fix(nav-bar): clicked nav item not focused or selected in some cases

reduce unnecessary calls to onFocus() and setFocused()
- in some cases, instead of focusing a nav item that was clicked on
  we would focus the selected nav item

Fixes #11747

Author: Splaktar

src/components/navBar/navBar.js

@@ -174,7 +174,7 @@ MdNavBarController.prototype._initTabs = function() {
 
   var self = this;
   this._$timeout(function() {
-    self._updateTabs(self.mdSelectedNavItem, undefined);
+    self._updateTabs(self.mdSelectedNavItem, null);
   });
 
   this._$scope.$watch('ctrl.mdSelectedNavItem', function(newValue, oldValue) {
@@ -189,7 +189,7 @@ MdNavBarController.prototype._initTabs = function() {
 /**
  * Set the current tab to be selected.
  * @param {string|undefined} newValue New current tab name.
- * @param {string|undefined} oldValue Previous tab name.
+ * @param {string|undefined|null} oldValue Previous tab name.
  * @private
  */
 MdNavBarController.prototype._updateTabs = function(newValue, oldValue) {
@@ -335,7 +335,7 @@ MdNavBarController.prototype._findTabReverse = function(fn, startIndex) {
  */
 MdNavBarController.prototype.onFocus = function() {
   var tab = this._getSelectedTab();
-  if (tab) {
+  if (tab && !tab._focused) {
     tab.setFocused(true);
   }
 };
@@ -595,9 +595,7 @@ function MdNavItem($mdAria, $$rAF, $mdUtil, $window) {
         });
 
         navButton.on('focus', function() {
-          if (!mdNavBar.getFocusedTab()) {
-            mdNavBar.onFocus();
-          }
+          mdNavItem._focused = true;
         });
 
         navButton.on('click', function() {