Implement ScrollTop and ScrollLeft getters:

Add new compositor message to get scroll_offset;
Add new layout query for computed value of overflow-x/y;
Implement layer_id method for ThreadSafeLayoutNode;
Add new layout query for layer_id;
Implement script interface for getting scrollTop and scrollLeft, as well as relavant helper functions.

components/script/dom/element.rs

@@ -19,6 +19,7 @@ use dom::bindings::codegen::Bindings::EventBinding::EventMethods;
 use dom::bindings::codegen::Bindings::HTMLInputElementBinding::HTMLInputElementMethods;
 use dom::bindings::codegen::Bindings::HTMLTemplateElementBinding::HTMLTemplateElementMethods;
 use dom::bindings::codegen::Bindings::NodeBinding::NodeMethods;
+use dom::bindings::codegen::Bindings::WindowBinding::WindowMethods;
 use dom::bindings::codegen::UnionTypes::NodeOrString;
 use dom::bindings::error::{Error, ErrorResult, Fallible};
 use dom::bindings::global::GlobalRef;
@@ -39,6 +40,7 @@ use dom::htmlanchorelement::HTMLAnchorElement;
 use dom::htmlbodyelement::{HTMLBodyElement, HTMLBodyElementLayoutHelpers};
 use dom::htmlbuttonelement::HTMLButtonElement;
 use dom::htmlcollection::HTMLCollection;
+use dom::htmlelement::HTMLElement;
 use dom::htmlfieldsetelement::HTMLFieldSetElement;
 use dom::htmlfontelement::{HTMLFontElement, HTMLFontElementLayoutHelpers};
 use dom::htmlhrelement::{HTMLHRElement, HTMLHRLayoutHelpers};
@@ -86,7 +88,7 @@ use string_cache::{Atom, Namespace, QualName};
 use style::element_state::*;
 use style::error_reporting::ParseErrorReporter;
 use style::properties::DeclaredValue;
-use style::properties::longhands::{self, background_image, border_spacing, font_family, font_size};
+use style::properties::longhands::{self, background_image, border_spacing, font_family, overflow_x, font_size};
 use style::properties::{PropertyDeclaration, PropertyDeclarationBlock, parse_style_attribute};
 use style::selector_impl::{NonTSPseudoClass, ServoSelectorImpl};
 use style::values::CSSFloat;
@@ -164,6 +166,35 @@ impl Element {
             document,
             ElementBinding::Wrap)
     }
+
+    // https://drafts.csswg.org/cssom-view/#css-layout-box
+    // Elements that have a computed value of the display property
+    // that is table-column or table-column-group
+    // FIXME: Currently, it is assumed to be true always
+    fn has_css_layout_box(&self) -> bool {
+        true
+    }
+
+    // https://drafts.csswg.org/cssom-view/#potentially-scrollable
+    fn potentially_scrollable(&self) -> bool {
+        self.has_css_layout_box() &&
+        !self.overflow_x_is_visible() &&
+        !self.overflow_y_is_visible()
+    }
+
+    // used value of overflow-x is "visible"
+    fn overflow_x_is_visible(&self) -> bool {
+        let window = window_from_node(self);
+        let overflow_pair = window.overflow_query(self.upcast::<Node>().to_trusted_node_address());
+        overflow_pair.x == overflow_x::computed_value::T::visible
+    }
+
+    // used value of overflow-y is "visible"
+    fn overflow_y_is_visible(&self) -> bool {
+        let window = window_from_node(self);
+        let overflow_pair = window.overflow_query(self.upcast::<Node>().to_trusted_node_address());
+        overflow_pair.y != overflow_x::computed_value::T::visible
+    }
 }
 
 #[allow(unsafe_code)]
@@ -1452,6 +1483,68 @@ impl ElementMethods for Element {
                      rect.size.height.to_f64_px())
     }
 
