src/vs/base/browser/ui/tree/indexTreeModel.ts/IndexTreeModel/setCollapsible
class IndexTreeModel:	setCollapsible(location: number[], collapsible?: boolean): boolean {
		const node = this.getTreeNode(location);

		if (typeof collapsible === 'undefined') {
			collapsible = !node.collapsible;
		}

		const update: CollapsibleStateUpdate = { collapsible };
		return this.eventBufferer.bufferEvents(() => this._setCollapseState(location, update));
	}