Events ignoring ack disposition receive synthetic acks.

content/browser/renderer_host/input/immediate_input_router.cc

Index: content/browser/renderer_host/input/immediate_input_router.cc
diff --git a/content/browser/renderer_host/input/immediate_input_router.cc b/content/browser/renderer_host/input/immediate_input_router.cc
index 444678fda27412acab88c21914cdf0a8ff814935..cd1dbd9984934c66aa687d9a9e005c330708d1d6 100644
--- a/content/browser/renderer_host/input/immediate_input_router.cc
+++ b/content/browser/renderer_host/input/immediate_input_router.cc
@@ -327,6 +327,13 @@ void ImmediateInputRouter::FilterAndSendWebInputEvent(
   next_mouse_move_.reset();
 
   OfferToHandlers(input_event, latency_info, is_keyboard_shortcut);
+  // If we don't care about the ack disposition, send the ack immediately.
+  if (WebInputEventTraits::IgnoresAckDisposition(input_event.type)) {
+    ProcessInputEventAck(input_event.type,
+                         INPUT_EVENT_ACK_STATE_IGNORED,
+                         latency_info,
+                         IGNORING_DISPOSITION);
+  }

[jdduke (slow), 2013/11/05 05:57:18]

This code should only be called if the event was sent to the renderer.  For the
synchronous handling cases (OverscrollController + Client) we may get valid
acks, in which case we should use those instead of the synthetic IGNORED ones. 
Please move to |OfferToRenderer()|.

[tdresser, 2013/11/05 18:57:02]

Done.

 }
 
 void ImmediateInputRouter::OfferToHandlers(const WebInputEvent& input_event,
@@ -424,6 +431,10 @@ void ImmediateInputRouter::OnInputEventAck(
 
   client_->DecrementInFlightEventCount();
 
+  // A synthetic ack will already have been sent for this event.
+  if (WebInputEventTraits::IgnoresAckDisposition(event_type))
+    return;
+
   ProcessInputEventAck(event_type, ack_result, latency_info, RENDERER);
   // WARNING: |this| may be deleted at this point.