Return main thread scroll layer in FindScrollLayerForDeviceViewportPoint

cc/trees/layer_tree_host_impl.cc

Index: cc/trees/layer_tree_host_impl.cc
diff --git a/cc/trees/layer_tree_host_impl.cc b/cc/trees/layer_tree_host_impl.cc
index 9d01bd653cb692523f0cfccf3bcc9f098ad0f097..6ac73a4ab3f435e5a75d90a6e7598b079b2f01b7 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -1659,7 +1659,6 @@ bool LayerTreeHostImpl::DrawLayers(FrameData* frame) {
     }
   }
 
-
   CompositorFrame compositor_frame;
   compositor_frame.metadata = std::move(metadata);
   resource_provider_->PrepareSendToParent(resources,
@@ -2576,6 +2575,28 @@ LayerImpl* LayerTreeHostImpl::FindScrollLayerForDeviceViewportPoint(
   return potentially_scrolling_layer_impl;
 }
 
+LayerImpl* LayerTreeHostImpl::FindScrollLayerByLayer(

[bokan, 2017/01/10 22:36:22]

This method avoids the walk up the parent layers looking for a scrollable one.
i.e. if the hit layer isn't itself scrollable this wont work. Instead, I think
you should modify FindScrollLayerForDeviceViewportPoint to return a layer even
if it's main thread scrolling. Since it returns whether the layer scrolls on
main, you can modify the other call sites that expect NULL to check
scrolls_on_main_thread.

+    LayerImpl* layer_impl) const {
+  ScrollTree& scroll_tree = active_tree_->property_trees()->scroll_tree;
+  LayerImpl* potentially_scrolling_layer_impl = nullptr;
+  if (layer_impl) {
+    ScrollNode* scroll_node = scroll_tree.Node(layer_impl->scroll_tree_index());
+    potentially_scrolling_layer_impl =
+        active_tree_->LayerById(scroll_node->owning_layer_id);
+  }
+
+  // Falling back to the viewport layer ensures generation of root overscroll
+  // notifications. We use the viewport's main scroll layer to represent the
+  // viewport in scrolling code.
+  if (!potentially_scrolling_layer_impl ||
+      potentially_scrolling_layer_impl == OuterViewportScrollLayer() ||
+      potentially_scrolling_layer_impl == InnerViewportScrollLayer()) {
+    potentially_scrolling_layer_impl = viewport()->MainScrollLayer();
+  }
+
+  return potentially_scrolling_layer_impl;
+}
+
 static bool IsClosestScrollAncestor(LayerImpl* child,
                                     LayerImpl* scroll_ancestor) {
   DCHECK(scroll_ancestor);
@@ -3243,9 +3264,6 @@ void LayerTreeHostImpl::MouseMoveAt(const gfx::Point& viewport_point) {
       active_tree_->FindLayerThatIsHitByPoint(device_viewport_point);
 
   // Check if mouse is over a scrollbar or not.
-  // TODO(sahel): get rid of this extera checking when
-  // FindScrollLayerForDeviceViewportPoint finds the proper layer for
-  // scrolling on main thread, as well.
   int new_id = Layer::INVALID_ID;
   if (layer_impl && layer_impl->ToScrollbarLayer())
     new_id = layer_impl->ToScrollbarLayer()->ScrollLayerId();
@@ -3253,11 +3271,7 @@ void LayerTreeHostImpl::MouseMoveAt(const gfx::Point& viewport_point) {
     // Mouse over a scrollbar.
     distance_to_scrollbar = 0;
   } else {
-    bool scroll_on_main_thread = false;
-    uint32_t main_thread_scrolling_reasons;
-    LayerImpl* scroll_layer_impl = FindScrollLayerForDeviceViewportPoint(
-        device_viewport_point, InputHandler::TOUCHSCREEN, layer_impl,
-        &scroll_on_main_thread, &main_thread_scrolling_reasons);
+    LayerImpl* scroll_layer_impl = FindScrollLayerByLayer(layer_impl);
 
     // Scrollbars for the viewport are registered with the outer viewport layer.
     if (scroll_layer_impl == InnerViewportScrollLayer())