Introduce is_native_anonymous and cull style traversal of doomed native anonymous content.

MozReview-Commit-ID: LqwmavJnqW

components/style/dom.rs

@@ -386,6 +386,10 @@ pub trait TElement : PartialEq + Debug + Sized + Copy + Clone + ElementExt + Pre
     unsafe fn unset_animation_only_dirty_descendants(&self) {
     }
 
+    /// Returns true if this element is native anonymous (only Gecko has native
+    /// anonymous content).
+    fn is_native_anonymous(&self) -> bool { false }
+
     /// Atomically stores the number of children of this node that we will
     /// need to process during bottom-up traversal.
     fn store_children_to_process(&self, n: isize);

components/style/gecko/wrapper.rs

@@ -47,6 +47,7 @@ use gecko_bindings::structs::EffectCompositor_CascadeLevel as CascadeLevel;
 use gecko_bindings::structs::NODE_HAS_ANIMATION_ONLY_DIRTY_DESCENDANTS_FOR_SERVO;
 use gecko_bindings::structs::NODE_HAS_DIRTY_DESCENDANTS_FOR_SERVO;
 use gecko_bindings::structs::NODE_IS_IN_NATIVE_ANONYMOUS_SUBTREE;
+use gecko_bindings::structs::NODE_IS_NATIVE_ANONYMOUS;
 use gecko_bindings::sugar::ownership::HasArcFFI;
 use parking_lot::RwLock;
 use parser::ParserContextExtraData;
@@ -543,6 +544,10 @@ impl<'le> TElement for GeckoElement<'le> {
         self.unset_flags(NODE_HAS_ANIMATION_ONLY_DIRTY_DESCENDANTS_FOR_SERVO as u32)
     }
 
+    fn is_native_anonymous(&self) -> bool {
+        self.flags() & (NODE_IS_NATIVE_ANONYMOUS as u32) != 0
+    }
+
     fn store_children_to_process(&self, _: isize) {
         // This is only used for bottom-up traversal, and is thus a no-op for Gecko.
     }

components/style/traversal.rs

@@ -182,6 +182,21 @@ pub trait DomTraversal<E: TElement> : Sync {
         match node.as_element() {
             None => Self::text_node_needs_traversal(node),
             Some(el) => {
+                // If the element is native-anonymous and an ancestor frame will
+                // be reconstructed, the child and all its descendants will be
+                // destroyed. In that case, we don't need to traverse the subtree.
+                if el.is_native_anonymous() {
+                    if let Some(parent) = el.parent_element() {
+                        let parent_data = parent.borrow_data().unwrap();
+                        if let Some(r) = parent_data.get_restyle() {
+                            if (r.damage | r.damage_handled()).contains(RestyleDamage::reconstruct()) {
+                                debug!("Element {:?} is in doomed NAC subtree - culling traversal", el);
+                                return false;
+                            }
+                        }
+                    }
+                }
+
                 // In case of animation-only traversal we need to traverse
                 // the element if the element has animation only dirty
                 // descendants bit, animation-only restyle hint or recascade.