如果你的iPad键盘在点击select2输入时隐藏了引导模式，你可以通过在select2输入初始化后添加以下规则来解决这个问题:

if (navigator.userAgent.match(/iPhone|iPad|iPod/i)) {
   var styleEl = document.createElement('style'), styleSheet;
   document.head.appendChild(styleEl);
   styleSheet = styleEl.sheet;
   styleSheet.insertRule(".modal { position:absolute; bottom:auto; }", 0);
   document.body.scrollTop = 0; // Only for Safari
}

摘自https://github.com/angular-ui/bootstrap/issues/1812#issuecomment-135119475

编辑:如果你的选项没有正确显示，你需要在初始化select2时使用dropdownParent属性:

$(".select2").select2({
    dropdownParent: $("#YOURMODALID")
});

祝你好运

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

好吧，我知道我迟到了。但让我和你们分享对我有效的方法。 tabindex和z-index解决方案不适合我。

设置select元素的父元素按照select2站点上列出的常见问题工作。

要使用bootstrap 4.0与服务器端(ajax或json数据内联)，你需要添加所有这些:

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

然后当modal打开时，创建select2:

  // when modal is open
  $('.modal').on('shown.bs.modal', function () {
            $('select').select2({
                  // ....
            });
  });

这个问题解决了工作为我单一Jquery函数

$('#myModal .select2').each(function() {  
   var $p = $(this).parent(); 
   $(this).select2({  
     dropdownParent: $p  
   });  
});