Store pristine element state rather than a set of changes.

This is the strategy we'll need to take for attributes, and so this change
puts us in a position to handle attributes and state the same way.

This does mean that we stop taking care to track the situations where our
state has reverted to the original state, with no net change. I think that's
probably of negligible value though.

6 files changed, 23 insertions, 35 deletions

diff --git a/components/layout/layout_task.rs b/components/layout/layout_task.rs
index 7a615b02f6b..bdf1b613562 100644
--- a/components/layout/layout_task.rs
+++ b/components/layout/layout_task.rs
@@ -1213,13 +1213,12 @@ impl LayoutTask {
             }
         }
 
-        let state_changes = document.drain_element_state_changes();
+        let modified_elements = document.drain_modified_elements();
         if !needs_dirtying {
-            for &(el, state_change) in state_changes.iter() {
-                debug_assert!(!state_change.is_empty());
+            for &(el, old_state) in modified_elements.iter() {
                 let hint = rw_data.stylist.restyle_hint_for_state_change(&el,
                                                                          el.get_state(),
-                                                                         state_change);
+                                                                         old_state);
                 el.note_restyle_hint(hint);
             }
         }
diff --git a/components/layout/wrapper.rs b/components/layout/wrapper.rs
index 04d3470fb64..ae8462426e8 100644
--- a/components/layout/wrapper.rs
+++ b/components/layout/wrapper.rs
@@ -375,10 +375,10 @@ impl<'le> LayoutDocument<'le> {
         self.as_node().children().find(LayoutNode::is_element)
     }
 
-    pub fn drain_element_state_changes(&self) -> Vec<(LayoutElement, ElementState)> {
+    pub fn drain_modified_elements(&self) -> Vec<(LayoutElement, ElementState)> {
         unsafe {
-            let changes = self.document.drain_element_state_changes();
-            Vec::from_iter(changes.iter().map(|&(el, state)|
+            let elements = self.document.drain_modified_elements();
+            Vec::from_iter(elements.iter().map(|&(el, state)|
                 (LayoutElement {
                     element: el,
                     chain: PhantomData,
diff --git a/components/script/dom/document.rs b/components/script/dom/document.rs
index 90e5697dd33..161a5639ceb 100644
--- a/components/script/dom/document.rs
+++ b/components/script/dom/document.rs
@@ -175,8 +175,8 @@ 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 ElementStates that have been changed since the last restyle.
-    element_state_changes: DOMRefCell<HashMap<JS<Element>, ElementState>>,
+    /// For each element that has had a state change since the last restyle, track the original state.
+    modified_elements: DOMRefCell<HashMap<JS<Element>, ElementState>>,
     /// http://w3c.github.io/touch-events/#dfn-active-touch-point
     active_touch_points: DOMRefCell<Vec<JS<Touch>>>,
 }
@@ -308,7 +308,7 @@ impl Document {
 
     pub fn needs_reflow(&self) -> bool {
         self.GetDocumentElement().is_some() &&
-        (self.upcast::<Node>().get_has_dirty_descendants() || !self.element_state_changes.borrow().is_empty())
+        (self.upcast::<Node>().get_has_dirty_descendants() || !self.modified_elements.borrow().is_empty())
     }
 
     /// Returns the first `base` element in the DOM that has an `href` attribute.
@@ -1230,7 +1230,7 @@ pub enum DocumentSource {
 #[allow(unsafe_code)]
 pub trait LayoutDocumentHelpers {
     unsafe fn is_html_document_for_layout(&self) -> bool;
-    unsafe fn drain_element_state_changes(&self) -> Vec<(LayoutJS<Element>, ElementState)>;
+    unsafe fn drain_modified_elements(&self) -> Vec<(LayoutJS<Element>, ElementState)>;
 }
 
 #[allow(unsafe_code)]
@@ -1242,9 +1242,9 @@ impl LayoutDocumentHelpers for LayoutJS<Document> {
 
