Watch expressions should not expand in drag & drop

fixes #16639

src/vs/workbench/parts/debug/electron-browser/debugViewer.ts

@@ -18,7 +18,7 @@ import { IMouseEvent, DragMouseEvent } from 'vs/base/browser/mouseEvent';
 import { getPathLabel } from 'vs/base/common/labels';
 import { IAction, IActionRunner } from 'vs/base/common/actions';
 import { IActionItem, Separator } from 'vs/base/browser/ui/actionbar/actionbar';
-import { ITree, IAccessibilityProvider, ContextMenuEvent, IDataSource, IRenderer, DRAG_OVER_ACCEPT, IDragAndDropData, IDragOverReaction } from 'vs/base/parts/tree/browser/tree';
+import { ITree, IAccessibilityProvider, ContextMenuEvent, IDataSource, IRenderer, DRAG_OVER_ACCEPT, DRAG_OVER_REJECT, IDragAndDropData, IDragOverReaction } from 'vs/base/parts/tree/browser/tree';
 import { InputBox, IInputValidationOptions } from 'vs/base/browser/ui/inputbox/inputBox';
 import { DefaultController, DefaultDragAndDrop } from 'vs/base/parts/tree/browser/treeDefaults';
 import { IActionProvider } from 'vs/base/parts/tree/browser/actionsRenderer';
@@ -1071,7 +1071,11 @@ export class WatchExpressionsDragAndDrop extends DefaultDragAndDrop {
 	}
 
 	public onDragOver(tree: ITree, data: IDragAndDropData, target: Expression | Model, originalEvent: DragMouseEvent): IDragOverReaction {
-		return DRAG_OVER_ACCEPT;
+		if (target instanceof Expression || target instanceof Model) {
+			return DRAG_OVER_ACCEPT;
+		}
+
+		return DRAG_OVER_REJECT;
 	}
 
 	public drop(tree: ITree, data: IDragAndDropData, target: Expression | Model, originalEvent: DragMouseEvent): void {