src/vs/base/browser/ui/tree/abstractTree.ts/TreeNodeList/setAnchor
class TreeNodeList:	override setAnchor(index: number | undefined, fromAPI = false): void {
		super.setAnchor(index);

		if (!fromAPI) {
			if (typeof index === 'undefined') {
				this.anchorTrait.set([]);
			} else {
				this.anchorTrait.set([this.element(index)]);
			}
		}
	}