Tie scroll anchoring adjustments to frame lifecycle instead of layout.

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>
@@ skipping 441 matching lines @@
   if (layer()->isRootLayer()) {
     frameView->frame().loader().saveScrollState();
     frameView->didChangeScrollOffset();
   }
 
   // All scrolls clear the fragment anchor.
   frameView->clearFragmentAnchor();
 
   // Clear the scroll anchor, unless it is the reason for this scroll.
   if (RuntimeEnabledFeatures::scrollAnchoringEnabled() &&
-      scrollType != AnchoringScroll)
+      scrollType != AnchoringScroll && scrollType != ClampingScroll)
     scrollAnchor()->clear();
 }
 
 IntSize PaintLayerScrollableArea::scrollOffsetInt() const {
   return flooredIntSize(m_scrollOffset);
 }
 
 ScrollOffset PaintLayerScrollableArea::scrollOffset() const {
   return m_scrollOffset;
 }
@@ skipping 306 matching lines @@
   // If a vertical scrollbar was removed, the min/max scroll offsets may have
   // changed, so the scroll offsets needs to be clamped.  If the scroll offset
   // did not change, but the scroll origin *did* change, we still need to notify
   // the scrollbars to update their dimensions.
 
   if (DelayScrollOffsetClampScope::clampingIsDelayed()) {
     DelayScrollOffsetClampScope::setNeedsClamp(this);
     return;
   }
 
-  // Restore before clamping because clamping clears the scroll anchor.
-  if (shouldPerformScrollAnchoring())
-    m_scrollAnchor.restore();
-
   if (scrollOriginChanged())
     setScrollOffsetUnconditionally(clampScrollOffset(scrollOffset()));
   else
-    ScrollableArea::setScrollOffset(scrollOffset(), ProgrammaticScroll);
+    ScrollableArea::setScrollOffset(scrollOffset(), ClampingScroll);
 
   setNeedsScrollOffsetClamp(false);
   resetScrollOriginChanged();
   m_scrollbarManager.destroyDetachedScrollbars();
 }
 
 bool PaintLayerScrollableArea::shouldPerformScrollAnchoring() const {
   return RuntimeEnabledFeatures::scrollAnchoringEnabled() &&
          m_scrollAnchor.hasScroller() &&
          layoutBox()->style()->overflowAnchor() != AnchorNone &&
@@ skipping 1144 matching lines @@
 
 void PaintLayerScrollableArea::DelayScrollOffsetClampScope::
     clampScrollableAreas() {
   for (auto& scrollableArea : *s_needsClamp)
     scrollableArea->clampScrollOffsetsAfterLayout();
   delete s_needsClamp;
   s_needsClamp = nullptr;
 }
 
 }  // namespace blink