Merging of touch event ack ids was inconsistent.

content/renderer/input/main_thread_event_queue.cc

 // Copyright 2016 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 "content/renderer/input/main_thread_event_queue.h"
 
 #include "base/metrics/field_trial.h"
 #include "base/metrics/histogram_macros.h"
 #include "base/strings/string_number_conversions.h"
 #include "content/common/input/event_with_latency_info.h"
@@ skipping 23 matching lines @@
     const ui::LatencyInfo& latency,
     InputEventDispatchType dispatch_type)
     : ScopedWebInputEventWithLatencyInfo(std::move(event), latency),
       dispatch_type_(dispatch_type),
       non_blocking_coalesced_count_(0),
       creation_timestamp_(base::TimeTicks::Now()),
       last_coalesced_timestamp_(creation_timestamp_) {}
 
 EventWithDispatchType::~EventWithDispatchType() {}
 
 void EventWithDispatchType::CoalesceWith(const EventWithDispatchType& other) {

[tdresser, 2017/02/22 15:53:47]

This method doesn't make it super clear whether you coalesce a newer event into
an older event, or an older event into a newer event, which I think is where
this confusion came from originally.

Can we add a comment indicating which is correct, and DCHECK that the timestamps
are valid?

[dtapuska, 2017/02/22 18:43:48]

The ScopedWebInputEventWithLatencyInfo already DCHECKS that the tracing id of
"other" is after "this" one; but I will add a comment.

-  if (other.dispatch_type_ == DISPATCH_TYPE_BLOCKING) {
+  if (dispatch_type_ == DISPATCH_TYPE_BLOCKING) {
     blocking_coalesced_event_ids_.push_back(
-        ui::WebInputEventTraits::GetUniqueTouchEventId(other.event()));
+        ui::WebInputEventTraits::GetUniqueTouchEventId(event()));
   } else {
     non_blocking_coalesced_count_++;
   }
   ScopedWebInputEventWithLatencyInfo::CoalesceWith(other);
+  dispatch_type_ = other.dispatch_type_;
   last_coalesced_timestamp_ = base::TimeTicks::Now();
 }
 
 MainThreadEventQueue::SharedState::SharedState()
     : sent_main_frame_request_(false) {}
 
 MainThreadEventQueue::SharedState::~SharedState() {}
 
 MainThreadEventQueue::MainThreadEventQueue(
     int routing_id,
@@ skipping 296 matching lines @@
     case blink::WebInputEvent::MouseWheel:
       return handle_raf_aligned_mouse_input_;
     case blink::WebInputEvent::TouchMove:
       return handle_raf_aligned_touch_input_;
     default:
       return false;
   }
 }
 
 }  // namespace content