Fix scrolling rails on Mac OSX.

content/browser/renderer_host/input/mouse_wheel_event_queue.cc

 // Copyright 2016 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.
 
 #include "content/browser/renderer_host/input/mouse_wheel_event_queue.h"
 
 #include "base/metrics/histogram_macros.h"
 #include "base/stl_util.h"
 #include "base/trace_event/trace_event.h"
 
@@ skipping 104 matching lines @@
         scroll_update.event.data.scrollUpdate.deltaX =
             scroll_update.event.data.scrollUpdate.deltaX > 0 ? 1 : -1;
       if (scroll_update.event.data.scrollUpdate.deltaY)
         scroll_update.event.data.scrollUpdate.deltaY =
             scroll_update.event.data.scrollUpdate.deltaY > 0 ? 1 : -1;
     } else {
       scroll_update.event.data.scrollUpdate.deltaUnits =
           event_sent_for_gesture_ack_->event.hasPreciseScrollingDeltas
               ? blink::WebGestureEvent::PrecisePixels
               : blink::WebGestureEvent::Pixels;
+
+      if (event_sent_for_gesture_ack_->event.railsMode ==
+          WebInputEvent::RailsModeVertical)
+        scroll_update.event.data.scrollUpdate.deltaX = 0;
+      if (event_sent_for_gesture_ack_->event.railsMode ==
+          WebInputEvent::RailsModeHorizontal)
+        scroll_update.event.data.scrollUpdate.deltaY = 0;
     }
 
     bool current_phase_ended = false;
     bool has_phase_info = false;
 
     if (event_sent_for_gesture_ack_->event.phase !=
             blink::WebMouseWheelEvent::PhaseNone ||
         event_sent_for_gesture_ack_->event.momentumPhase !=
             blink::WebMouseWheelEvent::PhaseNone) {
       has_phase_info = true;
@@ skipping 134 matching lines @@
   scroll_begin.event.data.scrollBegin.targetViewport = false;
   scroll_begin.event.data.scrollBegin.deltaHintUnits =
       gesture_update.event.data.scrollUpdate.deltaUnits;
 
   needs_scroll_begin_ = false;
   needs_scroll_end_ = true;
   client_->SendGestureEvent(scroll_begin);
 }
 
 }  // namespace content