Use children changed events to monitor active descendant changes.

chrome/browser/resources/chromeos/chromevox/cvox2/background/desktop_automation_handler.js

Index: chrome/browser/resources/chromeos/chromevox/cvox2/background/desktop_automation_handler.js
diff --git a/chrome/browser/resources/chromeos/chromevox/cvox2/background/desktop_automation_handler.js b/chrome/browser/resources/chromeos/chromevox/cvox2/background/desktop_automation_handler.js
index bafccde71b42f6697fd976cb259b42ee6912db70..0b1380b8a34ebc8876addab98f770a5f94da5093 100644
--- a/chrome/browser/resources/chromeos/chromevox/cvox2/background/desktop_automation_handler.js
+++ b/chrome/browser/resources/chromeos/chromevox/cvox2/background/desktop_automation_handler.js
@@ -47,6 +47,7 @@ DesktopAutomationHandler = function(node) {
   this.addListener_(e.alert, this.onAlert);
   this.addListener_(e.ariaAttributeChanged, this.onEventIfInRange);
   this.addListener_(e.checkedStateChanged, this.onCheckedStateChanged);
+  this.addListener_(e.childrenChanged, this.onActiveDescendantChanged);
   this.addListener_(e.focus, this.onFocus);
   this.addListener_(e.hover, this.onHover);
   this.addListener_(e.loadComplete, this.onLoadComplete);
@@ -178,7 +179,7 @@ DesktopAutomationHandler.prototype = {
    * @param {!AutomationEvent} evt
    */
   onActiveDescendantChanged: function(evt) {
-    if (!evt.target.activeDescendant)
+    if (!evt.target.activeDescendant || !evt.target.state.focused)
       return;
     this.onEventDefault(new chrome.automation.AutomationEvent(
         EventType.focus, evt.target.activeDescendant));