Mac: Add rails for scrolling.

content/browser/renderer_host/input/mouse_wheel_rails_filter_mac.cc

Index: content/browser/renderer_host/input/mouse_wheel_rails_filter_mac.cc
diff --git a/content/browser/renderer_host/input/mouse_wheel_rails_filter_mac.cc b/content/browser/renderer_host/input/mouse_wheel_rails_filter_mac.cc
new file mode 100644
index 0000000000000000000000000000000000000000..49e424f09f688c85f2c1d5d25940565955c45ae5
--- /dev/null
+++ b/content/browser/renderer_host/input/mouse_wheel_rails_filter_mac.cc
@@ -0,0 +1,37 @@
+// 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;

[jdduke (slow), 2015/03/19 00:21:53]

Without hooking this to time, the decay becomes frequency-sensitive (i.e.,
you'll get different behavior if events arrive at 60hz vs 120hz for the same
overall motion) which seems a little suspect.

In practice, this may not matter so much, but I wonder if it would be that much
additional work to fix (assuming wheel events are provided sane timestamps).

[ccameron, 2015/03/19 00:42:26]

The timestamps are sane, but the main goal of this is to fix up the last
trailing 2 or 3 events at the end of a scroll, where the delta gets quantized to
0 in the desired axis occasionally. The bulk of the scroll can determine the
mode with the with the x>y test. These last few events also tend to come in at a
lower rate than the other events in the scroll (which I guess makes sense --
they're only appearing as they pass the quantization threshold), so time isn't
so much of a factor.

+
+  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(fabsf(event.deltaX), fabsf(event.deltaY));

[jdduke (slow), 2015/03/19 00:21:53]

std::abs

[ccameron, 2015/03/19 00:42:26]

Done.

+
+  if (decayed_delta_.y() >= decayed_delta_.x())
+    return WebInputEvent::RailsModeVertical;
+  return WebInputEvent::RailsModeHorizontal;
+}
+
+}  // namespace content