Wheel scrolling with smooth scrolling enabled would cause an extra animation

ui/events/blink/input_handler_proxy.cc

 // Copyright 2013 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.
 
 #include "ui/events/blink/input_handler_proxy.h"
 
 #include <stddef.h>
 
 #include <algorithm>
 
@@ skipping 579 matching lines @@
       blink::WebGestureEvent::ScrollUnits::Page) {
     scroll_status.thread = cc::InputHandler::SCROLL_ON_MAIN_THREAD;
     scroll_status.main_thread_scrolling_reasons =
         cc::MainThreadScrollingReason::kContinuingMainThreadScroll;
   } else if (gesture_event.data.scrollBegin.targetViewport) {
     scroll_status = input_handler_->RootScrollBegin(&scroll_state,
                                                     cc::InputHandler::GESTURE);
   } else if (smooth_scroll_enabled_ &&
              gesture_event.data.scrollBegin.deltaHintUnits ==
                  blink::WebGestureEvent::ScrollUnits::Pixels) {
-    gfx::Vector2dF scroll_delta(-gesture_event.data.scrollBegin.deltaXHint,
-                                -gesture_event.data.scrollBegin.deltaYHint);
     scroll_status = input_handler_->ScrollAnimated(
-        gfx::Point(gesture_event.x, gesture_event.y), scroll_delta);
+        gfx::Point(gesture_event.x, gesture_event.y), gfx::Vector2dF());

[tdresser, 2016/02/22 20:14:49]

Does this need to be a ScrollAnimated?
Can we just always call input_handler_->ScrollBegin instead?

[dtapuska, 2016/02/22 20:15:59]

I first attempted to do that and since a scroll animator isn't created it was
failing.

[ymalik, 2016/02/22 20:35:32]

Why do we need this branch to begin with if we're setting scroll delta to 0?
 
ScrollAnimated calls ScrollBegin, so where exactly does the call fail? 

[dtapuska, 2016/02/22 20:39:03]

ScrollAnimated is successful; but the next ScrollAnimated call doesn't work;
because it is already inside a scroll event.

We need to know if the GestureScrollBegin can scroll or not; I guess I could
call BeginScroll/EndScroll combination. Do you recommend that instead? Ideally I
want to get into the ScrollAnimationCreate code path inside
layer_tree_host_impl.cc

   } else {
     scroll_status =
         input_handler_->ScrollBegin(&scroll_state, cc::InputHandler::GESTURE);
   }
   UMA_HISTOGRAM_ENUMERATION("Renderer4.CompositorScrollHitTestResult",
                             scroll_status.thread,
                             cc::InputHandler::LAST_SCROLL_STATUS + 1);
 
   RecordMainThreadScrollingReasons(gesture_event.type,
                                    scroll_status.main_thread_scrolling_reasons);
@@ skipping 682 matching lines @@
   // trigger a scroll, e.g., with a trivial time delta between fling updates.
   // Return true in this case to prevent early fling termination.
   if (std::abs(clipped_increment.width) < kScrollEpsilon &&
       std::abs(clipped_increment.height) < kScrollEpsilon)
     return true;
 
   return did_scroll;
 }
 
 }  // namespace ui