fix(tabs): properly detect text changes

* Currently the MutationObserver is watching for attribute changes and subtree modifications.
  It also watches for text content changes, but that's not working properly for span elements with `<md-tab-label>` elements.
  See https://bugzilla.mozilla.org/show_bug.cgi?id=1138368

The MutationObserver should watch for `characterData` to properly detect text changes.

Fixes #8667.

Closes #8803

src/components/tabs/js/tabsDummyWrapperDirective.js

@@ -21,7 +21,15 @@ function MdTabsDummyWrapper ($mdUtil) {
         ctrl.updatePagination();
         ctrl.updateInkBarStyles();
       });
-      var config = { childList: true, subtree: true };
+
+      var config = {
+        childList: true,
+        subtree: true,
+        // Per https://bugzilla.mozilla.org/show_bug.cgi?id=1138368, browsers will not fire
+        // the childList mutation, once a <span> element's innerText changes.
+        // The characterData of the <span> element will change.
+        characterData: true
+      };
 
       observer.observe(element[0], config);
 

src/components/tabs/tabs.spec.js

@@ -233,7 +233,7 @@ describe('<md-tabs>', function () {
     });
 
     it('updates pagination and ink styles when string labels change', function(done) {
-      inject(function($rootScope) {
+      inject(function($rootScope, $timeout) {
         // Setup our initial label
         $rootScope.$apply('label = "Some Label"');
 
@@ -242,21 +242,72 @@ describe('<md-tabs>', function () {
         var tabs = setup(template);
         var ctrl = tabs.controller('mdTabs');
 
-        // Setup spies
-        spyOn(ctrl, 'updatePagination');
-        spyOn(ctrl, 'updateInkBarStyles');
-
-        // Change the label
-        $rootScope.$apply('label="Another Label"');
+        // Flush the tabs controller timeout for initialization.
+        $timeout.flush();
 
-        // Use window.setTimeout to add our expectations to the end of the call stack, after the
+        // After the first timeout the mutation observer should have been fired once, because
+        // the initialization of the dummy tabs, already causes some mutations.
+        // Use setTimeout to add our expectations to the end of the call stack, after the
         // MutationObservers have already fired
-        window.setTimeout(function() {
-          // Fire expectations
-          expect(ctrl.updatePagination.calls.count()).toBe(1);
-          expect(ctrl.updateInkBarStyles.calls.count()).toBe(1);
+        setTimeout(function() {
+          // Setup spies
+          spyOn(ctrl, 'updatePagination');
+          spyOn(ctrl, 'updateInkBarStyles');
+
+          // Update the label to trigger a new update of the pagination and InkBar styles.
+          $rootScope.$apply('label = "Another Label"');
+
+          // Use setTimeout to add our expectations to the end of the call stack, after the
+          // MutationObservers have already fired
+          setTimeout(function() {
+            expect(ctrl.updatePagination).toHaveBeenCalledTimes(1);
+            expect(ctrl.updateInkBarStyles).toHaveBeenCalledTimes(1);
+
+            done();
+          });
+        });
+      })
+    });
 
-          done();
+    it('updates pagination and ink styles when content label changes', function(done) {
+      inject(function($rootScope, $timeout) {
+        // Setup our initial label
+        $rootScope.$apply('label = "Default Label"');
+
+        // Init our variables
+        var template = '' +
+          '<md-tabs>' +
+            '<md-tab>' +
+              '<md-tab-label>{{ label }}</md-tab-label>' +
+            '</md-tab>' +
+          '</md-tabs>';
+
+        var tabs = setup(template);
+        var ctrl = tabs.controller('mdTabs');
+
+        // Flush the tabs controller timeout for initialization.
+        $timeout.flush();
+
+        // After the first timeout the mutation observer should have been fired once, because
+        // the initialization of the dummy tabs, already causes some mutations.
+        // Use setTimeout to add our expectations to the end of the call stack, after the
+        // MutationObservers have already fired
+        setTimeout(function() {
+          // Setup spies
+          spyOn(ctrl, 'updatePagination');
+          spyOn(ctrl, 'updateInkBarStyles');
+
+          // Update the label to trigger a new update of the pagination and InkBar styles.
+          $rootScope.$apply('label = "New Label"');
+
+          // Use setTimeout to add our expectations to the end of the call stack, after the
+          // MutationObservers have already fired
+          setTimeout(function() {
+            expect(ctrl.updatePagination).toHaveBeenCalledTimes(1);
+            expect(ctrl.updateInkBarStyles).toHaveBeenCalledTimes(1);
+
+            done();
+          });
         });
       })
     });