fix($animate): make sure to run a post-digest reflow for parentless animations

Closes #12400

Author: matsko

src/ngAnimate/animation.js

@@ -106,7 +106,16 @@ var $$AnimationProvider = ['$animateProvider', function($animateProvider) {
           result = result.concat(row);
         }
 
-        return result;
+        var terminalAnimations = [];
+        var parentAnimations = [];
+        forEach(result, function(result) {
+          if (result.terminal) {
+            terminalAnimations.push(result);
+          } else {
+            parentAnimations.push(result);
+          }
+        });
+        return [parentAnimations, terminalAnimations];
       }
     }
 
@@ -220,10 +229,22 @@ var $$AnimationProvider = ['$animateProvider', function($animateProvider) {
         var anim = sortAnimations(toBeSortedAnimations);
         var finalLevel = anim.length - 1;
 
-        forEach(anim, function(entry) {
-          if (!entry.terminal) {
+        // sortAnimations will return two lists of animations. The first list
+        // is all of the parent animations that are likely class-based and the
+        // second list is a collection of the rest. Before we run the second
+        // list we must ensure that atleast one reflow has been passed such that
+        // the preparation classes (ng-enter, class-add, etc...) have been applied
+        // to their associated element.
+        if (anim[0].length) {
+          forEach(anim[0], function(entry) {
             $$forceReflow();
-          }
+            entry.fn();
+          });
+        } else {
+          $$forceReflow();
+        }
+
+        forEach(anim[1], function(entry) {
           entry.fn();
         });
       });

test/ngAnimate/integrationSpec.js

@@ -335,6 +335,30 @@ describe('ngAnimate integration tests', function() {
         expect($animate.reflows).toBe(0);
       });
     });
+
+    iit('should always issue atleast one reflow incase there are no parent class-based animations', function() {
+      module('ngAnimateMock');
+      inject(function($animate, $compile, $rootScope, $rootElement, $$rAF, $document) {
+        element = jqLite(
+          '<div ng-repeat="item in items" ng-class="{someAnimation:exp}">' +
+            '{{ item }}' +
+          '</div>'
+        );
+
+        $rootElement.append(element);
+        jqLite($document[0].body).append($rootElement);
+
+        $compile(element)($rootScope);
+        $rootScope.$digest();
+        expect($animate.reflows).toBe(0);
+
+        $rootScope.exp = true;
+        $rootScope.items = [1,2,3,4,5,6,7,8,9,10];
+        $rootScope.$digest();
+
+        expect($animate.reflows).toBe(1);
+      });
+    });
   });
 
   describe('JS animations', function() {