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

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 4a945a224384de89251c3b1d7a2b888f932c4999..a8001b7da199c0ea05167aef8a253f106578e191 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -2140,6 +2140,17 @@ LayerImpl* LayerTreeHostImpl::FindScrollLayerForDeviceViewportPoint(
   return potentially_scrolling_layer_impl;
 }
 
+// Similar to LayerImpl::HasAncestor, but takes into account scroll parents.
+static bool HasScrollAncestor(LayerImpl* child, LayerImpl* scroll_ancestor) {
+  DCHECK(scroll_ancestor);
+  for (LayerImpl* ancestor = child; ancestor;
+       ancestor = NextScrollLayer(ancestor)) {
+    if (ancestor->scrollable())
+      return ancestor == scroll_ancestor;
+  }
+  return false;
+}
+
 InputHandler::ScrollStatus LayerTreeHostImpl::ScrollBegin(
     const gfx::Point& viewport_point,
     InputHandler::ScrollInputType type) {
@@ -2159,6 +2170,15 @@ InputHandler::ScrollStatus LayerTreeHostImpl::ScrollBegin(
   LayerImpl* layer_impl = LayerTreeHostCommon::FindLayerThatIsHitByPoint(
       device_viewport_point,
       active_tree_->RenderSurfaceLayerList());
+
+  if (layer_impl) {
+    LayerImpl* scroll_layer_impl =
+        LayerTreeHostCommon::FindFirstScrollingLayerThatIsHitByPoint(
+            device_viewport_point, active_tree_->RenderSurfaceLayerList());
+    if (scroll_layer_impl && !HasScrollAncestor(layer_impl, scroll_layer_impl))
+      return ScrollUnknown;
+  }
+
   bool scroll_on_main_thread = false;
   LayerImpl* potentially_scrolling_layer_impl =
       FindScrollLayerForDeviceViewportPoint(device_viewport_point,