Question unity treeview expanding and collapsing manually.

You could register a ClickEvent on your items to detect the click and do the expand/collapse from there.
Here's some documentation on how to do it: https://docs.unity3d.com/Manual/UIE-Click-Events.html
It could also be done just on a PointerDownEvent or PointerUpEvent.

It could look something like this (not tested).

Code (CSharp):

1. treeView.makeItem = () =>
2. {
3.     var element = new MyElement();
4.     element.RegisterCallback<ClickEvent>(OnItemClicked);
5.     return element;
6. };
7. treeView.bindItem = (element, i) =>
8. {
9.     ((MyElement)element).index = i;
10. };
11.  
12. void OnItemClicked(ClickEvent evt)
13. {
14.     var element = evt.target as MyElement;
15.     treeView.viewController.ExpandItemByIndex(element.index, false);
16. }