src/vs/base/browser/ui/tree/abstractTree.ts/AbstractTree/setAnchor
class AbstractTree:	setAnchor(element: TRef | undefined): void {
		if (typeof element === 'undefined') {
			return this.view.setAnchor(undefined);
		}

		const node = this.model.getNode(element);
		this.anchor.set([node]);

		const index = this.model.getListIndex(element);

		if (index > -1) {
			this.view.setAnchor(index, true);
		}
	}