Auto merge of #12563 - emilio:stylo, r=bholley,jdm,pcwalton

stylo: Improve restyling performance

This commit adds hooks to the Servo style traversal to avoid traversing all the
DOM for every restyle. Additionally it changes the behavior of the dirty flag to
be propagated top down, to prevent extra overhead when an element is dirtied.

This commit doesn't aim to change the behavior on Servo just yet, since Servo does extra job when dirtying the node related with DOM revision counters that might be necessary.

CC @asajeffrey for the DOM revision counters stuff. When a node is dirty, do all its descendants really need to increment the revision counter, or is this an unintended effect? My intuition is that this is hurting performance quite a lot for servo.

r? @bholley

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components/layout_thread/lib.rs

@@ -1105,12 +1105,16 @@ impl LayoutThread {
                        .unwrap();
             }
             if data.document_stylesheets.iter().any(|sheet| sheet.dirty_on_viewport_size_change) {
-                for node in node.traverse_preorder() {
+                let mut iter = node.traverse_preorder();
+
+                let mut next = iter.next();
+                while let Some(node) = next {
                     if node.needs_dirty_on_viewport_size_changed() {
-                        node.dirty_self();
-                        node.dirty_descendants();
-                        // TODO(shinglyu): We can skip the traversal if the descendants were already
-                        // dirtied
+                        // NB: The dirty bit is propagated down the tree.
+                        unsafe { node.set_dirty(true); }
+                        next = iter.next_skipping_children();
+                    } else {
+                        next = iter.next();
                     }
                 }
             }
@@ -1122,7 +1126,9 @@ impl LayoutThread {
         let needs_reflow = viewport_size_changed && !needs_dirtying;
         unsafe {
             if needs_dirtying {
-                LayoutThread::dirty_all_nodes(node);
+                // NB: The dirty flag is propagated down during the restyle
+                // process.
+                node.set_dirty(true);
             }
         }
         if needs_reflow {
@@ -1476,16 +1482,6 @@ impl LayoutThread {
         }
     }
 
-    unsafe fn dirty_all_nodes<N: LayoutNode>(node: N) {
-        for node in node.traverse_preorder() {
-            // TODO(cgaebel): mark nodes which are sensitive to media queries as
-            // "changed":
-            // > node.set_changed(true);
-            node.set_dirty(true);
-            node.set_dirty_descendants(true);
-        }
-    }
-
     fn reflow_all_nodes(flow: &mut Flow) {
         debug!("reflowing all nodes!");
         flow::mut_base(flow).restyle_damage.insert(REPAINT | STORE_OVERFLOW | REFLOW);