Merging of touch event ack ids was inconsistent.

content/renderer/input/main_thread_event_queue.cc

Index: content/renderer/input/main_thread_event_queue.cc
diff --git a/content/renderer/input/main_thread_event_queue.cc b/content/renderer/input/main_thread_event_queue.cc
index b629b408ae2ed0a912d116d889855904c9d3a89b..9341efab8891d001dfdd53bfc8cf86d792e3f849 100644
--- a/content/renderer/input/main_thread_event_queue.cc
+++ b/content/renderer/input/main_thread_event_queue.cc
@@ -42,13 +42,14 @@ EventWithDispatchType::EventWithDispatchType(
 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();
 }