Xinxin
Use hook prop `eachDroppable`, calc dragging node's depth and hoveing node's depth, return boolean to control if droppable.
I recommend using [he-tree](https://hetree.phphe.com/). There is a prop for that: [maxLevel](https://hetree.phphe.com/v2/api#maxlevel)
`@open:node="openTrigger"`
use css make it height 0
Not enough information.
set min-height with css for tree, such as 100px.