Fix mouse wheel over-scrolls when display is scaled and scroll is paginated

ui/events/gesture_event_details.h

 // Copyright 2014 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.
 
 #ifndef UI_EVENTS_GESTURE_DETECTION_GESTURE_EVENT_DETAILS_H_
 #define UI_EVENTS_GESTURE_DETECTION_GESTURE_EVENT_DETAILS_H_
 
 #include <string.h>
 
 #include "base/logging.h"
 #include "ui/events/event_constants.h"
 #include "ui/events/events_base_export.h"
 #include "ui/gfx/geometry/rect.h"
 #include "ui/gfx/geometry/rect_conversions.h"
 
 namespace ui {
 
 struct EVENTS_BASE_EXPORT GestureEventDetails {
  public:
+  // The definition of ScrollUnits below is consistent with that in
+  // //src/third_party/WebKit/public/platform/WebGestureEvent.h
+  enum class ScrollUnits {
+    PRECISE_PIXELS = 0,  // generated by high precision devices.
+    PIXELS,              // large pixel jump duration; should animate to delta.

[sky, 2017/02/17 22:52:31]

Are you sure this isn't DIPs and what you are calling PRECISE_PIXELS is pixels?

[chengx, 2017/02/17 23:19:18]

I agree the names here, which originate from WebGestureEvent.h, is confusing.
Here is the thing. We didn't have ScrollUnits here for GestureEventDetails, so
when GestureEventDetails was used to create WebGestureEvent in
blink_event_util.cc, as in 
https://cs.chromium.org/chromium/src/ui/events/blink/blink_event_util.cc?type...
WebGestureEvent used the default value for the scroll units. The default value
is PrecisePixels, as documented in
https://cs.chromium.org/chromium/src/third_party/WebKit/public/platform/WebGe...
and
https://cs.chromium.org/chromium/src/third_party/WebKit/public/platform/WebGe...

So I think using PRECISE_PIXELS (same as PrecisePixels in WebGestureEvent.h just
using ALL_CAPS) here is fine. Besides, I tried PIXELS as default and some try
bots failed, which is expected because of inconsistency.

The names in the enum here are consistent with the ones in WebGestureEvent.h,
which have been used in a lot of places throughout chrome. If you think we
should rename them, probably we need to do it in another CL. 

+    PAGE                 // page (visible viewport) based scrolling.
+  };
+
   GestureEventDetails();
   explicit GestureEventDetails(EventType type);
-  GestureEventDetails(EventType type, float delta_x, float delta_y);
+
+  // ScrollUnits::PRECISE_PIXELS is the default in WebGestureEvent.h too.
+  GestureEventDetails(EventType type,
+                      float delta_x,
+                      float delta_y,
+                      ScrollUnits units = ScrollUnits::PRECISE_PIXELS);
 
   // The caller is responsible for ensuring that the gesture data from |other|
   // is compatible and sufficient for that expected by gestures of |type|.
   GestureEventDetails(EventType type, const GestureEventDetails& other);
 
   EventType type() const { return type_; }
 
   GestureDeviceType device_type() const { return device_type_; }
   void set_device_type(GestureDeviceType device_type) {
     device_type_ = device_type;
@@ skipping 18 matching lines @@
   float scroll_x_hint() const {
     DCHECK_EQ(ET_GESTURE_SCROLL_BEGIN, type_);
     return data_.scroll_begin.x_hint;
   }
 
   float scroll_y_hint() const {
     DCHECK_EQ(ET_GESTURE_SCROLL_BEGIN, type_);
     return data_.scroll_begin.y_hint;
   }
 
+  ScrollUnits scroll_begin_units() const {
+    DCHECK_EQ(ET_GESTURE_SCROLL_BEGIN, type_);
+    return data_.scroll_begin.delta_hint_units;
+  }
+
   float scroll_x() const {
     DCHECK_EQ(ET_GESTURE_SCROLL_UPDATE, type_);
     return data_.scroll_update.x;
   }
 
   float scroll_y() const {
     DCHECK_EQ(ET_GESTURE_SCROLL_UPDATE, type_);
     return data_.scroll_update.y;
   }
 
+  ScrollUnits scroll_update_units() const {
+    DCHECK_EQ(ET_GESTURE_SCROLL_UPDATE, type_);
+    return data_.scroll_update.delta_units;
+  }
+
   float velocity_x() const {
     DCHECK_EQ(ET_SCROLL_FLING_START, type_);
     return data_.fling_velocity.x;
   }
 
   float velocity_y() const {
     DCHECK_EQ(ET_SCROLL_FLING_START, type_);
     return data_.fling_velocity.y;
   }
 
@@ skipping 74 matching lines @@
 
  private:
   EventType type_;
   union Details {
     Details();
     struct {  // SCROLL start details.
       // Distance that caused the scroll to start.  Generally redundant with
       // the x/y values from the first scroll_update.
       float x_hint;
       float y_hint;
+      ScrollUnits delta_hint_units;
     } scroll_begin;
 
     struct {  // SCROLL delta.
       float x;
       float y;
+      ScrollUnits delta_units;
       // Whether any previous scroll update in the current scroll sequence was
       // suppressed because the underlying touch was consumed.
       bool previous_update_in_sequence_prevented;
     } scroll_update;
 
     float scale;  // PINCH scale.
 
     struct {  // FLING velocity.
       float x;
       float y;
@@ skipping 23 matching lines @@
   int touch_points_;  // Number of active touch points in the gesture.
 
   // Bounding box is an axis-aligned rectangle that contains all the
   // enclosing rectangles of the touch-points in the gesture.
   gfx::RectF bounding_box_;
 };
 
 }  // namespace ui
 
 #endif  // UI_EVENTS_GESTURE_DETECTION_GESTURE_EVENT_DETAILS_H_