From 4de2d39f63965e82e88b12a7bc668fea20fbe7dc Mon Sep 17 00:00:00 2001
From: Andreas Svensson <andreas@syranide.com>
Date: Wed, 1 Jan 2014 16:00:38 +0100
Subject: [PATCH] Fix WheelEvent incorrectly inverting sign of deltaY

---
 src/event/synthetic/SyntheticWheelEvent.js      | 17 ++++++++++-------
 .../__tests__/SyntheticWheelEvent-test.js       |  4 ++--
 2 files changed, 12 insertions(+), 9 deletions(-)

diff --git a/src/event/synthetic/SyntheticWheelEvent.js b/src/event/synthetic/SyntheticWheelEvent.js
index b18d208f27..e25d7239a4 100644
--- a/src/event/synthetic/SyntheticWheelEvent.js
+++ b/src/event/synthetic/SyntheticWheelEvent.js
@@ -27,7 +27,6 @@ var SyntheticMouseEvent = require('SyntheticMouseEvent');
  */
 var WheelEventInterface = {
   deltaX: function(event) {
-    // NOTE: IE<9 does not support x-axis delta.
     return (
       'deltaX' in event ? event.deltaX :
       // Fallback to `wheelDeltaX` for Webkit and normalize (right is positive).
@@ -36,15 +35,19 @@ var WheelEventInterface = {
   },
   deltaY: function(event) {
     return (
-      // Normalize (up is positive).
-      'deltaY' in event ? -event.deltaY :
-      // Fallback to `wheelDeltaY` for Webkit.
-      'wheelDeltaY' in event ? event.wheelDeltaY :
-      // Fallback to `wheelDelta` for IE<9.
-      'wheelDelta' in event ? event.wheelDelta : 0
+      'deltaY' in event ? event.deltaY :
+      // Fallback to `wheelDeltaY` for Webkit and normalize (down is positive).
+      'wheelDeltaY' in event ? -event.wheelDeltaY :
+      // Fallback to `wheelDelta` for IE<9 and normalize (down is positive).
+      'wheelDelta' in event ? -event.wheelDelta : 0
     );
   },
   deltaZ: null,
+  
+  // Browsers without "deltaMode" is reporting in raw wheel delta where one
+  // notch on the scroll is always +/- 120, roughly equivalent to pixels.
+  // A good approximation of DOM_DELTA_LINE (1) is 5% of viewport size or
+  // ~40 pixels, for DOM_DELTA_SCREEN (2) it is 87.5% of viewport size.
   deltaMode: null
 };
 
diff --git a/src/event/synthetic/__tests__/SyntheticWheelEvent-test.js b/src/event/synthetic/__tests__/SyntheticWheelEvent-test.js
index c14afa895c..14bda40b69 100644
--- a/src/event/synthetic/__tests__/SyntheticWheelEvent-test.js
+++ b/src/event/synthetic/__tests__/SyntheticWheelEvent-test.js
@@ -46,11 +46,11 @@ describe('SyntheticWheelEvent', function() {
   it('should normalize properties from the WheelEvent interface', function() {
     var standardEvent = createEvent({deltaX: 10, deltaY: -50});
     expect(standardEvent.deltaX).toBe(10);
-    expect(standardEvent.deltaY).toBe(50);
+    expect(standardEvent.deltaY).toBe(-50);
 
     var webkitEvent = createEvent({wheelDeltaX: -10, wheelDeltaY: 50});
     expect(webkitEvent.deltaX).toBe(10);
-    expect(webkitEvent.deltaY).toBe(50);
+    expect(webkitEvent.deltaY).toBe(-50);
   });
 
   it('should be able to `preventDefault` and `stopPropagation`', function() {