Coalesced mouse wheel event has the position of the most recent event.

content/common/input/event_with_latency_info.cc

Index: content/common/input/event_with_latency_info.cc
diff --git a/content/common/input/event_with_latency_info.cc b/content/common/input/event_with_latency_info.cc
index a2b306b7eaaa71e6683cc6c23f287f498f6891a2..9aec6328399900b210a0efcd39eb50d05d75160b 100644
--- a/content/common/input/event_with_latency_info.cc
+++ b/content/common/input/event_with_latency_info.cc
@@ -97,10 +97,19 @@ void Coalesce(const WebMouseWheelEvent& event_to_coalesce,
       GetUnacceleratedDelta(event->deltaY, event->accelerationRatioY) +
       GetUnacceleratedDelta(event_to_coalesce.deltaY,
                             event_to_coalesce.accelerationRatioY);

[tdresser, 2016/08/19 12:43:30]

WebMouseWheelEvent is a WebMouseEvent.
Could we call WebMouseEvent::Coalesce in here, and then only deal explicitly
with the deltas and wheelTicks?

That way if someone adds a new field to WebMouseEvent, we don't need to remember
to write the coalescing logic in both places.

[sahel, 2016/08/19 15:17:56]

I see your point here, but this way we guarantee that the coalesced event is the
new one with merged delta. It will be true if some logic is added to either of
the classes.

-  event->deltaX += event_to_coalesce.deltaX;
-  event->deltaY += event_to_coalesce.deltaY;
-  event->wheelTicksX += event_to_coalesce.wheelTicksX;
-  event->wheelTicksY += event_to_coalesce.wheelTicksY;
+  float old_deltaX = event->deltaX;
+  float old_deltaY = event->deltaY;
+  float old_wheelTicksX = event->wheelTicksX;
+  float old_wheelTicksY = event->wheelTicksY;
+  float old_movementX = event->movementX;
+  float old_movementY = event->movementY;
+  *event = event_to_coalesce;
+  event->deltaX += old_deltaX;
+  event->deltaY += old_deltaY;
+  event->wheelTicksX += old_wheelTicksX;
+  event->wheelTicksY += old_wheelTicksY;
+  event->movementX += old_movementX;
+  event->movementY += old_movementY;
   event->accelerationRatioX =
       GetAccelerationRatio(event->deltaX, unaccelerated_x);
   event->accelerationRatioY =