Fix for crbug.com/111185.

content/browser/renderer_host/render_widget_host.cc

Index: content/browser/renderer_host/render_widget_host.cc
diff --git a/content/browser/renderer_host/render_widget_host.cc b/content/browser/renderer_host/render_widget_host.cc
index 644eca8a47e6df1c778aea1522e3db1ff391a028..30a81853c9ce2f4190a781ff7ac8241d5dcc6545 100644
--- a/content/browser/renderer_host/render_widget_host.cc
+++ b/content/browser/renderer_host/render_widget_host.cc
@@ -77,6 +77,7 @@ bool ShouldCoalesceMouseWheelEvents(const WebMouseWheelEvent& last_event,
 RenderWidgetHost::RenderWidgetHost(content::RenderProcessHost* process,
                                    int routing_id)
     : renderer_initialized_(false),
+      hung_renderer_delay_ms_(kHungRendererDelayMs),
       renderer_accessible_(false),
       view_(NULL),
       process_(process),
@@ -94,6 +95,7 @@ RenderWidgetHost::RenderWidgetHost(content::RenderProcessHost* process,
       touch_event_is_queued_(false),
       needs_repainting_on_restore_(false),
       is_unresponsive_(false),
+      event_count_(0),
       in_get_backing_store_(false),
       view_being_painted_(false),
       ignore_input_events_(false),
@@ -538,13 +540,13 @@ void RenderWidgetHost::StartHangMonitorTimeout(TimeDelta delay) {
 void RenderWidgetHost::RestartHangMonitorTimeout() {
   // Setting to null will cause StartHangMonitorTimeout to restart the timer.
   time_when_considered_hung_ = Time();
-  StartHangMonitorTimeout(TimeDelta::FromMilliseconds(kHungRendererDelayMs));
+  StartHangMonitorTimeout(
+      TimeDelta::FromMilliseconds(hung_renderer_delay_ms_));
 }
 
 void RenderWidgetHost::StopHangMonitorTimeout() {
   time_when_considered_hung_ = Time();
   RendererIsResponsive();
-
   // We do not bother to stop the hung_renderer_timer_ here in case it will be
   // started again shortly, which happens to be the common use case.
 }
@@ -719,7 +721,9 @@ void RenderWidgetHost::ForwardInputEvent(const WebInputEvent& input_event,
   // after this line.
   next_mouse_move_.reset();
 
-  StartHangMonitorTimeout(TimeDelta::FromMilliseconds(kHungRendererDelayMs));
+  RecordInputEvent();
+  StartHangMonitorTimeout(
+      TimeDelta::FromMilliseconds(hung_renderer_delay_ms_));
 }
 
 void RenderWidgetHost::ForwardTouchEvent(
@@ -1149,7 +1153,8 @@ void RenderWidgetHost::OnMsgInputEventAck(WebInputEvent::Type event_type,
   UMA_HISTOGRAM_TIMES("MPArch.RWH_InputEventDelta", delta);
 
   // Cancel pending hung renderer checks since the renderer is responsive.
-  StopHangMonitorTimeout();
+  if (RecordInputEventAck())
+    StopHangMonitorTimeout();
 
   int type = static_cast<int>(event_type);
   if (type < WebInputEvent::Undefined) {
@@ -1525,6 +1530,27 @@ bool RenderWidgetHost::GotResponseToLockMouseRequest(bool allowed) {
   }
 }
 
+void RenderWidgetHost::RecordInputEvent() {
+  // TODO: remove this check once the compositor thread
+  // is used by default.
+  if (!CommandLine::ForCurrentProcess()->HasSwitch(

[darin (slow to review), 2012/01/24 17:58:01]

I'm not sure it is a good idea to check command line flags on each input event. 
That seems wasteful.  Also, why conditionalize this at all?

+          switches::kEnableThreadedCompositing)) {
+    return;
+  }
+  event_count_++;
+}
+
+bool RenderWidgetHost::RecordInputEventAck() {
+  // TODO: remove this check once the compositor thread
+  // is used by default.
+  if (!CommandLine::ForCurrentProcess()->HasSwitch(
+          switches::kEnableThreadedCompositing)) {
+    return true;
+  }
+  return --event_count_ == 0;

[darin (slow to review), 2012/01/24 17:58:01]

The name of this function, RecordInputEventAck, doesn't make its return value
immediately obvious.  It makes me think that you should just have the caller
check event_count_ directly.

It may also be nice to give this variable a slightly more descriptive name. 
Maybe: in_flight_event_count_

You should also take care to reset this event counter when we recover from a
crash.

+}
+
+
 // static
 void RenderWidgetHost::AcknowledgeSwapBuffers(int32 route_id, int gpu_host_id) {
   GpuProcessHostUIShim* ui_shim = GpuProcessHostUIShim::FromID(gpu_host_id);