src/vs/workbench/api/common/extHostTreeViews.ts/ExtHostTreeViews/addAdditionalTransferItems
class ExtHostTreeViews:	private async addAdditionalTransferItems(treeDataTransfer: vscode.DataTransfer, treeView: ExtHostTreeView<any>,
		sourceTreeItemHandles: string[], token: CancellationToken, operationUuid?: string): Promise<vscode.DataTransfer | undefined> {
		const existingTransferOperation = this.treeDragAndDropService.removeDragOperationTransfer(operationUuid);
		if (existingTransferOperation) {
			(await existingTransferOperation)?.forEach((value, key) => {
				if (value) {
					treeDataTransfer.set(key, value);
				}
			});
		} else if (operationUuid && treeView.handleDrag) {
			const willDropPromise = treeView.handleDrag(sourceTreeItemHandles, treeDataTransfer, token);
			this.treeDragAndDropService.addDragOperationTransfer(operationUuid, willDropPromise);
			await willDropPromise;
		}
		return treeDataTransfer;
	}