4 files changed, 21 insertions, 42 deletions

diff --git a/components/layout_thread/dom_wrapper.rs b/components/layout_thread/dom_wrapper.rs
index 5cf29e8e159..5c5fc4be052 100644
--- a/components/layout_thread/dom_wrapper.rs
+++ b/components/layout_thread/dom_wrapper.rs
@@ -151,10 +151,10 @@ impl<'ln> NodeInfo for ServoLayoutNode<'ln> {
     }
 }
 
-#[derive(Clone, Copy)]
+#[derive(Clone, Copy, PartialEq)]
 enum Impossible { }
 
-#[derive(Clone, Copy)]
+#[derive(Clone, Copy, PartialEq)]
 pub struct ShadowRoot<'lr>(Impossible, PhantomData<&'lr ()>);
 
 impl<'lr> TShadowRoot for ShadowRoot<'lr> {
diff --git a/components/style/dom.rs b/components/style/dom.rs
index 59c31572c56..cfd3c80e890 100644
--- a/components/style/dom.rs
+++ b/components/style/dom.rs
@@ -321,7 +321,7 @@ fn fmt_subtree<F, N: TNode>(f: &mut fmt::Formatter, stringify: &F, n: N, indent:
 }
 
 /// The ShadowRoot trait.
-pub trait TShadowRoot : Sized + Copy + Clone {
+pub trait TShadowRoot : Sized + Copy + Clone + PartialEq {
     /// The concrete node type.
     type ConcreteNode: TNode<ConcreteShadowRoot = Self>;
 
@@ -390,15 +390,6 @@ pub trait TElement
         depth
     }
 
-    /// The style scope of this element is a node that represents which rules
-    /// apply to the element.
-    ///
-    /// In Servo, where we don't know about Shadow DOM or XBL, the style scope
-    /// is always the document.
-    fn style_scope(&self) -> Self::ConcreteNode {
-        self.as_node().owner_doc().as_node()
-    }
-
     /// Get this node's parent element from the perspective of a restyle
     /// traversal.
     fn traversal_parent(&self) -> Option<Self> {
diff --git a/components/style/gecko/wrapper.rs b/components/style/gecko/wrapper.rs
index b4e7862bce8..321673d7c81 100644
--- a/components/style/gecko/wrapper.rs
+++ b/components/style/gecko/wrapper.rs
@@ -135,6 +135,13 @@ impl<'ld> TDocument for GeckoDocument<'ld> {
 #[derive(Clone, Copy)]
 pub struct GeckoShadowRoot<'lr>(pub &'lr structs::ShadowRoot);
 
+impl<'lr> PartialEq for GeckoShadowRoot<'lr> {
+    #[inline]
+    fn eq(&self, other: &Self) -> bool {
+        self.0 as *const _ == other.0 as *const _
+    }
+}
+
 impl<'lr> TShadowRoot for GeckoShadowRoot<'lr> {
     type ConcreteNode = GeckoNode<'lr>;
 
@@ -1040,29 +1047,6 @@ impl<'le> TElement for GeckoElement<'le> {
         self.before_or_after_pseudo(/* is_before = */ false)
     }
 
-    /// Ensure this accurately represents the rules that an element may ever
-    /// match, even in the native anonymous content case.
-    fn style_scope(&self) -> Self::ConcreteNode {
-        if self.implemented_pseudo_element().is_some() {
-            return self.closest_non_native_anonymous_ancestor().unwrap().style_scope();
-        }
-
-        if self.is_in_native_anonymous_subtree() {
-            return self.as_node().owner_doc().as_node();
-        }
-
-        if self.xbl_binding().is_some() || self.shadow_root().is_some() {
-            return self.as_node();
-        }
-
-        if let Some(parent) = self.xbl_binding_parent() {
-            return parent.as_node();
-        }
-
-        self.as_node().owner_doc().as_node()
-    }
-
-
     #[inline]
     fn is_html_element(&self) -> bool {
         self.namespace_id() == (structs::root::kNameSpaceID_XHTML as i32)
diff --git a/components/style/sharing/mod.rs b/components/style/sharing/mod.rs
index 6b3bbb9bccc..788ad3f5e3d 100644
--- a/components/style/sharing/mod.rs
+++ b/components/style/sharing/mod.rs
@@ -679,12 +679,14 @@ impl<E: TElement> StyleSharingCache<E> {
             return None;
         }
 
-        // Note that in the XBL case, we should be able to assert that the
-        // scopes are different, since two elements with different XBL bindings
-        // need to necessarily have different style (and thus children of them
-        // would never pass the parent check).
-        if target.element.style_scope() != candidate.element.style_scope() {
-            trace!("Miss: Different style scopes");
+        // If two elements belong to different shadow trees, different rules may
+        // apply to them, from the respective trees.
+        //
+        // Note that we don't need the same for XBL case, since two elements
+        // with different XBL bindings need to necessarily have different style
+        // (and thus children of them would never pass the parent check).
+        if target.element.containing_shadow() != candidate.element.containing_shadow() {
+            trace!("Miss: Different containing shadow roots");
             return None;
         }
 
@@ -695,7 +697,9 @@ impl<E: TElement> StyleSharingCache<E> {
         // shadow root, they could match different rules, due to the slot being
         // assigned to yet another slot in another shadow root.
         if target.element.assigned_slot() != candidate.element.assigned_slot() {
-            trace!("Miss: Different style scopes");
+            // TODO(emilio): We could have a look at whether the shadow roots
+            // actually have slotted rules and such.
+            trace!("Miss: Different assigned slots");
             return None;
         }