Send the WebMouseWheelEvents unconverted if Shift is down.

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"
 #include "ui/events/blink/web_input_event_traits.h"
@@ skipping 81 matching lines @@
     scroll_update.timeStampSeconds =
         event_sent_for_gesture_ack_->event.timeStampSeconds;
 
     scroll_update.x = event_sent_for_gesture_ack_->event.x;
     scroll_update.y = event_sent_for_gesture_ack_->event.y;
     scroll_update.globalX = event_sent_for_gesture_ack_->event.globalX;
     scroll_update.globalY = event_sent_for_gesture_ack_->event.globalY;
     scroll_update.type = WebInputEvent::GestureScrollUpdate;
     scroll_update.sourceDevice = blink::WebGestureDeviceTouchpad;
     scroll_update.resendingPluginId = -1;
-    scroll_update.data.scrollUpdate.deltaX =
-        event_sent_for_gesture_ack_->event.deltaX;
-    scroll_update.data.scrollUpdate.deltaY =
-        event_sent_for_gesture_ack_->event.deltaY;
+
+    // Swap X & Y if Shift is down and when there is no horizontal movement.
+    if ((event_sent_for_gesture_ack_->event.modifiers &
+         WebInputEvent::ShiftKey) != 0 &&
+        event_sent_for_gesture_ack_->event.deltaX == 0) {
+      scroll_update.data.scrollUpdate.deltaX =
+          event_sent_for_gesture_ack_->event.deltaY;
+      scroll_update.data.scrollUpdate.deltaY =
+          event_sent_for_gesture_ack_->event.deltaX;
+    } else {
+      scroll_update.data.scrollUpdate.deltaX =
+          event_sent_for_gesture_ack_->event.deltaX;
+      scroll_update.data.scrollUpdate.deltaY =
+          event_sent_for_gesture_ack_->event.deltaY;
+    }
     // Only OSX populates the phase and momentumPhase; so
     // |inertialPhase| will be UnknownMomentumPhase on all other platforms.
     if (event_sent_for_gesture_ack_->event.momentumPhase !=
         blink::WebMouseWheelEvent::PhaseNone) {
       scroll_update.data.scrollUpdate.inertialPhase =
           WebGestureEvent::MomentumPhase;
     } else if (event_sent_for_gesture_ack_->event.phase !=
                blink::WebMouseWheelEvent::PhaseNone) {
       scroll_update.data.scrollUpdate.inertialPhase =
           WebGestureEvent::NonMomentumPhase;
@@ skipping 174 matching lines @@
   scroll_begin.data.scrollBegin.deltaHintUnits =
       gesture_update.data.scrollUpdate.deltaUnits;
 
   needs_scroll_begin_ = false;
   needs_scroll_end_ = true;
   client_->ForwardGestureEventWithLatencyInfo(
       scroll_begin, ui::LatencyInfo(ui::SourceEventType::WHEEL));
 }
 
 }  // namespace content