Allow synthetic Tap gestures without a preceding TapDown

content/common/input/gesture_event_stream_validator.cc

 // Copyright 2014 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/common/input/gesture_event_stream_validator.h"
 
 #include "base/logging.h"
 #include "third_party/WebKit/public/web/WebInputEvent.h"
 
 using blink::WebInputEvent;
@@ skipping 44 matching lines @@
     case WebInputEvent::GesturePinchEnd:
       if (!pinching_)
         error_msg->append("Pinch end outside of pinch\n");
       pinching_ = false;
       break;
     case WebInputEvent::GestureTapDown:
       if (waiting_for_tap_end_)
         error_msg->append("Missing tap end event\n");
       waiting_for_tap_end_ = true;
       break;
-    case WebInputEvent::GestureTap:
+    case WebInputEvent::GestureTapUnconfirmed:
+      if (!waiting_for_tap_end_)
+        error_msg->append("Missing TapDown event before TapUnconfirmed\n");
+      break;
     case WebInputEvent::GestureTapCancel:
       if (!waiting_for_tap_end_)
-        error_msg->append("Missing GestureTapDown event\n");
+        error_msg->append("Missing TapDown event before TapCancel\n");
       waiting_for_tap_end_ = false;
       break;
+    case WebInputEvent::GestureTap:
     case WebInputEvent::GestureDoubleTap:
+      // Both Tap and DoubleTap gestures may be synthetically inserted, and do
+      // not require a preceding TapDown.

[tdresser, 2014/09/16 18:03:46]

Can this ever happen on Aura? 
We don't do tap disambiguation.

Do we synthesize tap events anywhere else?

It would be a bit of a shame to weaken the event validation on Aura if it isn't
necessary.

[jdduke (slow), 2014/09/16 19:00:46]

You tell me :)
TapDown is one of those nice-to-have synthetic events that isn't quite as
critical as a PinchBegin or ScrollBegin, so I'm fairly indifferent here.

       waiting_for_tap_end_ = false;
       break;
     default:
       break;
   }
   return error_msg->empty();
 }
 
 }  // namespace content