$(document).on("focus", ".select2", function (e) {
    if (e.originalEvent) {
      var s2element = $(this).siblings("select:enabled");
      s2element.select2("open");
      // Set focus back to select2 element on closing.
      s2element.on("select2:closing", function () {
        if (s2element.val()) s2element.select2("focus");
      });
    }
  });

如果出现多选选择异常的话
补充一下：

$(document).on("focus", ".select2", function (e) {
    if (e.originalEvent) {
      var s2element = $(this).siblings("select:enabled");
      s2element.select2("open");
      // Set focus back to select2 element on closing.
      s2element.on("select2:closing", function () {
        if (s2element.val()) s2element.select2("focus");
      });
    }
  });