Introduce "navigation target classification" for spatial navigation

Source/core/page/FocusController.cpp

 /*
  * Copyright (C) 2006, 2007 Apple Inc. All rights reserved.
  * Copyright (C) 2008 Nuanti Ltd.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
@@ skipping 769 matching lines @@
     if (candidate.alignment == closest.alignment) {
         if (candidate.distance < closest.distance)
             closest = candidate;
         return;
     }
 
     if (candidate.alignment > closest.alignment)
         closest = candidate;
 }
 
+static inline bool hasFocusEventHandler(const Element& element)
+{
+    return element.hasEventListeners(EventTypeNames::focus)
+        || element.hasEventListeners(EventTypeNames::blur)
+        || element.hasEventListeners(EventTypeNames::focusin)
+        || element.hasEventListeners(EventTypeNames::focusout);
+}
+
+static inline NavigationClass classifyNavigationCandidate(const Element& element, FocusType type)
+{
+    if (element.isKeyboardFocusable())
+        return NavigationClassFocusable;
+
+    if (element.isFrameOwnerElement())
+        return NavigationClassFrameOwner;
+
+    if (canScrollInDirection(&element, type))
+        return NavigationClassScrollable;
+
+    // If the element is not "keyboard focusable", apply an extended heuristic,
+    // by checking for the presence of a click/keyboard/focus/blur event
+    // handler.
+    // This way it's possible to navigate to (focus) elements which the web
+    // designer intended to be active (made them respond to mentioned events).
+
+    if (element.hasEventListeners(EventTypeNames::click)
+        || element.hasEventListeners(EventTypeNames::keydown)
+        || element.hasEventListeners(EventTypeNames::keypress)
+        || element.hasEventListeners(EventTypeNames::keyup))
+        return NavigationClassEventHandler;
+
+    if (element.isSVGElement() && hasFocusEventHandler(element))

[c.shu, 2015/01/07 23:49:48]

I would either expand the four focus event listeners here or use a helper
function for the above click/key events as well, just to make it consistent.

+        return NavigationClassEventHandler;
+
+    return NavigationClassNone;
+}
+
 void FocusController::findFocusCandidateInContainer(Node& container, const LayoutRect& startingRect, FocusType type, FocusCandidate& closest)
 {
     Element* focusedElement = (focusedFrame() && toLocalFrame(focusedFrame())->document()) ? toLocalFrame(focusedFrame())->document()->focusedElement() : nullptr;
 
     Element* element = ElementTraversal::firstWithin(container);
     FocusCandidate current;
     current.rect = startingRect;
     current.focusableNode = focusedElement;
     current.visibleNode = focusedElement;
+    if (focusedElement) {
+        // FIXME: Result the computed class depends on the focus type.
+        current.targetClass = classifyNavigationCandidate(*focusedElement, type);
+    }
 
     for (; element; element = (element->isFrameOwnerElement() || canScrollInDirection(element, type))
         ? ElementTraversal::nextSkippingChildren(*element, &container)
         : ElementTraversal::next(*element, &container)) {
         if (element == focusedElement)
             continue;
 
-        if (!element->isKeyboardFocusable() && !element->isFrameOwnerElement() && !canScrollInDirection(element, type))
+        NavigationClass navigationClass = classifyNavigationCandidate(*element, type);
+        if (navigationClass == NavigationClassNone)
             continue;
 
-        FocusCandidate candidate = FocusCandidate(element, type);
+        FocusCandidate candidate(element, type, navigationClass);

[c.shu, 2015/01/07 23:49:48]

Can we do some additional code refactory here so that there's only one place to
call classifyNavigationCandidate()? I hope it's possible.

         if (candidate.isNull())
             continue;
 
         candidate.enclosingScrollableBox = &container;
         updateFocusCandidateIfNeeded(type, current, candidate, closest);
     }
 }
 
 bool FocusController::advanceFocusDirectionallyInContainer(Node* container, const LayoutRect& startingRect, FocusType type)
 {
@@ skipping 108 matching lines @@
     return consumed;
 }
 
 void FocusController::trace(Visitor* visitor)
 {
     visitor->trace(m_page);
     visitor->trace(m_focusedFrame);
 }
 
 } // namespace blink