Allow tree drop at root

Fixes microsoft#145904

Author: alexr00

src/vs/workbench/api/browser/mainThreadTreeViews.ts

@@ -175,9 +175,9 @@ class TreeViewDragAndDropController implements ITreeViewDragAndDropController {
 		readonly hasWillDrop: boolean,
 		private readonly _proxy: ExtHostTreeViewsShape) { }
 
-	async handleDrop(dataTransfer: IDataTransfer, targetTreeItem: ITreeItem, token: CancellationToken,
+	async handleDrop(dataTransfer: IDataTransfer, targetTreeItem: ITreeItem | undefined, token: CancellationToken,
 		operationUuid?: string, sourceTreeId?: string, sourceTreeItemHandles?: string[]): Promise<void> {
-		return this._proxy.$handleDrop(this.treeViewId, await DataTransferConverter.toDataTransferDTO(dataTransfer), targetTreeItem.handle, token, operationUuid, sourceTreeId, sourceTreeItemHandles);
+		return this._proxy.$handleDrop(this.treeViewId, await DataTransferConverter.toDataTransferDTO(dataTransfer), targetTreeItem?.handle, token, operationUuid, sourceTreeId, sourceTreeItemHandles);
 	}
 
 	async handleDrag(sourceTreeItemHandles: string[], operationUuid: string, token: CancellationToken): Promise<IDataTransfer | undefined> {

src/vs/workbench/api/common/extHost.protocol.ts

@@ -1330,7 +1330,7 @@ export interface ExtHostDocumentsAndEditorsShape {
 
 export interface ExtHostTreeViewsShape {
 	$getChildren(treeViewId: string, treeItemHandle?: string): Promise<ITreeItem[] | undefined>;
-	$handleDrop(destinationViewId: string, treeDataTransfer: DataTransferDTO, newParentTreeItemHandle: string, token: CancellationToken, operationUuid?: string, sourceViewId?: string, sourceTreeItemHandles?: string[]): Promise<void>;
+	$handleDrop(destinationViewId: string, treeDataTransfer: DataTransferDTO, targetHandle: string | undefined, token: CancellationToken, operationUuid?: string, sourceViewId?: string, sourceTreeItemHandles?: string[]): Promise<void>;
 	$handleDrag(sourceViewId: string, sourceTreeItemHandles: string[], operationUuid: string, token: CancellationToken): Promise<DataTransferDTO | undefined>;
 	$setExpanded(treeViewId: string, treeItemHandle: string, expanded: boolean): void;
 	$setSelection(treeViewId: string, treeItemHandles: string[]): void;

src/vs/workbench/api/common/extHostTreeViews.ts

@@ -135,7 +135,7 @@ export class ExtHostTreeViews implements ExtHostTreeViewsShape {
 		return treeView.getChildren(treeItemHandle);
 	}
 
-	async $handleDrop(destinationViewId: string, treeDataTransferDTO: DataTransferDTO, newParentItemHandle: string, token: CancellationToken,
+	async $handleDrop(destinationViewId: string, treeDataTransferDTO: DataTransferDTO, targetItemHandle: string | undefined, token: CancellationToken,
 		operationUuid?: string, sourceViewId?: string, sourceTreeItemHandles?: string[]): Promise<void> {
 		const treeView = this.treeViews.get(destinationViewId);
 		if (!treeView) {
@@ -146,7 +146,7 @@ export class ExtHostTreeViews implements ExtHostTreeViewsShape {
 		if ((sourceViewId === destinationViewId) && sourceTreeItemHandles) {
 			await this.addAdditionalTransferItems(treeDataTransfer, treeView, sourceTreeItemHandles, token, operationUuid);
 		}
-		return treeView.onDrop(treeDataTransfer, newParentItemHandle, token);
+		return treeView.onDrop(treeDataTransfer, targetItemHandle, token);
 	}
 
 	private async addAdditionalTransferItems(treeDataTransfer: IDataTransfer, treeView: ExtHostTreeView<any>,
@@ -452,9 +452,9 @@ class ExtHostTreeView<T> extends Disposable {
 		return !!this.dndController?.handleDrag;
 	}
 
-	async onDrop(treeDataTransfer: vscode.DataTransfer, targetHandleOrNode: TreeItemHandle, token: CancellationToken): Promise<void> {
-		const target = this.getExtensionElement(targetHandleOrNode);
-		if (!target || !this.dndController?.handleDrop) {
+	async onDrop(treeDataTransfer: vscode.DataTransfer, targetHandleOrNode: TreeItemHandle | undefined, token: CancellationToken): Promise<void> {
+		const target = targetHandleOrNode ? this.getExtensionElement(targetHandleOrNode) : undefined;
+		if ((!target && targetHandleOrNode) || !this.dndController?.handleDrop) {
 			return;
 		}
 		return asPromise(() => this.dndController?.handleDrop

src/vs/workbench/browser/parts/views/treeView.ts

@@ -1419,7 +1419,7 @@ export class CustomTreeViewDragAndDrop implements ITreeDragAndDrop<ITreeItem> {
 
 	async drop(data: IDragAndDropData, targetNode: ITreeItem | undefined, targetIndex: number | undefined, originalEvent: DragEvent): Promise<void> {
 		const dndController = this.dndController;
-		if (!originalEvent.dataTransfer || !dndController || !targetNode) {
+		if (!originalEvent.dataTransfer || !dndController) {
 			return;
 		}
 		const treeDataTransfer: IDataTransfer = new Map();
@@ -1443,7 +1443,7 @@ export class CustomTreeViewDragAndDrop implements ITreeDragAndDrop<ITreeItem> {
 				}
 			}
 
-			if (!originalEvent.dataTransfer || !targetNode) {
+			if (!originalEvent.dataTransfer) {
 				return;
 			}
 			for (const dataItem of originalEvent.dataTransfer.items) {

src/vs/workbench/common/views.ts

@@ -830,7 +830,7 @@ export interface ITreeViewDragAndDropController {
 	readonly dropMimeTypes: string[];
 	readonly dragMimeTypes: string[];
 	handleDrag(sourceTreeItemHandles: string[], operationUuid: string, token: CancellationToken): Promise<IDataTransfer | undefined>;
-	handleDrop(elements: IDataTransfer, target: ITreeItem, token: CancellationToken, operationUuid?: string, sourceTreeId?: string, sourceTreeItemHandles?: string[]): Promise<void>;
+	handleDrop(elements: IDataTransfer, target: ITreeItem | undefined, token: CancellationToken, operationUuid?: string, sourceTreeId?: string, sourceTreeItemHandles?: string[]): Promise<void>;
 }
 
 export interface IEditableData {

src/vscode-dts/vscode.d.ts

@@ -9860,10 +9860,10 @@ declare module 'vscode' {
 		 * Extensions should fire {@link TreeDataProvider.onDidChangeTreeData onDidChangeTreeData} for any elements that need to be refreshed.
 		 *
 		 * @param dataTransfer The data transfer items of the source of the drag.
-		 * @param target The target tree element that the drop is occurring on.
+		 * @param target The target tree element that the drop is occurring on. When undefined, the target is the root.
 		 * @param token A cancellation token indicating that the drop has been cancelled.
 		 */
-		handleDrop?(target: T, dataTransfer: DataTransfer, token: CancellationToken): Thenable<void> | void;
+		handleDrop?(target: T | undefined, dataTransfer: DataTransfer, token: CancellationToken): Thenable<void> | void;
 	}
 
 	/**