Mac: Add rails for scrolling.

content/browser/renderer_host/input/mouse_wheel_rails_filter_mac.cc

+// Copyright 2015 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_rails_filter_mac.h"
+
+using blink::WebInputEvent;
+using blink::WebMouseWheelEvent;
+
+namespace content {
+
+MouseWheelRailsFilterMac::MouseWheelRailsFilterMac() {
+}
+
+MouseWheelRailsFilterMac::~MouseWheelRailsFilterMac() {
+}
+
+WebInputEvent::RailsMode MouseWheelRailsFilterMac::UpdateRailsMode(
+    const WebMouseWheelEvent& event) {
+  // A somewhat-arbitrary decay constant for hysteresis.
+  const float kDecayConstant = 0.8f;
+
+  if (event.phase == WebMouseWheelEvent::PhaseBegan) {
+    decayed_delta_ = gfx::Vector2dF();
+  }
+  if (event.deltaX == 0 && event.deltaY == 0)
+    return WebInputEvent::RailsModeFree;
+
+  decayed_delta_.Scale(kDecayConstant);
+  decayed_delta_ +=
+      gfx::Vector2dF(std::abs(event.deltaX), std::abs(event.deltaY));
+
+  if (decayed_delta_.y() >= decayed_delta_.x())

[tdresser, 2015/03/19 13:03:40]

Do we want to rail this aggressively?
This makes diagonal scrolling impossible for scrollers which respect rails at
all, doesn't it?

[ccameron, 2015/03/19 19:33:14]

Exactly. This mirrors the behavior that Safari has.

(I have a suspicion that Safari has this behavior because WebKit does a
terribleterribleterrible job propagating diagonal scrolls -- we've inherited
this in Blink, and forcing things to an axis does make behavior much better for
Blink scrolls. For cc, it's less of an improvement).

[tdresser, 2015/03/19 20:57:51]

Yikes. I just tried it out on Safari - can't say I'm a fan, but we can always
revisit this later, possibly after unifying main and impl scrolling.

+    return WebInputEvent::RailsModeVertical;
+  return WebInputEvent::RailsModeHorizontal;
+}
+
+}  // namespace content