4 files changed, 46 insertions, 12 deletions

diff --git a/components/script/dom/bindings/cell.rs b/components/script/dom/bindings/cell.rs
index 9ec04e358e5..dabc88284b6 100644
--- a/components/script/dom/bindings/cell.rs
+++ b/components/script/dom/bindings/cell.rs
@@ -94,6 +94,13 @@ impl<T> DOMRefCell<T> {
             _ => None,
         }
     }
+
+    /// Version of the above that we use during restyle while the script task
+    /// is blocked.
+    pub fn borrow_mut_for_layout(&self) -> RefMut<T> {
+        debug_assert!(task_state::get().is_layout());
+        self.value.borrow_mut()
+    }
 }
 
 impl<T: JSTraceable> JSTraceable for DOMRefCell<T> {
diff --git a/components/script/dom/document.rs b/components/script/dom/document.rs
index b8819fc4c86..f9028e3616e 100644
--- a/components/script/dom/document.rs
+++ b/components/script/dom/document.rs
@@ -32,7 +32,7 @@ use dom::customevent::CustomEvent;
 use dom::documentfragment::DocumentFragment;
 use dom::documenttype::DocumentType;
 use dom::domimplementation::DOMImplementation;
-use dom::element::{Element, ElementCreator};
+use dom::element::{Element, ElementCreator, EventState};
 use dom::event::{Event, EventBubbles, EventCancelable};
 use dom::eventtarget::{EventTarget};
 use dom::htmlanchorelement::HTMLAnchorElement;
@@ -174,6 +174,8 @@ pub struct Document {
     /// This field is set to the document itself for inert documents.
     /// https://html.spec.whatwg.org/multipage/#appropriate-template-contents-owner-document
     appropriate_template_contents_owner_document: MutNullableHeap<JS<Document>>,
+    // The collection of EventStates that have been changed since the last restyle.
+    event_state_changes: DOMRefCell<HashMap<JS<Element>, EventState>>,
 }
 
 impl PartialEq for Document {
@@ -301,6 +303,11 @@ impl Document {
         }
     }
 
+    pub fn needs_reflow(&self) -> bool {
+        self.GetDocumentElement().is_some() &&
+        (self.upcast::<Node>().get_has_dirty_descendants() || !self.event_state_changes.borrow().is_empty())
+    }
+
     /// Returns the first `base` element in the DOM that has an `href` attribute.
     pub fn base_element(&self) -> Option<Root<HTMLBaseElement>> {
         self.base_element.get_rooted()
@@ -1178,6 +1185,7 @@ pub enum DocumentSource {
 #[allow(unsafe_code)]
 pub trait LayoutDocumentHelpers {
     unsafe fn is_html_document_for_layout(&self) -> bool;
+    unsafe fn drain_event_state_changes(&self) -> Vec<(LayoutJS<Element>, EventState)>;
 }
 
 #[allow(unsafe_code)]
@@ -1186,6 +1194,15 @@ impl LayoutDocumentHelpers for LayoutJS<Document> {
     unsafe fn is_html_document_for_layout(&self) -> bool {
         (*self.unsafe_get()).is_html_document
     }
+
+    #[inline]
+    #[allow(unrooted_must_root)]
+    unsafe fn drain_event_state_changes(&self) -> Vec<(LayoutJS<Element>, EventState)> {
+        let mut changes = (*self.unsafe_get()).event_state_changes.borrow_mut_for_layout();
+        let drain = changes.drain();
+        let layout_drain = drain.map(|(k, v)| (k.to_layout(), v));
+        Vec::from_iter(layout_drain)
+    }
 }
 
 impl Document {
@@ -1251,6 +1268,7 @@ impl Document {
             reflow_timeout: Cell::new(None),
             base_element: Default::default(),
             appropriate_template_contents_owner_document: Default::default(),
+            event_state_changes: DOMRefCell::new(HashMap::new()),
         }
     }
 
@@ -1315,6 +1333,20 @@ impl Document {
     pub fn get_element_by_id(&self, id: &Atom) -> Option<Root<Element>> {
         self.idmap.borrow().get(&id).map(|ref elements| (*elements)[0].root())
     }
+
+    pub fn record_event_state_change(&self, el: &Element, which: EventState) {
+        let mut map = self.event_state_changes.borrow_mut();
+        let empty;
+        {
+            let states = map.entry(JS::from_ref(el))
+                            .or_insert(EventState::empty());
+            states.toggle(which);
+            empty = states.is_empty();
+        }
+        if empty {
+            map.remove(&JS::from_ref(el));
+        }
+    }
 }
 
 
diff --git a/components/script/dom/element.rs b/components/script/dom/element.rs
index 1be58932305..0bdca0714a7 100644
--- a/components/script/dom/element.rs
+++ b/components/script/dom/element.rs
@@ -1836,6 +1836,9 @@ impl Element {
 
     fn set_state(&self, which: EventState, value: bool) {
         let mut state = self.event_state.get();
+        if state.contains(which) == value {
+            return
+        }
         match value {
             true => state.insert(which),
             false => state.remove(which),
@@ -1843,7 +1846,7 @@ impl Element {
         self.event_state.set(state);
 
         let node = self.upcast::<Node>();
-        node.dirty(NodeDamage::NodeStyleDamaged);
+        node.owner_doc().record_event_state_change(self, which);
     }
 
     pub fn get_active_state(&self) -> bool {
diff --git a/components/script/dom/window.rs b/components/script/dom/window.rs
index 761ede6edf8..abe693696fd 100644
--- a/components/script/dom/window.rs
+++ b/components/script/dom/window.rs
@@ -957,16 +957,8 @@ impl Window {
     ///
     /// TODO(pcwalton): Only wait for style recalc, since we have off-main-thread layout.
     pub fn reflow(&self, goal: ReflowGoal, query_type: ReflowQueryType, reason: ReflowReason) {
-        let document = self.Document();
-        let root = document.r().GetDocumentElement();
-        let root = match root.r() {
-            Some(root) => root,
-            None => return,
-        };
-
-        let root = root.upcast::<Node>();
-        if query_type == ReflowQueryType::NoQuery && !root.get_has_dirty_descendants() {
-            debug!("root has no dirty descendants; avoiding reflow (reason {:?})", reason);
+        if query_type == ReflowQueryType::NoQuery && !self.Document().needs_reflow() {
+            debug!("Document doesn't need reflow - skipping it (reason {:?})", reason);
             return
         }