Signal input flush when all flings have terminated

content/browser/renderer_host/input/gesture_event_queue.cc

Index: content/browser/renderer_host/input/gesture_event_queue.cc
diff --git a/content/browser/renderer_host/input/gesture_event_queue.cc b/content/browser/renderer_host/input/gesture_event_queue.cc
index a13fde15a429ec47c2b6eaa59361171706eb0bad..c7028f8a375eaef3f4f29b63f682a4d4f37d9424 100644
--- a/content/browser/renderer_host/input/gesture_event_queue.cc
+++ b/content/browser/renderer_host/input/gesture_event_queue.cc
@@ -62,7 +62,7 @@ GestureEventQueue::GestureEventQueue(
     TouchpadTapSuppressionControllerClient* touchpad_client,
     const Config& config)
     : client_(client),
-      fling_in_progress_(false),
+      active_fling_count_(0),
       scrolling_in_progress_(false),
       ignore_next_ack_(false),
       touchpad_tap_suppression_controller_(
@@ -92,8 +92,6 @@ void GestureEventQueue::QueueEvent(
 
 bool GestureEventQueue::ShouldDiscardFlingCancelEvent(
     const GestureEventWithLatencyInfo& gesture_event) const {
-  if (coalesced_gesture_events_.empty() && fling_in_progress_)
-    return false;
   GestureQueue::const_reverse_iterator it =
       coalesced_gesture_events_.rbegin();
   while (it != coalesced_gesture_events_.rend()) {
@@ -103,7 +101,9 @@ bool GestureEventQueue::ShouldDiscardFlingCancelEvent(
       return true;
     it++;
   }
-  return true;
+  // If there are no fling-affecting events in the queue, and there's still an
+  // active fling in the renderer, the cancel event should not be dropped.
+  return !active_fling_count_;
 }
 
 bool GestureEventQueue::ShouldForwardForBounceReduction(
@@ -175,12 +175,6 @@ bool GestureEventQueue::ShouldForwardForTapSuppression(
 void GestureEventQueue::QueueAndForwardIfNecessary(
     const GestureEventWithLatencyInfo& gesture_event) {
   switch (gesture_event.event.type) {
-    case WebInputEvent::GestureFlingCancel:
-      fling_in_progress_ = false;
-      break;
-    case WebInputEvent::GestureFlingStart:
-      fling_in_progress_ = true;
-      break;
     case WebInputEvent::GesturePinchUpdate:
     case WebInputEvent::GestureScrollUpdate:
       QueueScrollOrPinchAndForwardIfNecessary(gesture_event);
@@ -204,6 +198,11 @@ void GestureEventQueue::ProcessGestureAck(InputEventAckState ack_result,
     return;
   }
 
+  if (ack_result == INPUT_EVENT_ACK_STATE_CONSUMED &&
+      type == blink::WebInputEvent::GestureFlingStart) {
+    ++active_fling_count_;
+  }
+
   // It's possible that the ack for the second event in an in-flight, coalesced
   // Gesture{Scroll,Pinch}Update pair is received prior to the first event ack.
   // TODO(jdduke): Unify GSU/GPU pairs into a single event, crbug.com/359115.
@@ -269,8 +268,9 @@ TouchpadTapSuppressionController*
   return &touchpad_tap_suppression_controller_;
 }
 
-void GestureEventQueue::FlingHasBeenHalted() {
-  fling_in_progress_ = false;
+void GestureEventQueue::DidStopFlinging() {
+  DCHECK_GE(active_fling_count_, 0);
+  --active_fling_count_;
 }
 
 void GestureEventQueue::ForwardGestureEvent(