Scroll on main if impl-hit testing isn't guaranteed to be correct

content/renderer/input/input_handler_proxy.cc

Index: content/renderer/input/input_handler_proxy.cc
diff --git a/content/renderer/input/input_handler_proxy.cc b/content/renderer/input/input_handler_proxy.cc
index 3af29385945fe3051944246cfd951011670d1b8f..c8a8494a026ffe098682dce711b2466d48c8d61b 100644
--- a/content/renderer/input/input_handler_proxy.cc
+++ b/content/renderer/input/input_handler_proxy.cc
@@ -160,8 +160,11 @@ InputHandlerProxy::EventDisposition InputHandlerProxy::HandleInputEvent(
         // main thread. Change back to DROP_EVENT once we have synchronization
         // bugs sorted out.
         return DID_NOT_HANDLE;
+      case cc::InputHandler::ScrollUnknown:
       case cc::InputHandler::ScrollOnMainThread:
         return DID_NOT_HANDLE;
+      case cc::InputHandler::ScrollStatusCount:
+        NOTREACHED();

[jdduke (slow), 2014/04/16 17:41:54]

Would it be at all useful to break here (and below)?

     }
   } else if (event.type == WebInputEvent::GestureScrollBegin) {
     DCHECK(!gesture_scroll_on_impl_thread_);
@@ -174,6 +177,9 @@ InputHandlerProxy::EventDisposition InputHandlerProxy::HandleInputEvent(
     cc::InputHandler::ScrollStatus scroll_status = input_handler_->ScrollBegin(
         gfx::Point(gesture_event.x, gesture_event.y),
         cc::InputHandler::Gesture);
+    UMA_HISTOGRAM_ENUMERATION("Renderer4.CompositorScrollHitTestResult",
+                              scroll_status,
+                              cc::InputHandler::ScrollStatusCount);
     switch (scroll_status) {
       case cc::InputHandler::ScrollStarted:
         TRACE_EVENT_INSTANT0("input",
@@ -181,10 +187,13 @@ InputHandlerProxy::EventDisposition InputHandlerProxy::HandleInputEvent(
                              TRACE_EVENT_SCOPE_THREAD);
         gesture_scroll_on_impl_thread_ = true;
         return DID_HANDLE;
+      case cc::InputHandler::ScrollUnknown:
       case cc::InputHandler::ScrollOnMainThread:
         return DID_NOT_HANDLE;
       case cc::InputHandler::ScrollIgnored:
         return DROP_EVENT;
+      case cc::InputHandler::ScrollStatusCount:
+        NOTREACHED();
     }
   } else if (event.type == WebInputEvent::GestureScrollUpdate) {
 #ifndef NDEBUG
@@ -330,6 +339,7 @@ InputHandlerProxy::HandleGestureFling(
       input_handler_->ScheduleAnimation();
       return DID_HANDLE;
     }
+    case cc::InputHandler::ScrollUnknown:
     case cc::InputHandler::ScrollOnMainThread: {
       TRACE_EVENT_INSTANT0("input",
                            "InputHandlerProxy::HandleGestureFling::"
@@ -351,6 +361,8 @@ InputHandlerProxy::HandleGestureFling(
       }
       return DROP_EVENT;
     }
+    case cc::InputHandler::ScrollStatusCount:
+      NOTREACHED();
   }
   return DID_NOT_HANDLE;
 }