extensions/references-view/src/tree.ts/TreeDndDelegate/handleDrag
class TreeDndDelegate:	handleDrag(source: undefined[], data: vscode.DataTransfer) {
		if (this._delegate) {
			const urls: string[] = [];
			for (const item of source) {
				const uri = this._delegate.getDragUri(item);
				if (uri) {
					urls.push(uri.toString());
				}
			}
			if (urls.length > 0) {
				data.set('text/uri-list', new vscode.DataTransferItem(urls.join('\r\n')));
			}
		}
	}