overscroll: Do not overscroll if the beginning of a scroll was consumed.

content/browser/renderer_host/overscroll_controller.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_H_
 #define CONTENT_BROWSER_RENDERER_HOST_OVERSCROLL_CONTROLLER_H_
 
 #include "base/basictypes.h"
 #include "base/compiler_specific.h"
 #include "third_party/WebKit/Source/WebKit/chromium/public/WebInputEvent.h"
@@ skipping 44 matching lines @@
   void set_delegate(OverscrollControllerDelegate* delegate) {
     delegate_ = delegate;
   }
 
   // Resets internal states.
   void Reset();
 
  private:
   friend class MockRenderWidgetHost;
 
+  // Different scrolling states.
+  enum ScrollState {
+    STATE_UNKNOWN,
+    STATE_PENDING,
+    STATE_CONTENT_SCROLLING,
+    STATE_OVERSCROLLING,
+  };
+
   // Returns true if the event indicates that the in-progress overscroll gesture
   // can now be completed.
   bool DispatchEventCompletesAction(
       const WebKit::WebInputEvent& event) const;
 
   // Returns true to indicate that dispatching the event should reset the
   // overscroll gesture status.
   bool DispatchEventResetsState(const WebKit::WebInputEvent& event) const;
 
   // Processes an event and updates internal state for overscroll.
@@ skipping 14 matching lines @@
   // Returns whether the input event should be forwarded to the
   // GestureEventFilter.
   bool ShouldForwardToGestureFilter(const WebKit::WebInputEvent& event) const;
 
   // The RenderWidgetHost that owns this overscroll controller.
   RenderWidgetHostImpl* render_widget_host_;
 
   // The current state of overscroll gesture.
   OverscrollMode overscroll_mode_;
 
+  // Used to keep track of the scrolling state.
+  // If scrolling starts, and some scroll events are consumed at the beginning
+  // of the scroll (i.e. some content on the web-page was scrolled), then do not
+  // process any of the subsequent scroll events for generating overscroll
+  // gestures.
+  ScrollState scroll_state_;
+
   // The amount of overscroll in progress. These values are invalid when
   // |overscroll_mode_| is set to OVERSCROLL_NONE.
   float overscroll_delta_x_;
   float overscroll_delta_y_;
 
   // The delegate that receives the overscroll updates. The delegate is not
   // owned by this controller.
   OverscrollControllerDelegate* delegate_;
 
   DISALLOW_COPY_AND_ASSIGN(OverscrollController);
 };
 
 }  // namespace content
 
 #endif  // CONTENT_BROWSER_RENDERER_HOST_OVERSCROLL_CONTROLLER_H_