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

		const additionalDataTransfer = new VSDataTransfer();
		additionalDataTransferDTO.items.forEach(([type, item]) => {
			additionalDataTransfer.replace(type, createStringDataTransferItem(item.asString));
		});
		return additionalDataTransfer;
	}