MacViews: Support continuous scrolling and horizontal scrolling

ui/events/cocoa/events_mac.mm

Index: ui/events/cocoa/events_mac.mm
diff --git a/ui/events/cocoa/events_mac.mm b/ui/events/cocoa/events_mac.mm
index b9f010559ef392a0cf732d796760f1548884ee82..cb85adb64448375786303a580ebb5c27f0a6fbef 100644
--- a/ui/events/cocoa/events_mac.mm
+++ b/ui/events/cocoa/events_mac.mm
@@ -6,6 +6,8 @@
 
 #include <Cocoa/Cocoa.h>
 
+#import "base/mac/mac_util.h"
+#import "base/mac/sdk_forward_declarations.h"
 #include "base/logging.h"
 #include "base/time/time.h"
 #include "build/build_config.h"
@@ -130,14 +132,18 @@ int GetChangedMouseButtonFlagsFromNative(
 }
 
 gfx::Vector2d GetMouseWheelOffset(const base::NativeEvent& event) {
-  // Empirically, a value of 0.1 is typical for one mousewheel click. Positive
-  // values when scrolling up or to the left. Scrolling quickly results in a
-  // higher delta per click, up to about 15.0. (Quartz documentation suggests
-  // +/-10).
-  // Multiply by 1000 to vaguely approximate WHEEL_DELTA on Windows (120).
-  const CGFloat kWheelDeltaMultiplier = 1000;
-  return gfx::Vector2d(kWheelDeltaMultiplier * [event deltaX],
-                       kWheelDeltaMultiplier * [event deltaY]);
+  if (base::mac::IsOSLionOrLater() && [event hasPreciseScrollingDeltas]) {
+      return gfx::Vector2d([event scrollingDeltaX], [event scrollingDeltaY]);

[Andre, 2014/08/29 23:31:34]

Scrolling feels pretty good on Magic Mouse without any multiplier.

[tapted, 2014/09/02 09:10:25]

nit: It might be worth documenting what the numbers are like. The function
returns integers so some things might be interested in the extra precision.

[Andre, 2014/09/02 17:05:05]

Done.

+  } else {
+    // Empirically, a value of 0.1 is typical for one mousewheel click. Positive
+    // values when scrolling up or to the left. Scrolling quickly results in a
+    // higher delta per click, up to about 15.0. (Quartz documentation suggests
+    // +/-10).
+    // Multiply by 1000 to vaguely approximate WHEEL_DELTA on Windows (120).
+    const CGFloat kWheelDeltaMultiplier = 1000;
+    return gfx::Vector2d(kWheelDeltaMultiplier * [event deltaX],
+                         kWheelDeltaMultiplier * [event deltaY]);
+  }
 }
 
 base::NativeEvent CopyNativeEvent(const base::NativeEvent& event) {