style: Ensure QuerySelector only processes the light tree.

MozReview-Commit-ID: 7Nw1SEuWNaC

components/style/dom_apis.rs

@@ -133,6 +133,8 @@ where
     Q: SelectorQuery<E>,
     Q::Output: 'a,
 {
+    fn light_tree_only(&self) -> bool { true }
+
     fn collect_invalidations(
         &mut self,
         element: E,

components/style/invalidation/element/invalidator.rs

@@ -24,6 +24,11 @@ where
     /// that would originate it.
     fn invalidates_on_eager_pseudo_element(&self) -> bool { false }
 
+    /// Whether the invalidation processor only cares about light-tree
+    /// descendants of a given element, that is, doesn't invalidate
+    /// pseudo-elements, NAC, or XBL anon content.
+    fn light_tree_only(&self) -> bool { false }
+
     /// The matching context that should be used to process invalidations.
     fn matching_context(&mut self) -> &mut MatchingContext<'a, E::Impl>;
 
@@ -460,6 +465,11 @@ where
             }
         }
 
+        if self.processor.light_tree_only() {
+            let node = self.element.as_node();
+            return self.invalidate_dom_descendants_of(node, invalidations);
+        }
+
         let mut any_descendant = false;
 
         if let Some(anon_content) = self.element.xbl_binding_anonymous_content() {