How do I get vss sdk treeview to collapse by default?

How can I get a combo control of type TreeView.ComboTreeBehaviorName to have its nodes collapsed by default?

I've tried looking at the documentation here and here, but it seems completely unhelpful to me.

I've also tried inspecting the types (in vss.d.ts) to see if there's a property I can set:

• I found maxAutoExpandDropWidth which claims that it Specifies the max size when auto-expand drop bigger than combo, but setting it to 0 or 1 doesn't seem to have any effect.
• I found dropOptions?: IComboDropOptions; but it doesn't seem to have any properties for expand/collapse either.
• I tried adding a property collapsed: true to the IComboOptions
• The signature of the Controls.Enhancement.enhance method seems to suggest that I can pass some options as part of the type property on the IComboOptions, but I tried that and couldn't do it because the type property wants a string, not an object.
• Setting node.expanded = false for all nodes before passing the tree to the source property

Here are some relevant code excerpts:

import * as Controls_Combos from "VSS/Controls/Combos";
import * as Controls from "VSS/Controls";
import * as TreeView from "VSS/Controls/TreeView";



 this._$areaInput = $("<input type='text' id='inputAreaPicker' />")
                .val(someValueThatDoesnotMatter)
                .bind("blur", (e) => {
                    this._updateSomeOtherField();
                    this._validate();
                });
}

...

        <Controls_Combos.Combo>Controls.Enhancement.enhance(
            Controls_Combos.Combo,
            this._$areaInput,
            <Controls_Combos.IComboOptions> {
                type: TreeView.ComboTreeBehaviorName,
                source: ConvertToTreeNodes(someItems),  // loads multi-level tree successfully
                mode: 'drop',
                allowEdit: false,
                maxAutoExpandDropWidth: 1, // seems to have no effect
                collapsed: true  // no effect
            }
        );


export function ConvertToTreeNodes(items): TreeView.TreeNode[] {
    // let _this = this;
    return $.map(items, function (item) {
        let node = new TreeView.TreeNode(item.name);
        node.id = item.id;
        if (item.children && item.children.length > 0) {
            node.addRange(ConvertToTreeNodes(item.children));
        }
        node.expanded = false;
        return node;
    });
}

The answer is an undocumented property called 'treeLevel':

<Controls_Combos.Combo>Controls.Enhancement.enhance(
            Controls_Combos.Combo,
            this._$areaInput,
            <Controls_Combos.IComboOptions> {
                type: TreeView.ComboTreeBehaviorName,
                source: ConvertToTreeNodes(someItems),
                mode: 'drop',
                allowEdit: false,
                treeLevel: 0 // collapse to first level by default

            }
        );

I've requested that it be documented