src/vs/workbench/contrib/notebook/browser/contrib/outline/notebookOutline.ts/OutlineEntry/updateMarkers
class OutlineEntry:	updateMarkers(markerService: IMarkerService): void {
		if (this.cell.cellKind === CellKind.Code) {
			// a code cell can have marker
			const marker = markerService.read({ resource: this.cell.uri, severities: MarkerSeverity.Error | MarkerSeverity.Warning });
			if (marker.length === 0) {
				this._markerInfo = undefined;
			} else {
				const topSev = marker.find(a => a.severity === MarkerSeverity.Error)?.severity ?? MarkerSeverity.Warning;
				this._markerInfo = { topSev, count: marker.length };
			}
		} else {
			// a markdown cell can inherit markers from its children
			let topChild: MarkerSeverity | undefined;
			for (const child of this.children) {
				child.updateMarkers(markerService);
				if (child.markerInfo) {
					topChild = !topChild ? child.markerInfo.topSev : Math.max(child.markerInfo.topSev, topChild);
				}
			}
			this._markerInfo = topChild && { topSev: topChild, count: 0 };
		}
	}