fix(fabric.Object) ISSUE-6340 infinite recursion on groups (#6416)

src/mixins/stateful.mixin.js

@@ -11,6 +11,7 @@
     props.forEach(function(prop) {
       tmpObj[prop] = origin[prop];
     });
+
     extend(origin[destination], tmpObj, deep);
   }
 
@@ -42,7 +43,8 @@
         key = keys[i];
         // since clipPath is in the statefull cache list and the clipPath objects
         // would be iterated as an object, this would lead to possible infinite recursion
-        if (key === 'canvas') {
+        // we do not want to compare those.
+        if (key === 'canvas' || key === 'group') {
           continue;
         }
         if (!_isEqual(origValue[key], currentValue[key])) {

src/util/lang_object.js

@@ -4,7 +4,7 @@
    * this does not and cannot compete with generic utils.
    * Does not clone or extend fabric.Object subclasses.
    * This is mostly for internal use and has extra handling for fabricJS objects
-   * it skips the canvas property in deep cloning.
+   * it skips the canvas and group properties in deep cloning.
    * @memberOf fabric.util.object
    * @param {Object} destination Where to copy to
    * @param {Object} source Where to copy from
@@ -28,8 +28,10 @@
       }
       else if (source && typeof source === 'object') {
         for (var property in source) {
-          if (property === 'canvas') {
-            destination[property] = extend({ }, source[property]);
+          if (property === 'canvas' || property === 'group') {
+            // we do not want to clone this props at all.
+            // we want to keep the keys in the copy
+            destination[property] = null;
           }
           else if (source.hasOwnProperty(property)) {
             destination[property] = extend({ }, source[property], deep);