Implement CSS: scroll-boundary-behavior

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 3e781b39b5701f7e6e04ea8a14a629e3d36b1102..9a96ea2bfad674b8f7603afbee704627df059a2f 100644
--- a/cc/trees/layer_tree_host_impl.cc
+++ b/cc/trees/layer_tree_host_impl.cc
@@ -3055,6 +3055,7 @@ InputHandler::ScrollStatus LayerTreeHostImpl::ScrollAnimated(
       MainThreadScrollingReason::kNotScrollingOnMain;
   ScrollTree& scroll_tree = active_tree_->property_trees()->scroll_tree;
   ScrollNode* scroll_node = scroll_tree.CurrentlyScrollingNode();
+  bool x_dominated = std::abs(scroll_delta.x()) > std::abs(scroll_delta.y());

[majidvp, 2017/07/05 21:30:03]

nit: x_dominant is more appropriate. Also we should add a comment
explaining why we look at the dominant axis when we have to decide
how to handle the propagation. It is not immediately obvious.

[sunyunjia, 2017/07/14 02:59:00]

Done.

 
   if (scroll_node) {
     // Flash the overlay scrollbar even if the scroll dalta is 0.
@@ -3134,6 +3135,14 @@ InputHandler::ScrollStatus LayerTreeHostImpl::ScrollAnimated(
       }
 
       pending_delta -= scroll_delta;
+
+      if ((x_dominated &&
+           scroll_node->scroll_boundary_behavior.x !=
+               ScrollBoundaryBehavior::kScrollBoundaryBehaviorTypeAuto) ||
+          (!x_dominated &&
+           scroll_node->scroll_boundary_behavior.y !=
+               ScrollBoundaryBehavior::kScrollBoundaryBehaviorTypeAuto))
+        break;

[majidvp, 2017/07/05 21:30:03]

Please move this to its own function e.g., CanBubbleScroll(node, delta).

Should help readability, and also can be reused below.
 

[sunyunjia, 2017/07/14 02:59:00]

Done.

     }
   }
   scroll_state.set_is_ending(true);
@@ -3369,6 +3378,12 @@ void LayerTreeHostImpl::DistributeScrollDelta(ScrollState* scroll_state) {
       viewport()->MainScrollLayer()
           ? scroll_tree.Node(viewport()->MainScrollLayer()->scroll_tree_index())
           : nullptr;
+  bool x_dominated =
+      std::abs(scroll_state->delta_x()) > std::abs(scroll_state->delta_y());
+  if (scroll_state->is_beginning()) {
+    x_dominated = (std::abs(scroll_state->delta_x_hint()) >
+                   std::abs(scroll_state->delta_y_hint()));
+  }
   if (scroll_node) {
     // TODO(bokan): The loop checks for a null parent but don't we still want to
     // distribute to the root scroll node?
@@ -3383,11 +3398,17 @@ void LayerTreeHostImpl::DistributeScrollDelta(ScrollState* scroll_state) {
         break;
       }
 
-      if (!scroll_node->scrollable)
-        continue;
-
-      if (CanConsumeDelta(scroll_node, *scroll_state))
+      if (scroll_node->scrollable &&
+          CanConsumeDelta(scroll_node, *scroll_state))
         current_scroll_chain.push_front(scroll_node);

[majidvp, 2017/07/05 21:30:03]

Do I understand this correctly that the reason the previous early out
is removed is to allow the case where the node is not "scrollable"
but has a boundary behavior. 

If this is so, is this even possible?

[sunyunjia, 2017/07/14 02:59:00]

Done.

+
+      if ((x_dominated &&
+           scroll_node->scroll_boundary_behavior.x !=
+               ScrollBoundaryBehavior::kScrollBoundaryBehaviorTypeAuto) ||
+          (!x_dominated &&
+           scroll_node->scroll_boundary_behavior.y !=
+               ScrollBoundaryBehavior::kScrollBoundaryBehaviorTypeAuto))
+        break;

[majidvp, 2017/07/05 21:30:03]

ditto. This should be a function called from both places.

[sunyunjia, 2017/07/14 02:59:00]

Done.

     }
   }
   active_tree_->SetCurrentlyScrollingNode(