src/vs/base/browser/ui/list/listWidget.ts/List/setAnchor
class List:	setAnchor(index: number | undefined): void {
		if (typeof index === 'undefined') {
			this.anchor.set([]);
			return;
		}

		if (index < 0 || index >= this.length) {
			throw new ListError(this.user, `Invalid index ${index}`);
		}

		this.anchor.set([index]);
	}