PanelEditor: fix button position when dragging row (#27666)

public/app/features/dashboard/panel_editor/QueryEditorRows.tsx

@@ -108,6 +108,7 @@ export class QueryEditorRows extends PureComponent<Props> {
                     inMixedMode={props.datasource.meta.mixed}
                   />
                 ))}
+                {provided.placeholder}
               </div>
             );
           }}