OverscrollController consumes scroll updates only during gesture-nav.

content/browser/renderer_host/overscroll_controller_delegate.h

 // Copyright (c) 2012 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 CONTENT_BROWSER_RENDERER_HOST_OVERSCROLL_CONTROLLER_DELEGATE_H_
 #define CONTENT_BROWSER_RENDERER_HOST_OVERSCROLL_CONTROLLER_DELEGATE_H_
 
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
 #include "content/browser/renderer_host/overscroll_controller.h"
 #include "ui/gfx/rect.h"
 
 namespace content {
 
 // The delegate receives overscroll gesture updates from the controller and
 // should perform appropriate actions.
 class OverscrollControllerDelegate {
  public:
   OverscrollControllerDelegate() {}
   virtual ~OverscrollControllerDelegate() {}
 
   // Get the bounds of the view corresponding to the delegate. Overscroll-ending
   // events will only be processed if the visible bounds are non-empty.
   virtual gfx::Rect GetVisibleBounds() const = 0;
 
-  // This is called for each update in the overscroll amount.
-  virtual void OnOverscrollUpdate(float delta_x, float delta_y) = 0;
+  // This is called for each update in the overscroll amount. Returns true if
+  // the delegate consumed the event.
+  virtual bool OnOverscrollUpdate(float delta_x, float delta_y) = 0;
 
   // This is called when the overscroll completes.
   virtual void OnOverscrollComplete(OverscrollMode overscroll_mode) = 0;
 
   // This is called when the direction of the overscroll changes.
   virtual void OnOverscrollModeChange(OverscrollMode old_mode,
                                       OverscrollMode new_mode) = 0;
 
  private:
   DISALLOW_COPY_AND_ASSIGN(OverscrollControllerDelegate);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_RENDERER_HOST_OVERSCROLL_CONTROLLER_DELEGATE_H_