src/vs/workbench/contrib/debug/browser/watchExpressionsView.ts/WatchExpressionsDragAndDrop/onDragOver
class WatchExpressionsDragAndDrop:	onDragOver(data: IDragAndDropData): boolean | ITreeDragOverReaction {
		if (!(data instanceof ElementsDragAndDropData)) {
			return false;
		}

		const expressions = (data as ElementsDragAndDropData<IExpression>).elements;
		return expressions.length > 0 && expressions[0] instanceof Expression;
	}