fix tree dnd drag image

fixes #12311

src/vs/base/parts/tree/browser/tree.ts

@@ -558,6 +558,11 @@ export interface IDragAndDrop {
 	 */
 	getDragURI(tree: ITree, element: any): string;
 
+	/**
+	 * Returns a label to display when dragging the element.
+	 */
+	getDragLabel?(tree: ITree, elements: any[]): string;
+
 	/**
 	 * Sent when the drag operation is starting.
 	 */

src/vs/base/parts/tree/browser/treeView.ts

@@ -1277,17 +1277,20 @@ export class TreeView extends HeightMap {
 		e.dataTransfer.effectAllowed = 'copyMove';
 		e.dataTransfer.setData('URL', item.uri);
 		if(e.dataTransfer.setDragImage) {
-			if (elements.length === 1 && item.element) {
-				e.dataTransfer.setDragImage(item.element, e.offsetX || 6, e.offsetY || 6);
-
-			} else if (elements.length > 1) {
-				var dragImage = document.createElement('div');
-				dragImage.className = 'monaco-tree-drag-image';
-				dragImage.textContent = '' + elements.length;
-				document.body.appendChild(dragImage);
-				e.dataTransfer.setDragImage(dragImage, -10, -10);
-				setTimeout(() => document.body.removeChild(dragImage), 0);
+			let label: string;
+
+			if (this.context.dnd.getDragLabel) {
+				label = this.context.dnd.getDragLabel(this.context.tree, elements);
+			} else {
+				label = String(elements.length);
 			}
+
+			const dragImage = document.createElement('div');
+			dragImage.className = 'monaco-tree-drag-image';
+			dragImage.textContent = label;
+			document.body.appendChild(dragImage);
+			e.dataTransfer.setDragImage(dragImage, -10, -10);
+			setTimeout(() => document.body.removeChild(dragImage), 0);
 		}
 
 		this.currentDragAndDropData = new dnd.ElementsDragAndDropData(elements);

src/vs/workbench/parts/files/browser/views/explorerViewer.ts

@@ -757,6 +757,15 @@ export class FileDragAndDrop implements IDragAndDrop {
 		return stat.resource.toString();
 	}
 
+	getDragLabel(tree: ITree, elements: any[]): string {
+		if (elements.length > 1) {
+			return String(elements.length);
+		}
+
+		const stat = elements[0] as FileStat;
+		return paths.basename(stat.resource.fsPath);
+	}
+
 	public onDragStart(tree: ITree, data: IDragAndDropData, originalEvent: DragMouseEvent): void {
 		const sources: FileStat[] = data.getData();
 		let source: FileStat = null;