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..1cb371f8b5fdb25263d4c183d75cce44e0ec0c1b 100644
--- a/content/browser/renderer_host/input/immediate_input_router.cc
+++ b/content/browser/renderer_host/input/immediate_input_router.cc
@@ -339,6 +339,14 @@ void ImmediateInputRouter::OfferToHandlers(const WebInputEvent& input_event,
     return;
 
   OfferToRenderer(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);
+  }
 }
 
 bool ImmediateInputRouter::OfferToOverscrollController(
@@ -424,6 +432,10 @@ 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.