     #[inline]
     #[allow(unrooted_must_root)]
-    unsafe fn drain_element_state_changes(&self) -> Vec<(LayoutJS<Element>, ElementState)> {
-        let mut changes = (*self.unsafe_get()).element_state_changes.borrow_mut_for_layout();
-        let drain = changes.drain();
+    unsafe fn drain_modified_elements(&self) -> Vec<(LayoutJS<Element>, ElementState)> {
+        let mut elements = (*self.unsafe_get()).modified_elements.borrow_mut_for_layout();
+        let drain = elements.drain();
         let layout_drain = drain.map(|(k, v)| (k.to_layout(), v));
         Vec::from_iter(layout_drain)
     }
@@ -1313,7 +1313,7 @@ impl Document {
             reflow_timeout: Cell::new(None),
             base_element: Default::default(),
             appropriate_template_contents_owner_document: Default::default(),
-            element_state_changes: DOMRefCell::new(HashMap::new()),
+            modified_elements: DOMRefCell::new(HashMap::new()),
             active_touch_points: DOMRefCell::new(Vec::new()),
         }
     }
@@ -1380,18 +1380,9 @@ impl Document {
         self.idmap.borrow().get(&id).map(|ref elements| Root::from_ref(&*(*elements)[0]))
     }
 
-    pub fn record_element_state_change(&self, el: &Element, which: ElementState) {
-        let mut map = self.element_state_changes.borrow_mut();
-        let empty;
-        {
-            let states = map.entry(JS::from_ref(el))
-                            .or_insert(ElementState::empty());
-            states.toggle(which);
-            empty = states.is_empty();
-        }
-        if empty {
-            map.remove(&JS::from_ref(el));
-        }
+    pub fn element_state_will_change(&self, el: &Element) {
+        let mut map = self.modified_elements.borrow_mut();
+        map.entry(JS::from_ref(el)).or_insert(el.get_state());
     }
 }
 
diff --git a/components/script/dom/element.rs b/components/script/dom/element.rs
index 7d605ecd133..d420e9b5405 100644
--- a/components/script/dom/element.rs
+++ b/components/script/dom/element.rs
@@ -1814,14 +1814,13 @@ impl Element {
         if state.contains(which) == value {
             return
         }
+        let node = self.upcast::<Node>();
+        node.owner_doc().element_state_will_change(self);
         match value {
             true => state.insert(which),
             false => state.remove(which),
         };
         self.state.set(state);
-
-        let node = self.upcast::<Node>();
-        node.owner_doc().record_element_state_change(self, which);
     }
 
     pub fn get_active_state(&self) -> bool {
diff --git a/components/style/restyle_hints.rs b/components/style/restyle_hints.rs
index fe143c3e315..c180c13dcde 100644
--- a/components/style/restyle_hints.rs
+++ b/components/style/restyle_hints.rs
@@ -195,11 +195,10 @@ impl StateDependencySet {
         StateDependencySet { deps: Vec::new() }
     }
 
-    pub fn compute_hint<E>(&self, el: &E, current_state: ElementState, state_changes: ElementState)
+    pub fn compute_hint<E>(&self, el: &E, current_state: ElementState, old_state: ElementState)
                           -> RestyleHint where E: Element, E: Clone {
         let mut hint = RestyleHint::empty();
-        let mut old_state = current_state;
-        old_state.toggle(state_changes);
+        let state_changes = current_state ^ old_state;
         for dep in &self.deps {
             if state_changes.intersects(dep.state) {
                 let old_el: ElementWrapper<E> = ElementWrapper::new_with_override(el.clone(), old_state);
diff --git a/components/style/selector_matching.rs b/components/style/selector_matching.rs
index 53f1a0d3589..8cc56e693eb 100644
--- a/components/style/selector_matching.rs
+++ b/components/style/selector_matching.rs
@@ -174,10 +174,10 @@ impl Stylist {
 
     pub fn restyle_hint_for_state_change<E>(&self, element: &E,
                                             current_state: ElementState,
-                                            state_change: ElementState)
+                                            old_state: ElementState)
                                             -> RestyleHint
                                             where E: Element + Clone {
-        self.state_deps.compute_hint(element, current_state, state_change)
+        self.state_deps.compute_hint(element, current_state, old_state)
     }
 
     pub fn set_device(&mut self, device: Device) {