src/vs/base/browser/ui/tree/compressedObjectTreeModel.ts/CompressibleObjectTreeModel/onDidChangeRenderNodeCount
class CompressibleObjectTreeModel:	get onDidChangeRenderNodeCount(): Event<ITreeNode<T | null, TFilterData>> {
		return Event.map(this.model.onDidChangeRenderNodeCount, node => this.nodeMapper.map(node));
	}