Bug: 分组内插件拖动排序位置偏移

复现方法：新建一个插件组，新增几个插件，拖动一个到另外两个插件之间，松开后插件没有移动到那两个插件之间，而是移动到更上方

boilerplate 自带的插件组内拖动是没有问题的，观察到新增的插件组发现多了一个 $label，猜测是这个导致的

Reproduce the issue by creating a plugin group, adding several plugins, and dragging one between two others. Compare the behavior with the boilerplate group and inspect the added group's `$label`; done means the dragged plugin lands between the two selected plugins.

Written by the indexing model from the issue text.