Selectively disable rubber banding on mac.

Source/platform/mac/ScrollElasticityController.mm

 /*
  * Copyright (C) 2011 Apple Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 81 matching lines @@
         multiplier = [[NSUserDefaults standardUserDefaults] floatForKey:@"NSScrollWheelMultiplier"];
         if (multiplier <= 0)
             multiplier = 1;
     }
     return multiplier;
 }
 
 ScrollElasticityController::ScrollElasticityController(ScrollElasticityControllerClient* client)
     : m_client(client)
     , m_inScrollGesture(false)
+    , m_hasScrolled(false)
     , m_momentumScrollInProgress(false)
     , m_ignoreMomentumScrolls(false)
     , m_lastMomentumScrollTimestamp(0)
     , m_startTime(0)
     , m_snapRubberbandTimerIsActive(false)
 {
 }
 
 bool ScrollElasticityController::handleWheelEvent(const PlatformWheelEvent& wheelEvent)
 {
+    // When a new gesture begins, reset all state that might have been carried
+    // over from the last event.
     if (wheelEvent.phase() == PlatformWheelEventPhaseBegan) {
-        // First, check if we should rubber-band at all.
-        if (m_client->pinnedInDirection(FloatSize(-wheelEvent.deltaX(), 0)) &&
-            !shouldRubberBandInHorizontalDirection(wheelEvent))
-            return false;
-
         m_inScrollGesture = true;
+        m_hasScrolled = false;
         m_momentumScrollInProgress = false;
         m_ignoreMomentumScrolls = false;
         m_lastMomentumScrollTimestamp = 0;
         m_momentumVelocity = FloatSize();
 
         IntSize stretchAmount = m_client->stretchAmount();
         m_stretchScrollForce.setWidth(reboundDeltaForElasticDelta(stretchAmount.width()));
         m_stretchScrollForce.setHeight(reboundDeltaForElasticDelta(stretchAmount.height()));
         m_overflowScrollDelta = FloatSize();
 
         stopSnapRubberbandTimer();
 
-        return true;
+        return shouldHandleEvent(wheelEvent);
     }
 
-    if (wheelEvent.phase() == PlatformWheelEventPhaseEnded) {
+    if (wheelEvent.phase() == PlatformWheelEventPhaseEnded || wheelEvent.phase() == PlatformWheelEventPhaseCancelled) {

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

this change wasn't necessary to make this CL work, but it appeared to be an
oversight from the original author. The phase PlatformWheelEventPhaseCancelled
is not mentioned anywhere in this file.

         bool wasRubberBandInProgress = isRubberBandInProgress();
         // Call snapRubberBand() even if isRubberBandInProgress() is false. For example,
         // m_inScrollGesture may be true (and needs to be reset on a phase end) even if
         // isRubberBandInProgress() is not (e.g. the overhang area is empty).
         snapRubberBand();
         return wasRubberBandInProgress;
     }
 
     bool isMomentumScrollEvent = (wheelEvent.momentumPhase() != PlatformWheelEventPhaseNone);
     if (m_ignoreMomentumScrolls && (isMomentumScrollEvent || m_snapRubberbandTimerIsActive)) {
         if (wheelEvent.momentumPhase() == PlatformWheelEventPhaseEnded) {
             m_ignoreMomentumScrolls = false;
             return true;
         }
         return false;
     }
 
+    if (!shouldHandleEvent(wheelEvent))
+        return false;
+
     float deltaX = m_overflowScrollDelta.width();
     float deltaY = m_overflowScrollDelta.height();
 
     // Reset overflow values because we may decide to remove delta at various points and put it into overflow.
     m_overflowScrollDelta = FloatSize();
 
     IntSize stretchAmount = m_client->stretchAmount();
     bool isVerticallyStretched = stretchAmount.height();
     bool isHorizontallyStretched = stretchAmount.width();
 
@@ skipping 72 matching lines @@
                         deltaX = 0;
                     } else
                         m_overflowScrollDelta.setWidth(m_overflowScrollDelta.width() + deltaX);
                 }
                 shouldStretch = true;
             }
         }
     }
 
     if (deltaX != 0 || deltaY != 0) {
+        m_hasScrolled = true;
         if (!(shouldStretch || isVerticallyStretched || isHorizontallyStretched)) {
             if (deltaY != 0) {
                 deltaY *= scrollWheelMultiplier();
                 m_client->immediateScrollBy(FloatSize(0, deltaY));
             }
             if (deltaX != 0) {
                 deltaX *= scrollWheelMultiplier();
                 m_client->immediateScrollBy(FloatSize(deltaX, 0));
             }
         } else {
@@ skipping 152 matching lines @@
         m_startTime = 0;
         m_stretchScrollForce = FloatSize();
         return;
     }
 
     m_startTime = [NSDate timeIntervalSinceReferenceDate];
     m_client->startSnapRubberbandTimer();
     m_snapRubberbandTimerIsActive = true;
 }
 
-bool ScrollElasticityController::shouldRubberBandInHorizontalDirection(const PlatformWheelEvent& wheelEvent)
+bool ScrollElasticityController::shouldHandleEvent(const PlatformWheelEvent& wheelEvent)
 {
-    if (wheelEvent.deltaX() > 0)
-        return m_client->shouldRubberBandInDirection(ScrollLeft);
-    if (wheelEvent.deltaX() < 0)
-        return m_client->shouldRubberBandInDirection(ScrollRight);
+    if (m_hasScrolled)
+        return true;
+    if (!m_client->pinnedInDirection(FloatSize(-wheelEvent.deltaX(), 0)))
+        return true;
+
+    if (wheelEvent.deltaX() > 0 && !wheelEvent.canRubberbandLeft())
+        return false;
+    if (wheelEvent.deltaX() < 0 && !wheelEvent.canRubberbandRight())
+        return false;
 
     return true;
 }
 
 } // namespace WebCore
 
 #endif // USE(RUBBER_BANDING)