src/vs/workbench/api/browser/mainThreadTreeViews.ts/TreeViewDragAndDropController/handleDrag
class TreeViewDragAndDropController:	async handleDrag(sourceTreeItemHandles: string[], operationUuid: string, token: CancellationToken): Promise<IDataTransfer | undefined> {
		if (!this.hasWillDrop) {
			return;
		}
		const additionalTransferItems = await this._proxy.$handleDrag(this.treeViewId, sourceTreeItemHandles, operationUuid, token);
		if (!additionalTransferItems) {
			return;
		}
		return DataTransferConverter.toDataTransfer(additionalTransferItems);
	}