Make scroll anchoring adjustment when the bounds of a scroller is changed

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 3629 matching lines @@
         positionScrollbarLayers();
         updateScrollCorner();
     }
 
     adjustScrollPositionFromUpdateScrollbars();
 }
 
 void FrameView::adjustScrollPositionFromUpdateScrollbars()
 {
     DoublePoint adjustedScrollPosition = clampScrollPosition(scrollPositionDouble());
-
     if (adjustedScrollPosition != scrollPositionDouble() || scrollOriginChanged()) {
-        ScrollableArea::setScrollPosition(adjustedScrollPosition, ProgrammaticScroll);
+        // Since the scroll bounds have changed and the scroller is at a position
+        // that is outside its new bounds. Calling ScrollAnchor::restore

[skobes, 2016/07/14 14:49:25]

If the scroll bounds have changed, we will relayout.  Why wouldn't we have
already restored by this point?

+        // ensures that no visible jump is seen when clamping.
+        if (shouldPerformScrollAnchoring() && m_scrollAnchor.hasScroller()) {
+            m_scrollAnchor.restore();
+            adjustedScrollPosition = clampScrollPosition(scrollPositionDouble());
+        }
+        if (adjustedScrollPosition != scrollPositionDouble())
+            ScrollableArea::setScrollPosition(adjustedScrollPosition, ProgrammaticScroll);
         resetScrollOriginChanged();
     }
 }
 
 IntRect FrameView::rectToCopyOnScroll() const
 {
     IntRect scrollViewRect = convertToRootFrame(IntRect((shouldPlaceVerticalScrollbarOnLeft() && verticalScrollbar()) ? verticalScrollbar()->width() : 0, 0, visibleWidth(), visibleHeight()));
     if (hasOverlayScrollbars()) {
         int verticalScrollbarWidth = (verticalScrollbar() && !hasLayerForVerticalScrollbar()) ? verticalScrollbar()->width() : 0;
         int horizontalScrollbarHeight = (horizontalScrollbar() && !hasLayerForHorizontalScrollbar()) ? horizontalScrollbar()->height() : 0;
@@ skipping 612 matching lines @@
 }
 
 bool FrameView::canThrottleRendering() const
 {
     if (!RuntimeEnabledFeatures::renderingPipelineThrottlingEnabled())
         return false;
     return m_subtreeThrottled || (m_hiddenForThrottling && m_crossOriginForThrottling);
 }
 
 } // namespace blink