Selectively disable rubber banding on mac.

Source/platform/mac/ScrollElasticityController.mm

Index: Source/platform/mac/ScrollElasticityController.mm
diff --git a/Source/platform/mac/ScrollElasticityController.mm b/Source/platform/mac/ScrollElasticityController.mm
index 0ab305b6ab91823e3ff09e4e30feebf082a77ac4..e4529d34c5ed63b48d219a8f9e6ff0e596ab9660 100644
--- a/Source/platform/mac/ScrollElasticityController.mm
+++ b/Source/platform/mac/ScrollElasticityController.mm
@@ -107,9 +107,22 @@ ScrollElasticityController::ScrollElasticityController(ScrollElasticityControlle
 {
 }
 
-bool ScrollElasticityController::handleWheelEvent(const PlatformWheelEvent& wheelEvent)
+// |canRubberbandLeft| and |canRubberbandRight| prevent the start of rubber-banding in their respective directions.
+// Once rubber banding has started, the two parameters have no effect.
+bool ScrollElasticityController::handleWheelEvent(const PlatformWheelEvent& wheelEvent, bool canRubberbandLeft, bool canRubberbandRight)
 {
     if (wheelEvent.phase() == PlatformWheelEventPhaseBegan) {
+        // Don't allow rubber banding to start in a direction that explicitly disallows rubberbanding.
+        if (!canRubberbandLeft && wheelEvent.deltaX() > 0) {
+          printf("cannot rubberband left");
+          return false;
+        }
+        if (!canRubberbandRight && wheelEvent.deltaX() < 0) {
+          printf("cannot rubberband right");
+          return false;
+        }
+
+
         // First, check if we should rubber-band at all.
         if (m_client->pinnedInDirection(FloatSize(-wheelEvent.deltaX(), 0)) &&
             !shouldRubberBandInHorizontalDirection(wheelEvent))

[Nico, 2014/03/14 17:51:46]

Should the checks be in shouldRubberBandInHorizontalDirection()?

[erikchen, 2014/03/20 18:04:26]

I've moved all the relevant logic into the method. I am also calling the method
before starting rubber-banding from a momentum event, which is a behavioral
change which I believe to be accurate.

@@ -185,6 +198,13 @@ bool ScrollElasticityController::handleWheelEvent(const PlatformWheelEvent& whee
 
     // If we are starting momentum scrolling then do some setup.
     if (!m_momentumScrollInProgress && (momentumPhase == PlatformWheelEventPhaseBegan || momentumPhase == PlatformWheelEventPhaseChanged)) {
+        // Momentum events should not trigger rubber banding in a disallowed direction.
+        if (!canRubberbandLeft && wheelEvent.deltaX() > 0) {
+          return false;
+        }
+        if (!canRubberbandRight && wheelEvent.deltaX() < 0) {
+          return false;
+        }
         m_momentumScrollInProgress = true;
         // Start the snap rubber band timer if it's not running. This is needed to
         // snap back from the over scroll caused by momentum events.