src/vs/workbench/browser/parts/views/treeView.ts/CustomTreeViewDragAndDrop/getDragLabel
class CustomTreeViewDragAndDrop:	getDragLabel?(elements: ITreeItem[]): string | undefined {
		if (elements.length > 1) {
			return String(elements.length);
		}
		const element = elements[0];
		return element.label ? element.label.label : (element.resourceUri ? this.labelService.getUriLabel(URI.revive(element.resourceUri)) : undefined);
	}