Make rootScroller the layout viewport.

third_party/WebKit/Source/core/page/scrolling/RootScrollerController.h

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef RootScrollerController_h
 #define RootScrollerController_h
 
 #include "core/CoreExport.h"
 #include "platform/heap/Handle.h"
 
@@ skipping 18 matching lines @@
 //
 // If the root scroller element is a valid element to become the root scroller,
 // it will be promoted to the effective root scroller. If it is not valid, the
 // effective root scroller will fall back to a default Element (see
 // defaultEffectiveRootScroller()). The rules for what makes an element a valid
 // root scroller are set in isValidRootScroller(). The validity of the current
 // root scroller is re-checked after each layout.
 class CORE_EXPORT RootScrollerController
     : public GarbageCollected<RootScrollerController> {
 public:
-    // Creates a RootScrollerController for the given document. An optional
-    // ViewportScrollCallback can be provided. If it is, RootScrollerController
-    // will ensure that the effectiveRootScroller element always has this set as
-    // the apply scroll callback.
-    static RootScrollerController* create(
-        Document& document,
-        ViewportScrollCallback* applyScrollCallback)
+    // Creates a RootScrollerController for the given document. You should use
+    // setViewportScrollCallback to provide this class with a scroll callback
+    // that RootScrollerController will keep applied to the current RootScroller
+    // so that special actions can occur on scrolling.
+    static RootScrollerController* create(Document& document)
     {
-        return new RootScrollerController(document, applyScrollCallback);
+        return new RootScrollerController(document);
     }
 
     DECLARE_TRACE();
 
     // Sets the element that will be used as the root scroller. This can be
     // nullptr, in which case we'll use the default element (documentElement) as
     // the effective root scroller.
     void set(Element*);
 
     // Returns the element currently set as the root scroller from script. This
@@ skipping 15 matching lines @@
     void didUpdateLayout();
 
     // TODO(bokan): Temporarily exposed to allow ScrollCustomization to
     // differentiate between real custom callback and the built-in viewport
     // apply scroll.
     const ViewportScrollCallback* viewportScrollCallback()
     {
         return m_viewportApplyScroll;
     }
 
+    void setViewportScrollCallback(ViewportScrollCallback*);
+
 private:
-    RootScrollerController(Document&, ViewportScrollCallback*);
+    RootScrollerController(Document&);
 
     Element* defaultEffectiveRootScroller();
 
     // Ensures the effective root scroller is currently valid and replaces it
     // with the default if not.
     void updateEffectiveRootScroller();
-    void moveViewportApplyScroll(Element* target);
+
+    // Returns true if the move was successful.
+    bool moveViewportApplyScroll(Element* target);
 
     WeakMember<Document> m_document;
     Member<ViewportScrollCallback> m_viewportApplyScroll;
 
     WeakMember<Element> m_rootScroller;
+
+    // The element currently being used as the root scroller. If
+    // m_viewportApplyScroll has been set, this element is guaranteed to have it
+    // set as its applyScroll callback. This can be nullptr during
+    // initialization and will not be set until m_viewportApplyScroll is
+    // provided.
     WeakMember<Element> m_effectiveRootScroller;
 };
 
 } // namespace blink
 
 #endif // RootScrollerController_h