Use ScrollAnchor in FrameView and PaintLayerScrollableArea.

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@web.de>
  *   Randall Jesup <rjesup@wgate.com>
@@ skipping 74 matching lines @@
     : m_layer(layer)
     , m_inResizeMode(false)
     , m_scrollsOverflow(false)
     , m_inOverflowRelayout(false)
     , m_nextTopmostScrollChild(0)
     , m_topmostScrollChild(0)
     , m_needsCompositedScrolling(false)
     , m_scrollbarManager(*this)
     , m_scrollCorner(nullptr)
     , m_resizer(nullptr)
+    , m_scrollAnchor(this)
 #if ENABLE(ASSERT)
     , m_hasBeenDisposed(false)
 #endif
 {
     Node* node = box().node();
     if (node && node->isElementNode()) {
         // We save and restore only the scrollOffset as the other scroll values are recalculated.
         Element* element = toElement(node);
         m_scrollOffset = element->savedLayerScrollOffset();
         if (!m_scrollOffset.isZero())
@@ skipping 49 matching lines @@
     clearScrollAnimators();
 
 #if ENABLE(ASSERT)
     m_hasBeenDisposed = true;
 #endif
 }
 
 DEFINE_TRACE(PaintLayerScrollableArea)
 {
     visitor->trace(m_scrollbarManager);
+    visitor->trace(m_scrollAnchor);
     ScrollableArea::trace(visitor);
 }
 
 HostWindow* PaintLayerScrollableArea::hostWindow() const
 {
     if (Page* page = box().frame()->page())
         return &page->chromeClient();
     return nullptr;
 }
 
@@ skipping 147 matching lines @@
 {
     IntSize scrollDimensions = maximumScrollPosition() - minimumScrollPosition();
     return (orientation == HorizontalScrollbar) ? scrollDimensions.width() : scrollDimensions.height();
 }
 
 void PaintLayerScrollableArea::setScrollOffset(const IntPoint& newScrollOffset, ScrollType scrollType)
 {
     setScrollOffset(DoublePoint(newScrollOffset), scrollType);
 }
 
-void PaintLayerScrollableArea::setScrollOffset(const DoublePoint& newScrollOffset, ScrollType)
+void PaintLayerScrollableArea::setScrollOffset(const DoublePoint& newScrollOffset, ScrollType scrollType)
 {
     if (scrollOffset() == toDoubleSize(newScrollOffset))
         return;
 
     DoubleSize scrollDelta = scrollOffset() - toDoubleSize(newScrollOffset);
     m_scrollOffset = toDoubleSize(newScrollOffset);
 
     LocalFrame* frame = box().frame();
     ASSERT(frame);
 
@@ skipping 60 matching lines @@
     box().view()->clearHitTestCache();
 
     // Inform the FrameLoader of the new scroll position, so it can be restored when navigating back.
     if (layer()->isRootLayer()) {
         frameView->frame().loader().saveScrollState();
         frame->loader().client()->didChangeScrollOffset();
     }
 
     // All scrolls clear the fragment anchor.
     frameView->clearFragmentAnchor();
+
+    // Clear the scroll anchor, unless it is the reason for this scroll.
+    if (scrollAnchoringEnabled() && scrollType != AnchoringScroll)

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

It's a bummer we need to copy-paste this. Maybe put this as a function on
FrameView, then this and the FrameView code could both call something like:
frameView->clearScrollAnchor();

[skobes, 2016/02/02 22:39:11]

Yes, this is why we want root layer scrolling. :)

It wouldn't really make sense to call frameView->clearScrollAnchor() here, since
we want to clear our own scroll anchor and not the FrameView's.

+        scrollAnchor().clear();
 }
 
 IntPoint PaintLayerScrollableArea::scrollPosition() const
 {
     return IntPoint(flooredIntSize(m_scrollOffset));
 }
 
 DoublePoint PaintLayerScrollableArea::scrollPositionDouble() const
 {
     return DoublePoint(m_scrollOffset);
@@ skipping 1020 matching lines @@
         return false;
 
     return frame->settings()->viewportMetaEnabled();
 }
 
 Widget* PaintLayerScrollableArea::widget()
 {
     return box().frame()->view();
 }
 
+bool PaintLayerScrollableArea::scrollAnchoringEnabled() const
+{
+    if (Frame* frame = box().frame()) {

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

Nit: If you add a scrollAnchoringEnabled method to Frame, you could probably
drop the versions of that from this class and from FrameView.

[skobes, 2016/02/02 22:39:11]

Hmm, this doesn't save us very much, and it increases the number of classes that
need to know about scroll anchoring.

In general I wish we would change the way settings are plumbed to make it easier
to access.  There are too many indirections and ad hoc null checks.

+        if (Settings* settings = frame->settings())
+            return settings->scrollAnchoringEnabled();
+    }
+    return false;
+}
+
 PaintLayerScrollableArea::ScrollbarManager::ScrollbarManager(PaintLayerScrollableArea& scrollableArea)
     : m_scrollableArea(&scrollableArea)
     , m_canDetachScrollbars(0)
     , m_hBarIsAttached(0)
     , m_vBarIsAttached(0)
 {
 }
 
 void PaintLayerScrollableArea::ScrollbarManager::dispose()
 {
@@ skipping 85 matching lines @@
 }
 
 DEFINE_TRACE(PaintLayerScrollableArea::ScrollbarManager)
 {
     visitor->trace(m_scrollableArea);
     visitor->trace(m_hBar);
     visitor->trace(m_vBar);
 }
 
 } // namespace blink