Use ScrollAnchor in FrameView and PaintLayerScrollableArea.

third_party/WebKit/Source/core/layout/LayoutObject.cpp

Index: third_party/WebKit/Source/core/layout/LayoutObject.cpp
diff --git a/third_party/WebKit/Source/core/layout/LayoutObject.cpp b/third_party/WebKit/Source/core/layout/LayoutObject.cpp
index 2af844ee0717fcfb7e676f32add5cf3ea26bfc18..0d10b80251ba2d220b429cacc1f7ddccfdf633be 100644
--- a/third_party/WebKit/Source/core/layout/LayoutObject.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutObject.cpp
@@ -2671,6 +2671,32 @@ void LayoutObject::willBeRemovedFromTree()
     // Update cached boundaries in SVG layoutObjects if a child is removed.
     if (parent()->isSVG())
         parent()->setNeedsBoundariesUpdate();
+
+    clearScrollAnchorIfNeeded();
+}
+
+void LayoutObject::clearScrollAnchorIfNeeded() const
+{
+    if (!scrollAnchoringEnabled())
+        return;
+
+    PaintLayer* layer = parent()->enclosingLayer();
+
+    // Walk up the layer tree to clear any scroll anchors that reference us.
+    // TODO(skobes): Should this be a hash lookup or a data member instead?

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

I was hoping this TODO would explicitly point out that this is n^2.

Now that I'm a bit more awake, I can think of a potential solution. I agree that
a hash table or data member is bad. I think we need a bit on LayoutObject as to
whether this object is a scroll anchor. That way we only do the ancestor walk
for things that actually are scroll anchors, which removes the n^2. That's easy
enough to do that maybe we don't even need the TODO and you can roll it into
this patch?

+    while (layer) {
+        if (PaintLayerScrollableArea* scrollableArea = layer->scrollableArea()) {
+            ScrollAnchor& anchor = scrollableArea->scrollAnchor();
+            if (anchor.anchorObject() == this)
+                anchor.clear();
+        }
+        layer = layer->parent();
+    }
+    if (FrameView* view = frameView()) {
+        ScrollAnchor& anchor = view->scrollAnchor();
+        if (anchor.anchorObject() == this)
+            anchor.clear();
+    }
 }
 
 void LayoutObject::removeFromLayoutFlowThread()
@@ -3537,6 +3563,13 @@ void LayoutObject::clearObjectPaintProperties()
     objectPaintPropertiesMap().remove(this);
 }
 
+bool LayoutObject::scrollAnchoringEnabled() const
+{
+    if (Settings* settings = document().settings())
+        return settings->scrollAnchoringEnabled();
+    return false;
+}
+
 } // namespace blink
 
 #ifndef NDEBUG