在我的情况下，我这样做，它工作。我使用bundle来加载它，在这种情况下，它为我工作

 $(document).on('select2:select', '#Id_Producto', function (evt) {
   // Here your code...
  });

为了更好地理解tabindex元素是如何工作的，完成公认的回答:

The tabindex global attribute is an integer indicating if the element can take input focus (is focusable), if it should participate to sequential keyboard navigation, and if so, at what position. It can take several values:   -a negative value means that the element should be focusable, but should not be reachable via sequential keyboard navigation;   -0 means that the element should be focusable and reachable via sequential keyboard navigation, but its relative order is defined by the platform convention;   -a positive value means should be focusable and reachable via sequential keyboard navigation; its relative order is defined by the value of the attribute: the sequential follow the increasing number of the tabindex. If several elements share the same tabindex, their relative order follows their relative position in the document.

来自:Mozilla开发人员网络

我在github上找到了select2的解决方案

https://github.com/ivaynberg/select2/issues/1436

对于bootstrap 3，解决方案是:

$.fn.modal.Constructor.prototype.enforceFocus = function() {};

Bootstrap 4重命名了enforceFocus方法为_enforceFocus，所以你需要修补它:

$.fn.modal.Constructor.prototype._enforceFocus = function() {};

从以上链接复制的解释:

Bootstrap向focusin事件注册了一个监听器，它检查被聚焦的元素是覆盖本身还是它的后代——如果不是，它只是重新聚焦在覆盖上。随着select2下拉框被附加到正文，这有效地防止你在文本框中输入任何东西。 您可以通过覆盖在模态上注册事件的enforceFocus函数来快速修复这个问题

我以前遇到过这个问题，我用yii2解决了它

$.fn.modal.Constructor.prototype.enforceFocus = $.noop;

基于@pymarco的答案，我写了这个解决方案，它不是完美的，但解决了select2焦点问题，并保持制表符序列在模态内工作

    $.fn.modal.Constructor.prototype.enforceFocus = function () {
        $(document)
        .off('focusin.bs.modal') // guard against infinite focus loop
        .on('focusin.bs.modal', $.proxy(function (e) {
            if (this.$element[0] !== e.target && !this.$element.has(e.target).length && !$(e.target).closest('.select2-dropdown').length) {
                this.$element.trigger('focus')
            }
        }, this))
    }

你不应该像大多数答案中建议的那样，将下拉框固定在情态动词上。 如果你的select2在模态的底部，你需要向下滚动来找到它，那么你就会有一个下拉的位置问题，因为你把它粘在了模态上。

相反，您应该将下拉父目录设置为输入父目录。在你的例子中，它应该是这样的

$(document).ready(function() {
  $("#vdn_number").select2({
    dropdownParent: $("#vdn_number").parent()
  });
});