Create a sequential focus ordering list

This list will hold elements that have their SEQUENTIALLY_FOCUSABLE
flag set. The elements are added to the list in tree order.

components/script/dom/document.rs

@@ -238,6 +238,8 @@ pub struct Document {
     dom_complete: Cell<u64>,
     load_event_start: Cell<u64>,
     load_event_end: Cell<u64>,
+    /// Vector to store focusable elements
+    focus_list: DOMRefCell<Vec<JS<Element>>>,
     /// https://html.spec.whatwg.org/multipage/#concept-document-https-state
     https_state: Cell<HttpsState>,
     touchpad_pressure_phase: Cell<TouchpadPressurePhase>,
@@ -496,6 +498,11 @@ impl Document {
         }
     }
 
+    /// Add a focusable element to this document's ordering focus list.
+    pub fn add_focusable_element(&self, element: &Element) {
+        self.focus_list.borrow_mut().push(JS::from_ref(element));
+    }
+
     /// Associate an element present in this document with the provided id.
     pub fn register_named_element(&self, element: &Element, id: Atom) {
         debug!("Adding named element to document {:p}: {:p} id={}",
@@ -1799,6 +1806,7 @@ impl Document {
             dom_complete: Cell::new(Default::default()),
             load_event_start: Cell::new(Default::default()),
             load_event_end: Cell::new(Default::default()),
+            focus_list: DOMRefCell::new(vec![]),
             https_state: Cell::new(HttpsState::None),
             touchpad_pressure_phase: Cell::new(TouchpadPressurePhase::BeforeClick),
             origin: origin,

components/script/dom/element.rs

@@ -2203,6 +2203,19 @@ impl VirtualMethods for Element {
             return;
         }
 
+        if self.is_focusable_area() {
+            let doc = document_from_node(self);
+            let children = self.upcast::<Node>().traverse_preorder();
+
+            for child in children {
+                if let Some(element) = child.downcast::<Element>() {
+                    if element.is_focusable_area() && !element.disabled_state() {
+                        doc.add_focusable_element(element);
+                    }
+                }
+            }
+        }
+
         if let Some(ref value) = *self.id_attribute.borrow() {
             let doc = document_from_node(self);
             doc.register_named_element(self, value.clone());