Auto merge of #18104 - aethanyc:fix-nth-child-xbl-bug1382102, r=emilio

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

This was reviewed in https://bugzilla.mozilla.org/show_bug.cgi?id=1382102

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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 {

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.

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
         }