style: Skip matching :nth-child if element is the root of anonymous subtree

This implement the logic in Gecko's nsNthIndexCache::GetNthIndex().

MozReview-Commit-ID: 8lzK9iIbfzo

3 files changed, 21 insertions, 2 deletions

diff --git a/components/selectors/matching.rs b/components/selectors/matching.rs
index 8f6b2fa6aa5..ead73c6a7fb 100644
--- a/components/selectors/matching.rs
+++ b/components/selectors/matching.rs
@@ -776,6 +776,10 @@ fn matches_generic_nth_child<E, F>(element: &E,
     where E: Element,
           F: FnMut(&E, ElementSelectorFlags),
 {
+    if element.ignores_nth_child_selectors() {
+        return false;
+    }
+
     flags_setter(element, if is_from_end {
         HAS_SLOW_SELECTOR
     } else {
diff --git a/components/selectors/tree.rs b/components/selectors/tree.rs
index 7827dc6bbe6..23238237b2f 100644
--- a/components/selectors/tree.rs
+++ b/components/selectors/tree.rs
@@ -86,6 +86,12 @@ pub trait Element: Sized + Debug {
     /// if the parent node is a `DocumentFragment`.
     fn is_root(&self) -> bool;
 
+    /// Returns whether this element should ignore matching nth child
+    /// selector.
+    fn ignores_nth_child_selectors(&self) -> bool {
+        false
+    }
+
     /// Return true if we want to stop lookup ancestor of the current
     /// element while matching complex selectors with descendant/child
     /// combinator.
diff --git a/components/style/gecko/wrapper.rs b/components/style/gecko/wrapper.rs
index e4ccac1a2ba..6b7e2ca6030 100644
--- a/components/style/gecko/wrapper.rs
+++ b/components/style/gecko/wrapper.rs
@@ -729,6 +729,12 @@ impl<'le> GeckoElement<'le> {
             debug!("(Element not styled, discarding hints)");
         }
     }
+
+    /// This logic is duplicated in Gecko's nsIContent::IsRootOfAnonymousSubtree.
+    fn is_root_of_anonymous_subtree(&self) -> bool {
+        use gecko_bindings::structs::NODE_IS_ANONYMOUS_ROOT;
+        self.flags() & (NODE_IS_ANONYMOUS_ROOT as u32) != 0
+    }
 }
 
 /// Converts flags from the layout used by rust-selectors to the layout used
@@ -1970,9 +1976,12 @@ impl<'le> ::selectors::Element for GeckoElement<'le> {
         node.owner_doc().mType == structs::root::nsIDocument_Type::eHTML
     }
 
+    fn ignores_nth_child_selectors(&self) -> bool {
+        self.is_root_of_anonymous_subtree()
+    }
+
     fn blocks_ancestor_combinators(&self) -> bool {
-        use gecko_bindings::structs::NODE_IS_ANONYMOUS_ROOT;
-        if self.flags() & (NODE_IS_ANONYMOUS_ROOT as u32) == 0 {
+        if !self.is_root_of_anonymous_subtree() {
             return false
         }