Spill item while dropping over other item in nested sortable

[jrief]

I'm attempting to integrate SortableJS for nested items. Inside this nested tree, some of those items may be "closed", hiding their own children. Sometimes I want to move a draggable item over an existing target item and drop it there. It then shall become the last child of that target item and since the latter is closed, become hidden. This is a typical behaviour for say, a directory browser.

I attempted to create such a sortable using

Sortable.create(elem, {
    ...
    invertSwap: true,
    invertedSwapThreshold: 0.2,
    removeOnSpill: true,
});

hoping that dropping over another target item is considered as "spill". This however is not the case, such a dropped item is inserted above or below the target item.

Is there any way to implement this feature?