pendingDelta = animator->desiredTargetPosition() - FloatPoint(m_scroller->scrollPositionDouble());
+            animator->cancelAnimation();
+            m_scroller->setScrollPosition(
+                m_scroller->scrollPositionDouble() + DoubleSize(adjustment),
+                AnchoringScroll);
+            animator->userScroll(ScrollByPixel, pendingDelta);
+        }
         // Update UMA metric.
         DEFINE_STATIC_LOCAL(EnumerationHistogram, adjustedOffsetHistogram,
             ("Layout.ScrollAnchor.AdjustedScrollOffset", 2));
         adjustedOffsetHistogram.count(1);
         UseCounter::count(scrollerLayoutBox(m_scroller)->document(), UseCounter::ScrollAnchored);
     }
 }
 
 void ScrollAnchor::clear()
 {
     LayoutObject* anchorObject = m_anchorObject;
     m_anchorObject = nullptr;
 
     if (anchorObject)
         anchorObject->maybeClearIsScrollAnchorObject();
 }
 
 } // namespace blink