PR for -> autofocus isn't supported #3696 (#5356)

* Update core.js

* Update core.js

* Update core.js

src/js/select2/core.js

@@ -276,6 +276,23 @@ define([
 
   Select2.prototype._registerEvents = function () {
     var self = this;
+    
+    this.on('focus', function () {
+      self.$container.addClass('select2-container--focus');
+
+      if (!self.$container.hasClass('select2-container--disabled') &&
+          !self.isOpen()) {
+        if (self.options.get('multiple')) {
+          window.setTimeout(function () {
+            self.open();
+          },
+          self.options.get('ajax') ? 300 : 100);
+        }
+        else {
+          self.open();
+        }
+      }
+    });
 
     this.on('open', function () {
       self.$container.addClass('select2-container--open');