Use ScrollAnchor in FrameView and PaintLayerScrollableArea.

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 135 matching lines @@
     , m_horizontalScrollbarLock(false)
     , m_verticalScrollbarLock(false)
     , m_scrollbarsAvoidingResizer(0)
     , m_scrollbarsSuppressed(false)
     , m_inUpdateScrollbars(false)
     , m_frameTimingRequestsDirty(true)
     , m_viewportIntersectionValid(false)
     , m_hiddenForThrottling(false)
     , m_crossOriginForThrottling(false)
     , m_isUpdatingAllLifecyclePhases(false)
+    , m_scrollAnchor(this)
 {
     ASSERT(m_frame);
     init();
 }
 
 PassRefPtrWillBeRawPtr<FrameView> FrameView::create(LocalFrame* frame)
 {
     RefPtrWillBeRawPtr<FrameView> view = adoptRefWillBeNoop(new FrameView(frame));
     view->show();
     return view.release();
@@ skipping 24 matching lines @@
 #if ENABLE(OILPAN)
     visitor->trace(m_frame);
     visitor->trace(m_fragmentAnchor);
     visitor->trace(m_scrollableAreas);
     visitor->trace(m_animatingScrollableAreas);
     visitor->trace(m_autoSizeInfo);
     visitor->trace(m_horizontalScrollbar);
     visitor->trace(m_verticalScrollbar);
     visitor->trace(m_children);
     visitor->trace(m_viewportScrollableArea);
+    visitor->trace(m_scrollAnchor);
 #endif
     Widget::trace(visitor);
     ScrollableArea::trace(visitor);
 }
 
 void FrameView::reset()
 {
     m_hasPendingLayout = false;
     m_doFullPaintInvalidation = false;
     m_layoutSchedulingEnabled = true;
@@ skipping 577 matching lines @@
 
     document->updateLayoutTreeIfNeeded();
     lifecycle().advanceTo(DocumentLifecycle::StyleClean);
 
     if (m_frame->isMainFrame() && !m_viewportScrollableArea) {
         ScrollableArea& visualViewport = m_frame->host()->visualViewport();
         ScrollableArea* layoutViewport = layoutViewportScrollableArea();
         ASSERT(layoutViewport);
         m_viewportScrollableArea = RootFrameViewport::create(visualViewport, *layoutViewport);
     }
+
+    if (scrollAnchoringEnabled())
+        m_scrollAnchor.save();
 }
 
 static inline void layoutFromRootObject(LayoutObject& root)
 {
     LayoutState layoutState(root);
     root.layout();
 }
 
 void FrameView::prepareLayoutAnalyzer()
 {
@@ skipping 716 matching lines @@
 void FrameView::setScrollPosition(const DoublePoint& scrollPoint, ScrollType scrollType, ScrollBehavior scrollBehavior)
 {
     DoublePoint newScrollPosition = clampScrollPosition(scrollPoint);
     if (newScrollPosition == scrollPositionDouble())
         return;
 
     if (scrollBehavior == ScrollBehaviorAuto)
         scrollBehavior = scrollBehaviorStyle();
 
     ScrollableArea::setScrollPosition(newScrollPosition, scrollType, scrollBehavior);
+
+    if (scrollAnchoringEnabled() && scrollType != AnchoringScroll)
+        m_scrollAnchor.clear();
 }
 
 void FrameView::didUpdateElasticOverscroll()
 {
     Page* page = frame().page();
     if (!page)
         return;
     FloatSize elasticOverscroll = page->chromeClient().elasticOverscroll();
     if (m_horizontalScrollbar) {
         float delta = elasticOverscroll.width() - m_horizontalScrollbar->elasticOverscroll();
@@ skipping 460 matching lines @@
 
     // Plugins could have torn down the page inside updateWidgetGeometries().
     if (!layoutView())
         return;
 
     scheduleUpdateWidgetsIfNecessary();
 
     if (ScrollingCoordinator* scrollingCoordinator = this->scrollingCoordinator())
         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();
 }
 
 bool FrameView::wasViewportResized()
 {
     ASSERT(m_frame);
     LayoutView* layoutView = this->layoutView();
     if (!layoutView)
         return false;
@@ skipping 2025 matching lines @@
         return false;
     return m_hiddenForThrottling && m_crossOriginForThrottling;
 }
 
 LayoutBox& FrameView::boxForScrollControlPaintInvalidation() const
 {
     ASSERT(layoutView());
     return *layoutView();
 }
 
+bool FrameView::scrollAnchoringEnabled() const
+{
+    if (Settings* settings = frame().settings())
+        return settings->scrollAnchoringEnabled();
+    return false;
+}
+
 } // namespace blink