servo: Merge #9968 - Implement scroll, scrollLeft, scrollTop and friends, addressing issue #9650 (from izgzhen:scroll); r=asajeffrey

This is a work in progress to solve https://github.com/servo/servo/issues/9650. Thanks a lot for helping the review.

- [x] scroll
- [x] scrollTo
- [x] scrollBy
- [x] scrollTop (setter and getter)
- [x] scrollLeft (setter and getter)

The setters will be implemented in another PR after this is merged.

Source-Repo: https://github.com/servo/servo
Source-Revision: 8d988f20c12e0a5267e79650a90310951c66ca77

servo/components/layout/query.rs

@@ -13,11 +13,12 @@ use flow;
 use flow_ref::FlowRef;
 use fragment::{Fragment, FragmentBorderBoxIterator, SpecificFragmentInfo};
 use gfx::display_list::OpaqueNode;
+use gfx_traits::{LayerId};
 use layout_thread::LayoutThreadData;
 use msg::constellation_msg::ConstellationChan;
 use opaque_node::OpaqueNodeMethods;
-use script::layout_interface::{ContentBoxResponse, ContentBoxesResponse, NodeGeometryResponse};
-use script::layout_interface::{HitTestResponse, LayoutRPC, OffsetParentResponse};
+use script::layout_interface::{ContentBoxResponse, NodeOverflowResponse, ContentBoxesResponse, NodeGeometryResponse};
+use script::layout_interface::{HitTestResponse, LayoutRPC, OffsetParentResponse, NodeLayerIdResponse};
 use script::layout_interface::{ResolvedStyleResponse, ScriptLayoutChan, MarginStyleResponse};
 use script_traits::LayoutMsg as ConstellationMsg;
 use script_traits::UntrustedNodeAddress;
@@ -112,12 +113,23 @@ impl LayoutRPC for LayoutRPCImpl {
         }
     }
 
+    fn node_overflow(&self) -> NodeOverflowResponse {
+        NodeOverflowResponse(self.0.lock().unwrap().overflow_response.0)
+    }
+
     fn node_scroll_area(&self) -> NodeGeometryResponse {
         NodeGeometryResponse {
             client_rect: self.0.lock().unwrap().scroll_area_response
         }
     }
 
+    fn node_layer_id(&self) -> NodeLayerIdResponse {
+        NodeLayerIdResponse {
+            layer_id: self.0.lock().unwrap().layer_id_response
+                          .expect("layer_id is not correctly fetched, see PR #9968")
+        }
+    }
+
     /// Retrieves the resolved value for a CSS style property.
     fn resolved_style(&self) -> ResolvedStyleResponse {
         let &LayoutRPCImpl(ref rw_data) = self;
@@ -425,6 +437,7 @@ impl FragmentBorderBoxIterator for UnioningFragmentScrollAreaIterator {
         let bottom_padding = (border_box.size.height - bottom_border - top_border).to_px();
         let top_padding = top_border.to_px();
         let left_padding = left_border.to_px();
+
         match self.level {
             Some(start_level) if level <= start_level => { self.is_child = false; }
             Some(_) => {
@@ -517,6 +530,11 @@ pub fn process_node_geometry_request<N: LayoutNode>(requested_node: N, layout_ro
     iterator.client_rect
 }
 
+pub fn process_node_layer_id_request<N: LayoutNode>(requested_node: N) -> LayerId {
+    let layout_node = requested_node.to_threadsafe();
+    layout_node.layer_id()
+}
+
 pub fn process_node_scroll_area_request< N: LayoutNode>(requested_node: N, layout_root: &mut FlowRef)
         -> Rect<i32> {
     let mut iterator = UnioningFragmentScrollAreaIterator::new(requested_node.opaque());
@@ -691,6 +709,14 @@ pub fn process_offset_parent_query<N: LayoutNode>(requested_node: N, layout_root
     }
 }
 
+pub fn process_node_overflow_request<N: LayoutNode>(requested_node: N) -> NodeOverflowResponse {
+    let layout_node = requested_node.to_threadsafe();
+    let style = &*layout_node.style();
+    let style_box = style.get_box();
+
+    NodeOverflowResponse(Some((Point2D::new(style_box.overflow_x, style_box.overflow_y.0))))
+}
+
 pub fn process_margin_style_query<N: LayoutNode>(requested_node: N)
         -> MarginStyleResponse {
     let layout_node = requested_node.to_threadsafe();