Process animation-only traversal.

All traversal processes are like this:

1. Traverse only elements that have this restyle hint (animation-only traversal)
   RESTYLE_CSS_ANIMATIONS is stripped off from restyle hints of the elements
2. Traverse all dirty elements (normal traversal)
3. Create a SequentialTask if we have updated CSS Animations properties in
   the normal traversal
4. Traverse elements that need to have updated animation style as a result of 3
   (second animation-only traversal)

components/style/traversal.rs

@@ -134,6 +134,8 @@ pub trait DomTraversal<E: TElement> : Sync {
     /// The traversal_flag is used in Gecko.
     /// If traversal_flag::UNSTYLED_CHILDREN_ONLY is specified, style newly-
     /// appended children without restyling the parent.
+    /// If traversal_flag::ANIMATION_ONLY is specified, style only elements for
+    /// animations.
     fn pre_traverse(root: E, stylist: &Stylist, traversal_flags: TraversalFlags)
                     -> PreTraverseToken
     {
@@ -158,7 +160,7 @@ pub trait DomTraversal<E: TElement> : Sync {
         }
 
         PreTraverseToken {
-            traverse: Self::node_needs_traversal(root.as_node()),
+            traverse: Self::node_needs_traversal(root.as_node(), traversal_flags.for_animation_only()),
             unstyled_children_only: false,
         }
     }
@@ -172,7 +174,7 @@ pub trait DomTraversal<E: TElement> : Sync {
     }
 
     /// Returns true if traversal is needed for the given node and subtree.
-    fn node_needs_traversal(node: E::ConcreteNode) -> bool {
+    fn node_needs_traversal(node: E::ConcreteNode, animation_only: bool) -> bool {
         // Non-incremental layout visits every node.
         if cfg!(feature = "servo") && opts::get().nonincremental_layout {
             return true;
@@ -181,6 +183,22 @@ pub trait DomTraversal<E: TElement> : Sync {
         match node.as_element() {
             None => Self::text_node_needs_traversal(node),
             Some(el) => {
+                // 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.
+                if animation_only {
+                    if el.has_animation_only_dirty_descendants() {
+                        return true;
+                    }
+
+                    let data = match el.borrow_data() {
+                        Some(d) => d,
+                        None => return false,
+                    };
+                    return data.get_restyle()
+                               .map_or(false, |r| r.hint.has_animation_hint() || r.recascade);
+                }
+
                 // If the dirty descendants bit is set, we need to traverse no
                 // matter what. Skip examining the ElementData.
                 if el.has_dirty_descendants() {
@@ -293,7 +311,7 @@ pub trait DomTraversal<E: TElement> : Sync {
         }
 
         for kid in parent.as_node().children() {
-            if Self::node_needs_traversal(kid) {
+            if Self::node_needs_traversal(kid, self.shared_context().animation_only_restyle) {
                 let el = kid.as_element();
                 if el.as_ref().and_then(|el| el.borrow_data())
                               .map_or(false, |d| d.has_styles())
@@ -471,21 +489,38 @@ pub fn recalc_style_at<E, D>(traversal: &D,
         None => empty_hint,
         Some(r) => {
             r.recascade = false;
-            mem::replace(&mut r.hint, empty_hint).propagate()
+            if r.hint.has_animation_hint() {
+                debug_assert!(context.shared.animation_only_restyle,
+                              "animation restyle hint should be handled during animation-only restyles");
+                // Drop animation restyle hint.
+                let propagated_hint = r.hint.propagate();
+                r.hint.remove_animation_hint();
+                propagated_hint
+            } else {
+                mem::replace(&mut r.hint, empty_hint).propagate()
+            }
         },
     };
-    debug_assert!(data.has_current_styles());
+    debug_assert!(data.has_current_styles() || context.shared.animation_only_restyle,
+                  "Should have computed style or haven't yet valid computed style in case of animation-only restyle");
     trace!("propagated_hint={:?}, inherited_style_changed={:?}", propagated_hint, inherited_style_changed);
 
     // Preprocess children, propagating restyle hints and handling sibling relationships.
     if traversal.should_traverse_children(&mut context.thread_local, element, &data, DontLog) &&
-       (element.has_dirty_descendants() || !propagated_hint.is_empty() || inherited_style_changed) {
+       ((!context.shared.animation_only_restyle && element.has_dirty_descendants()) ||
+        (context.shared.animation_only_restyle && element.has_animation_only_dirty_descendants()) ||
+        !propagated_hint.is_empty() ||
+        inherited_style_changed) {
         let damage_handled = data.get_restyle().map_or(RestyleDamage::empty(), |r| {
             r.damage_handled() | r.damage.handled_for_descendants()
         });
         preprocess_children(traversal, element, propagated_hint, damage_handled, inherited_style_changed);
     }
 
+    if context.shared.animation_only_restyle {
+        unsafe { element.unset_animation_only_dirty_descendants(); }
+    }
+
     // Make sure the dirty descendants bit is not set for the root of a
     // display:none subtree, even if the style didn't change (since, if
     // the style did change, we'd have already cleared it above).

ports/geckolib/glue.rs

@@ -83,8 +83,8 @@ use style::stylesheets::StylesheetLoader as StyleStylesheetLoader;
 use style::supports::parse_condition_or_declaration;
 use style::thread_state;
 use style::timer::Timer;
+use style::traversal::{ANIMATION_ONLY, UNSTYLED_CHILDREN_ONLY};
 use style::traversal::{resolve_style, DomTraversal, TraversalDriver, TraversalFlags};
-use style::traversal::UNSTYLED_CHILDREN_ONLY;
 use style_traits::ToCss;
 use super::stylesheet_loader::StylesheetLoader;
 
@@ -202,6 +202,11 @@ pub extern "C" fn Servo_TraverseSubtree(root: RawGeckoElementBorrowed,
         _ => TraversalFlags::empty(),
     };
 
+    if element.has_animation_only_dirty_descendants() ||
+       element.has_animation_restyle_hints() {
+        traverse_subtree(element, raw_data, traversal_flags | ANIMATION_ONLY);
+    }
+
     traverse_subtree(element, raw_data, traversal_flags);
 
     element.has_dirty_descendants() || element.mutate_data().unwrap().has_restyle()
@@ -1612,7 +1617,7 @@ pub extern "C" fn Servo_AssertTreeIsClean(root: RawGeckoElementBorrowed) {
 
     let root = GeckoElement(root);
     fn assert_subtree_is_clean<'le>(el: GeckoElement<'le>) {
-        debug_assert!(!el.has_dirty_descendants());
+        debug_assert!(!el.has_dirty_descendants() && !el.has_animation_only_dirty_descendants());
         for child in el.as_node().children() {
             if let Some(child) = child.as_element() {
                 assert_subtree_is_clean(child);