MDL-50304 atto/plugins/table: Fixed _addRowAfter function

lib/editor/atto/plugins/table/yui/build/moodle-atto_table-button/moodle-atto_table-button-debug.js

@@ -950,13 +950,11 @@ Y.namespace('M.atto_table').Button = Y.Base.create('button', Y.M.editor_atto.Edi
      * @private
      */
     _addRowAfter: function() {
-        var rowindex = this._getRowIndex(this._lastTarget);
-
-        var tablebody = this._lastTarget.ancestor('table').one('tbody');
+        var target = this._lastTarget.ancestor('tr'),
+            tablebody = this._lastTarget.ancestor('table').one('tbody');
         if (!tablebody) {
             // Not all tables have tbody.
             tablebody = this._lastTarget.ancestor('table');
-            rowindex += 1;
         }
 
         var firstrow = tablebody.one('tr');
@@ -979,7 +977,12 @@ Y.namespace('M.atto_table').Button = Y.Base.create('button', Y.M.editor_atto.Edi
             tablecell.setHTML(' ');
         });
 
-        tablebody.insert(newrow, rowindex);
+        if (target.ancestor('thead')) {
+            target = firstrow;
+            tablebody.insert(newrow, target);
+        } else {
+            target.insert(newrow, 'after');
+        }
 
         // Clean the HTML.
         this.markUpdated();