Use ScrollAnchor in FrameView and PaintLayerScrollableArea.

third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp

Index: third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
diff --git a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
index 90ff56744e3faba95093776f53ca75b508b374dd..c4e56b7a84a669d3acd960c73d366fc562bb390e 100644
--- a/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
+++ b/third_party/WebKit/Source/core/paint/PaintLayerScrollableArea.cpp
@@ -92,6 +92,7 @@ PaintLayerScrollableArea::PaintLayerScrollableArea(PaintLayer& layer)
     , m_scrollbarManager(*this)
     , m_scrollCorner(nullptr)
     , m_resizer(nullptr)
+    , m_scrollAnchor(this)
 #if ENABLE(ASSERT)
     , m_hasBeenDisposed(false)
 #endif
@@ -161,6 +162,7 @@ void PaintLayerScrollableArea::dispose()
 DEFINE_TRACE(PaintLayerScrollableArea)
 {
     visitor->trace(m_scrollbarManager);
+    visitor->trace(m_scrollAnchor);
     ScrollableArea::trace(visitor);
 }
 
@@ -328,7 +330,7 @@ void PaintLayerScrollableArea::setScrollOffset(const IntPoint& newScrollOffset,
     setScrollOffset(DoublePoint(newScrollOffset), scrollType);
 }
 
-void PaintLayerScrollableArea::setScrollOffset(const DoublePoint& newScrollOffset, ScrollType)
+void PaintLayerScrollableArea::setScrollOffset(const DoublePoint& newScrollOffset, ScrollType scrollType)
 {
     if (scrollOffset() == toDoubleSize(newScrollOffset))
         return;
@@ -409,6 +411,10 @@ void PaintLayerScrollableArea::setScrollOffset(const DoublePoint& newScrollOffse
 
     // 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
@@ -1449,6 +1455,15 @@ 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)