Use ScrollAnchor in FrameView and PaintLayerScrollableArea.

third_party/WebKit/Source/core/frame/FrameView.cpp

Index: third_party/WebKit/Source/core/frame/FrameView.cpp
diff --git a/third_party/WebKit/Source/core/frame/FrameView.cpp b/third_party/WebKit/Source/core/frame/FrameView.cpp
index a1c51cf1c3ee4242a27064ec410192fdfd4266da..b1d86689bc67bb9a4c531df7b1b5f07220cb72ea 100644
--- a/third_party/WebKit/Source/core/frame/FrameView.cpp
+++ b/third_party/WebKit/Source/core/frame/FrameView.cpp
@@ -153,6 +153,7 @@ FrameView::FrameView(LocalFrame* frame)
     , m_hiddenForThrottling(false)
     , m_crossOriginForThrottling(false)
     , m_isUpdatingAllLifecyclePhases(false)
+    , m_scrollAnchor(this)
 {
     ASSERT(m_frame);
     init();
@@ -197,6 +198,7 @@ DEFINE_TRACE(FrameView)
     visitor->trace(m_verticalScrollbar);
     visitor->trace(m_children);
     visitor->trace(m_viewportScrollableArea);
+    visitor->trace(m_scrollAnchor);
 #endif
     Widget::trace(visitor);
     ScrollableArea::trace(visitor);
@@ -794,6 +796,9 @@ void FrameView::performPreLayoutTasks()
         ASSERT(layoutViewport);
         m_viewportScrollableArea = RootFrameViewport::create(visualViewport, *layoutViewport);
     }
+
+    if (scrollAnchoringEnabled())
+        m_scrollAnchor.save();
 }
 
 static inline void layoutFromRootObject(LayoutObject& root)
@@ -1530,6 +1535,9 @@ void FrameView::setScrollPosition(const DoublePoint& scrollPoint, ScrollType scr
         scrollBehavior = scrollBehaviorStyle();
 
     ScrollableArea::setScrollPosition(newScrollPosition, scrollType, scrollBehavior);
+
+    if (scrollAnchoringEnabled() && scrollType != AnchoringScroll)
+        m_scrollAnchor.clear();
 }
 
 void FrameView::didUpdateElasticOverscroll()
@@ -2011,6 +2019,8 @@ void FrameView::performPostLayoutTasks()
         scrollingCoordinator->notifyGeometryChanged();
 
     scrollToFragmentAnchor();

[ojan, 2016/02/02 06:51:01]

We'll need to think of what the right behavior is when scrolling to a fragment
anchor scrolling fights with scroll anchoring. I haven't really thought about
it. Mind adding a TODO? Or maybe you've thought this through already.

[skobes, 2016/02/02 22:39:10]

Added TODO.  I think the right behavior is for the fragment anchor to override /
suppress the scroll anchor.  I also need to think about history restoration.  We
may want to scroll-anchor only if InitialScrollState::wasScrolledByUser is set.

[ojan, 2016/02/03 01:23:43]

I agree.
I'm less sure of that. If history or fragment anchor causes scroll and then an
image loads above you, it would still be jarring to have the content shift
around. So, at first glance, I think we want to scroll anchor in all cases and
to treat fragment/history scrolling the same way we would a user scroll.

+    if (scrollAnchoringEnabled())
+        m_scrollAnchor.restore();
 
     sendResizeEventIfNeeded();
 }
@@ -4056,4 +4066,11 @@ LayoutBox& FrameView::boxForScrollControlPaintInvalidation() const
     return *layoutView();
 }
 
+bool FrameView::scrollAnchoringEnabled() const
+{
+    if (Settings* settings = frame().settings())
+        return settings->scrollAnchoringEnabled();
+    return false;
+}
+
 } // namespace blink