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 46b59a63ca12db1ff5688c0aa073da4f21263577..5fb5ad67d24d98d286a9c8544a903f373f74b7d2 100644
--- a/content/browser/renderer_host/input/immediate_input_router.cc
+++ b/content/browser/renderer_host/input/immediate_input_router.cc
@@ -340,6 +340,13 @@ void ImmediateInputRouter::FilterAndSendWebInputEvent(
   }
 
   SendWebInputEvent(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,
+                         CLIENT);

[jdduke (slow), 2013/10/25 20:53:34]

Please create a new enum entry for this ack source instead of repurposing
CLIENT.

[tdresser, 2013/11/04 15:57:30]

Done.

+  }
 
   // Any input event cancels a pending mouse move event.
   next_mouse_move_.reset();
@@ -416,6 +423,10 @@ 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.