Selectively disable rubber banding on mac.

Source/platform/mac/ScrollElasticityController.h

Index: Source/platform/mac/ScrollElasticityController.h
diff --git a/Source/platform/mac/ScrollElasticityController.h b/Source/platform/mac/ScrollElasticityController.h
index 180e79c9335c9e0cd8c93f143b85de2e5f774eb4..911bff2f50724a3fb22ef88442ce489ec882db2c 100644
--- a/Source/platform/mac/ScrollElasticityController.h
+++ b/Source/platform/mac/ScrollElasticityController.h
@@ -44,10 +44,13 @@ protected:
 public:
     virtual bool allowsHorizontalStretching() = 0;
     virtual bool allowsVerticalStretching() = 0;
+    // The amount that the view is stretched past the normal allowable bounds.
+    // The "overhang" amount.
     virtual IntSize stretchAmount() = 0;
     virtual bool pinnedInDirection(const FloatSize&) = 0;
     virtual bool canScrollHorizontally() = 0;
     virtual bool canScrollVertically() = 0;
+    // TODO(erikchen): This method is no longer used. Remove all references.

[erikchen, 2014/03/24 22:12:05]

I left this as a TODO to try to keep this CL smaller and more self-contained.
Let me know if you'd like me to remove all 11 references to this method as a
part of this change.

[Alexei Svitkine (slow), 2014/03/25 15:25:40]

SGTM.

     virtual bool shouldRubberBandInDirection(ScrollDirection) = 0;
 
     // Return the absolute scroll position, not relative to the scroll origin.
@@ -65,6 +68,18 @@ class ScrollElasticityController {
 public:
     explicit ScrollElasticityController(ScrollElasticityControllerClient*);
 
+    // This method is responsible for both scrolling and rubber-banding.
+    //
+    // Events are passed by IPC from the embedder. Events on mac are grouped

[Alexei Svitkine (slow), 2014/03/25 15:25:40]

Nit: Mac

[erikchen, 2014/03/26 18:24:30]

Done.

+    // into "gestures". If this method returns 'true', then this object has
+    // handled the event. It expects the embedder to continue to forward events
+    // from the gesture.
+    //
+    // This method makes the assumption that there is only 1 input device being
+    // used at a time. If the user simultaneously uses multiple input devices,
+    // Cocoa does not correctly pass all the gestureBegin/End events. The state
+    // of this class is guaranteed to become eventually consistent, once the
+    // user stops using multiple input devices.
     bool handleWheelEvent(const PlatformWheelEvent&);
     void snapRubberBandTimerFired();
 
@@ -74,11 +89,30 @@ private:
     void stopSnapRubberbandTimer();
     void snapRubberBand();
 
-    bool shouldRubberBandInHorizontalDirection(const PlatformWheelEvent&);
+    // This method determines whether a given event should be handled. The
+    // logic for control events of gestures (PhaseBegan, PhaseEnded) is handled
+    // elsewhere.
+    //
+    // This class handles almost all wheel events. All of the following
+    // conditions must be met for this class to ignore an event:
+    // + No previous events in this gesture have caused any scrolling or rubber
+    // banding.
+    // + The event contains a horizontal component.
+    // + The client's view is pinned in the horizontal direction of the event.
+    // + The wheel event disallows rubber banding in the horizontal direction
+    // of the event.
+    bool shouldHandleEvent(const PlatformWheelEvent&);
 
     ScrollElasticityControllerClient* m_client;
 
+    // There is an active scroll gesture event. This parameter only gets set to
+    // false after the rubber band has been snapped, and before a new gesture
+    // has begun. A careful audit of the code may deprecate the need for this parameter.

[Alexei Svitkine (slow), 2014/03/25 15:25:40]

Nit: Wrap the end of this comment since everything else has consistent line
length. 

[erikchen, 2014/03/26 18:24:30]

Done.

     bool m_inScrollGesture;
+    // At least one event in the current gesture has been consumed and has
+    // caused the view to scroll or rubber band. All future events in this
+    // gesture will be consumed and overscrolls will cause rubberbanding.
+    bool m_hasScrolled;
     bool m_momentumScrollInProgress;
     bool m_ignoreMomentumScrolls;