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

content/renderer/input/input_handler_proxy.cc

 // Copyright 2013 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "content/renderer/input/input_handler_proxy.h"
 
 #include "base/auto_reset.h"
 #include "base/debug/trace_event.h"
 #include "base/logging.h"
 #include "base/metrics/histogram.h"
@@ skipping 142 matching lines @@
             gfx::Vector2dF(-wheel_event.deltaX, -wheel_event.deltaY));
         input_handler_->ScrollEnd();
         return did_scroll ? DID_HANDLE : DROP_EVENT;
       }
       case cc::InputHandler::ScrollIgnored:
         // TODO(jamesr): This should be DROP_EVENT, but in cases where we fail
         // to properly sync scrollability it's safer to send the event to the
         // 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_);
 #ifndef NDEBUG
     DCHECK(!expect_scroll_update_end_);
     expect_scroll_update_end_ = true;
 #endif
     const WebGestureEvent& gesture_event =
         *static_cast<const WebGestureEvent*>(&event);
     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",
                              "InputHandlerProxy::handle_input gesture scroll",
                              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
     DCHECK(expect_scroll_update_end_);
 #endif
 
     if (!gesture_scroll_on_impl_thread_ && !gesture_pinch_on_impl_thread_)
       return DID_NOT_HANDLE;
 
     const WebGestureEvent& gesture_event =
@@ skipping 125 matching lines @@
           WebFloatPoint(gesture_event.data.flingStart.velocityX,
                         gesture_event.data.flingStart.velocityY);
       fling_parameters_.point = WebPoint(gesture_event.x, gesture_event.y);
       fling_parameters_.globalPoint =
           WebPoint(gesture_event.globalX, gesture_event.globalY);
       fling_parameters_.modifiers = gesture_event.modifiers;
       fling_parameters_.sourceDevice = gesture_event.sourceDevice;
       input_handler_->ScheduleAnimation();
       return DID_HANDLE;
     }
+    case cc::InputHandler::ScrollUnknown:
     case cc::InputHandler::ScrollOnMainThread: {
       TRACE_EVENT_INSTANT0("input",
                            "InputHandlerProxy::HandleGestureFling::"
                            "scroll_on_main_thread",
                            TRACE_EVENT_SCOPE_THREAD);
       fling_may_be_active_on_main_thread_ = true;
       return DID_NOT_HANDLE;
     }
     case cc::InputHandler::ScrollIgnored: {
       TRACE_EVENT_INSTANT0(
           "input",
           "InputHandlerProxy::HandleGestureFling::ignored",
           TRACE_EVENT_SCOPE_THREAD);
       if (gesture_event.sourceDevice == WebGestureEvent::Touchpad) {
         // We still pass the curve to the main thread if there's nothing
         // scrollable, in case something
         // registers a handler before the curve is over.
         return DID_NOT_HANDLE;
       }
       return DROP_EVENT;
     }
+    case cc::InputHandler::ScrollStatusCount:
+      NOTREACHED();
   }
   return DID_NOT_HANDLE;
 }
 
 void InputHandlerProxy::Animate(base::TimeTicks time) {
   if (!fling_curve_)
     return;
 
   double monotonic_time_sec = InSecondsF(time);
   if (!fling_parameters_.startTime ||
@@ skipping 173 matching lines @@
   // trigger a scroll, e.g., with a trivial time delta between fling updates.
   // Return true in this case to prevent early fling termination.
   if (std::abs(clipped_increment.width) < kScrollEpsilon &&
       std::abs(clipped_increment.height) < kScrollEpsilon)
     return true;
 
   return did_scroll;
 }
 
 }  // namespace content