+    // https://drafts.csswg.org/cssom-view/#dom-element-scrolltop
+    fn ScrollTop(&self) -> f64 {
+        let node = self.upcast::<Node>();
+
+        // 1. Let document be the element’s node document.
+        let doc = node.owner_doc();
+
+        // 2. If the document is not the active document, return zero and terminate these steps.
+        if !doc.is_fully_active() {
+            return 0.0;
+        } else {
+            // 3. Let window be the value of document’s defaultView attribute.
+            let win = doc.DefaultView();
+
+            // 5. If the element is the root element and document is in quirks mode,
+            //    return zero and terminate these steps.
+            if *self.root_element() == *self {
+                if doc.quirks_mode() == Quirks {
+                    return 0.0;
+                }
+
+                // 6. If the element is the root element return the value of scrollY on window.
+                return (*win).ScrollY() as f64;
+            }
+
+            // 7. If the element is the HTML body element, document is in quirks mode,
+            //    and the element is not potentially scrollable, return the value of scrollY on window.
+            if doc.GetBody().r() == self.downcast::<HTMLElement>() &&
+               doc.quirks_mode() == Quirks &&
+               !self.potentially_scrollable() {
+                   return (*win).ScrollY() as f64;
+            }
+
+
+            // 8. If the element does not have any associated CSS layout box, return zero and terminate these steps.
+            if !self.has_css_layout_box() {
+                return 0.0;
+            }
+
+            // 9. Return the y-coordinate of the scrolling area at the alignment point
+            //    with the top of the padding edge of the element.
+            let point = node.scroll_offset();
+            return -point.y as f64;
+        }
+    }
+
+    // https://drafts.csswg.org/cssom-view/#dom-element-scrolltop
+    fn SetScrollTop(&self, _scroll_top: f64) {
+        unimplemented!()
+    }
+
+    // https://drafts.csswg.org/cssom-view/#dom-element-scrollleft
+    fn ScrollLeft(&self) -> f64 {
+        let point = self.upcast::<Node>().scroll_offset();
+        return -point.x as f64;
+    }
+
+    // https://drafts.csswg.org/cssom-view/#dom-element-scrollleft
+    fn SetScrollLeft(&self, _scroll_left: f64) {
+        unimplemented!()
+    }
+
     // https://drafts.csswg.org/cssom-view/#dom-element-scrollwidth
     fn ScrollWidth(&self) -> i32 {
         self.upcast::<Node>().scroll_area().size.width

components/script/dom/node.rs

@@ -614,6 +614,12 @@ impl Node {
         }
     }
 
+    pub fn scroll_offset(&self) -> Point2D<f32> {
+        let document = self.owner_doc();
+        let window = document.window();
+        window.scroll_offset_query(self.to_trusted_node_address())
+    }
+
     // https://dom.spec.whatwg.org/#dom-childnode-before
     pub fn before(&self, nodes: Vec<NodeOrString>) -> ErrorResult {
         // Step 1.

components/script/dom/webidls/Element.webidl

@@ -81,6 +81,8 @@ partial interface Element {
   DOMRectList getClientRects();
   DOMRect getBoundingClientRect();
 
+  attribute unrestricted double scrollTop;
+  attribute unrestricted double scrollLeft;
   readonly attribute long scrollWidth;
   readonly attribute long scrollHeight;
 

components/script/dom/window.rs

@@ -74,6 +74,7 @@ use std::sync::{Arc, Mutex};
 use string_cache::Atom;
 use style::context::ReflowGoal;
 use style::error_reporting::ParseErrorReporter;
+use style::properties::longhands::{overflow_x};
 use style::selector_impl::PseudoElement;
 use task_source::TaskSource;
 use task_source::dom_manipulation::{DOMManipulationTaskSource, DOMManipulationTask};
@@ -1122,6 +1123,25 @@ impl Window {
         self.layout_rpc.node_scroll_area().client_rect
     }
 
