Root nodes for the duration of their CSS transitions.

This ensures that we can pass a node address as part of the asynchronous
transition end notification, making it safe to fire the corresponding
DOM event on the node from the script thread. Without explicitly rooting
this node when the transition starts, we risk the node being GCed before
the transition is complete.

components/layout/query.rs

@@ -94,6 +94,9 @@ pub struct LayoutThreadData {
 
     /// A queued response for the list of nodes at a given point.
     pub nodes_from_point_response: Vec<UntrustedNodeAddress>,
+
+    /// A list of nodes that have just started a CSS transition.
+    pub newly_transitioning_nodes: Vec<UntrustedNodeAddress>,
 }
 
 pub struct LayoutRPCImpl(pub Arc<Mutex<LayoutThreadData>>);
@@ -204,6 +207,12 @@ impl LayoutRPC for LayoutRPCImpl {
         let mut rw_data = rw_data.lock().unwrap();
         mem::replace(&mut rw_data.pending_images, vec![])
     }
+
+    fn newly_transitioning_nodes(&self) -> Vec<UntrustedNodeAddress> {
+        let &LayoutRPCImpl(ref rw_data) = self;
+        let mut rw_data = rw_data.lock().unwrap();
+        mem::replace(&mut rw_data.newly_transitioning_nodes, vec![])
+    }
 }
 
 struct UnioningFragmentBorderBoxIterator {