Implement Wheel Gesture Scrolling on OSX.

third_party/WebKit/public/web/WebInputEvent.h

 /*
  * Copyright (C) 2009 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 495 matching lines @@
         struct {
             // Initial motion that triggered the scroll.
             // May be redundant with deltaX/deltaY in the first scrollUpdate.
             float deltaXHint;
             float deltaYHint;
             // Default initialized to ScrollUnits::PrecisePixels.
             ScrollUnits deltaHintUnits;
             // If true, this event will skip hit testing to find a scroll
             // target and instead just scroll the viewport.
             bool targetViewport;
+            // If true, this event comes after a non-inertial gesture
+            // scroll sequence; OSX has unique phases for normal and
+            // momentum scroll events. Should always be false for touch based
+            // input as it generates GestureFlingStart instead.
+            bool inertial;
+            // True if the begin was generated when phase information is available
+            // for touchpad and for every scroll update we are generating a GSB/GSU/GSE
+            // tuple. This is a temporary variable that forces the non-latching behavior
+            // until crbug.com/526463 is fully implemented.

[tdresser, 2016/03/08 14:28:53]

Mentioning that these synthesized begin events are used to force a hit test
might make this a bit clearer. 

Incorporating something like the following perhaps:

True if this event was synthesized in order to force a hit test, to avoid scroll
latching behavior until crbug.com/526463 is fully implemented.

[dtapuska, 2016/03/08 20:31:49]

Done.

+            bool synthetic;
         } scrollBegin;
 
         struct {
             float deltaX;
             float deltaY;
             float velocityX;
             float velocityY;
             // Whether any previous GestureScrollUpdate in the current scroll
             // sequence was suppressed (e.g., the causal touchmove was
             // preventDefault'ed). This bit is particularly useful for
             // determining whether the observed scroll update sequence captures
             // the entirety of the generative motion.
             bool previousUpdateInSequencePrevented;
             bool preventPropagation;
             bool inertial;
             // Default initialized to ScrollUnits::PrecisePixels.
             ScrollUnits deltaUnits;
         } scrollUpdate;
 
         struct {
             // The original delta units the scrollBegin and scrollUpdates
             // were sent as.
             ScrollUnits deltaUnits;
+            // If true, this event comes after a non-inertial gesture
+            // scroll sequence; OSX has unique phases for normal and

[tdresser, 2016/03/08 14:28:53]

Is this correct? If this is true, the event comes after an inertial gesture
scroll sequence, right?

[dtapuska, 2016/03/08 20:31:49]

correct; I had copied the text from the begin which isn't really correct.

+            // momentum scroll events. Should always be false for touch based
+            // input as it generates GestureFlingStart instead.
+            bool inertial;
+            // True if the begin was generated when phase information is available
+            // for touchpad and for every scroll update we are generating a GSB/GSU/GSE
+            // tuple. This is a temporary variable that forces the non-latching behavior
+            // until crbug.com/526463 is fully implemented.

[tdresser, 2016/03/08 14:28:53]

I'd mention that these are only needed to match with the synthesized GSB events
that we use to force hit testing to avoid scroll latching.

[dtapuska, 2016/03/08 20:31:49]

Done.

+            bool synthetic;
         } scrollEnd;
 
         struct {
             float velocityX;
             float velocityY;
             // If true, this event will skip hit testing to find a scroll
             // target and instead just scroll the viewport.
             bool targetViewport;
         } flingStart;
 
@@ skipping 55 matching lines @@
         , uniqueTouchEventId(0)
     {
     }
 };
 
 #pragma pack(pop)
 
 } // namespace blink
 
 #endif