+    pub fn overflow_query(&self, node: TrustedNodeAddress) -> Point2D<overflow_x::computed_value::T> {
+        self.reflow(ReflowGoal::ForScriptQuery,
+                    ReflowQueryType::NodeOverflowQuery(node),
+                    ReflowReason::Query);
+        self.layout_rpc.node_overflow().0.unwrap()
+    }
+
+    pub fn scroll_offset_query(&self, node: TrustedNodeAddress) -> Point2D<f32> {
+        self.reflow(ReflowGoal::ForScriptQuery,
+                    ReflowQueryType::NodeLayerIdQuery(node),
+                    ReflowReason::Query);
+        let layer_id = self.layout_rpc.node_layer_id().layer_id;
+        let pipeline_id = self.id;
+
+        let (send, recv) = ipc::channel::<Point2D<f32>>().unwrap();
+        self.compositor.send(ScriptToCompositorMsg::GetScrollOffset(pipeline_id, layer_id, send)).unwrap();
+        recv.recv().unwrap_or(Point2D::zero())
+    }
+
     pub fn resolved_style_query(&self,
                             element: TrustedNodeAddress,
                             pseudo: Option<PseudoElement>,
@@ -1476,6 +1496,8 @@ fn debug_reflow_events(id: PipelineId, goal: &ReflowGoal, query_type: &ReflowQue
         ReflowQueryType::ContentBoxesQuery(_n) => "\tContentBoxesQuery",
         ReflowQueryType::HitTestQuery(_n, _o) => "\tHitTestQuery",
         ReflowQueryType::NodeGeometryQuery(_n) => "\tNodeGeometryQuery",
+        ReflowQueryType::NodeLayerIdQuery(_n) => "\tNodeLayerIdQuery",
+        ReflowQueryType::NodeOverflowQuery(_n) => "\tNodeOverFlowQuery",
         ReflowQueryType::NodeScrollGeometryQuery(_n) => "\tNodeScrollGeometryQuery",
         ReflowQueryType::ResolvedStyleQuery(_, _, _) => "\tResolvedStyleQuery",
         ReflowQueryType::OffsetParentQuery(_n) => "\tOffsetParentQuery",

components/script/layout_interface.rs

@@ -23,7 +23,7 @@ use std::sync::Arc;
 use std::sync::mpsc::{Receiver, Sender, channel};
 use string_cache::Atom;
 use style::context::ReflowGoal;
-use style::properties::longhands::{margin_top, margin_right, margin_bottom, margin_left};
+use style::properties::longhands::{margin_top, margin_right, margin_bottom, margin_left, overflow_x};
 use style::selector_impl::PseudoElement;
 use style::servo::Stylesheet;
 use url::Url;
@@ -104,8 +104,12 @@ pub trait LayoutRPC {
     fn content_boxes(&self) -> ContentBoxesResponse;
     /// Requests the geometry of this node. Used by APIs such as `clientTop`.
     fn node_geometry(&self) -> NodeGeometryResponse;
+    /// Requests the overflow-x and overflow-y of this node. Used by `scrollTop` etc.
+    fn node_overflow(&self) -> NodeOverflowResponse;
     /// Requests the scroll geometry of this node. Used by APIs such as `scrollTop`.
     fn node_scroll_area(&self) -> NodeGeometryResponse;
+    /// Requests the layer id of this node. Used by APIs such as `scrollTop`
+    fn node_layer_id(&self) -> NodeLayerIdResponse;
     /// Requests the node containing the point of interest
     fn hit_test(&self) -> HitTestResponse;
     /// Query layout for the resolved value of a given CSS property
@@ -136,6 +140,8 @@ impl MarginStyleResponse {
     }
 }
 
+pub struct NodeOverflowResponse(pub Option<Point2D<overflow_x::computed_value::T>>);
+
 pub struct ContentBoxResponse(pub Rect<Au>);
 pub struct ContentBoxesResponse(pub Vec<Rect<Au>>);
 pub struct HitTestResponse {
@@ -144,6 +150,11 @@ pub struct HitTestResponse {
 pub struct NodeGeometryResponse {
     pub client_rect: Rect<i32>,
 }
+
+pub struct NodeLayerIdResponse {
+    pub layer_id: LayerId,
+}
+
 pub struct ResolvedStyleResponse(pub Option<String>);
 
 #[derive(Clone)]
@@ -167,8 +178,10 @@ pub enum ReflowQueryType {
     NoQuery,
     ContentBoxQuery(TrustedNodeAddress),
     ContentBoxesQuery(TrustedNodeAddress),
+    NodeOverflowQuery(TrustedNodeAddress),
     HitTestQuery(Point2D<f32>, bool),
     NodeGeometryQuery(TrustedNodeAddress),
+    NodeLayerIdQuery(TrustedNodeAddress),
     NodeScrollGeometryQuery(TrustedNodeAddress),
     ResolvedStyleQuery(TrustedNodeAddress, Option<PseudoElement>, Atom),
     OffsetParentQuery(TrustedNodeAddress),