src/vs/workbench/contrib/markers/browser/markersTreeViewer.ts/ResourceMarkersRenderer/updateCount
class ResourceMarkersRenderer:	private updateCount(node: ITreeNode<ResourceMarkers, ResourceMarkersFilterData>, templateData: IResourceMarkersTemplateData): void {
		templateData.count.setCount(node.children.reduce((r, n) => r + (n.visible ? 1 : 0), 0));
	}