Setting Accurate ScrollResult from Blink for Elastic Scroll.

content/renderer/render_widget.cc

Index: content/renderer/render_widget.cc
diff --git a/content/renderer/render_widget.cc b/content/renderer/render_widget.cc
index de16016217bd6bb6eb96a4e6aa2b4e6bbc4226a9..d0a6ca3df3f0ba043ec98ea153ae6b26af675688 100644
--- a/content/renderer/render_widget.cc
+++ b/content/renderer/render_widget.cc
@@ -1208,7 +1208,10 @@ void RenderWidget::OnHandleInputEvent(const blink::WebInputEvent* input_event,
   // that they can be used to produce the elastic overscroll effect on Mac.
   if (input_event->type == WebInputEvent::MouseWheel) {
     ObserveWheelEventAndResult(
-        static_cast<const WebMouseWheelEvent&>(*input_event), processed);
+        static_cast<const WebMouseWheelEvent&>(*input_event),
+        event_overscroll ? event_overscroll->latest_overscroll_delta
+                         : gfx::Vector2dF(),
+        processed);
   }
 
   bool frame_pending = compositor_ && compositor_->BeginMainFrameRequested();
@@ -2308,23 +2311,15 @@ bool RenderWidget::WillHandleGestureEvent(
 
 void RenderWidget::ObserveWheelEventAndResult(
     const blink::WebMouseWheelEvent& wheel_event,
+    const gfx::Vector2dF& wheel_unused_delta,
     bool event_processed) {
   if (!compositor_deps_->IsElasticOverscrollEnabled())
     return;
 
-  // Blink does not accurately compute scroll bubbling or overscroll. For now,
-  // assume that an unprocessed event was entirely an overscroll, and that a
-  // processed event was entirely scroll.
-  // TODO(ccameron): Retrieve an accurate scroll result from Blink.
-  // http://crbug.com/442859
   cc::InputHandlerScrollResult scroll_result;
-  if (event_processed) {
-    scroll_result.did_scroll = true;
-  } else {
-    scroll_result.did_overscroll_root = true;
-    scroll_result.unused_scroll_delta =
-        gfx::Vector2dF(-wheel_event.deltaX, -wheel_event.deltaY);
-  }
+  scroll_result.did_scroll = event_processed;
+  scroll_result.did_overscroll_root = !wheel_unused_delta.IsZero();

[jdduke (slow), 2015/07/07 15:06:17]

Can you make sure we're not getting small residual "unused" deltas? In cc/, the
unused delta can be ~.001, and I'm not sure we're filtering those out on the
Blink side.

[MuVen, 2015/07/07 15:36:45]

Done.

+  scroll_result.unused_scroll_delta = wheel_unused_delta;
 
   RenderThreadImpl* render_thread = RenderThreadImpl::current();
   InputHandlerManager* input_handler_manager =