@lumino/widgets/style/accordionpanel.css

Lumino Widgets

/*
 * Copyright (c) Jupyter Development Team.
 * Distributed under the terms of the Modified BSD License.
 */

.lm-AccordionPanel[data-orientation='horizontal'] > .lm-AccordionPanel-title {
  /* Title is rotated for horizontal accordion panel using CSS */
  display: block;
  transform-origin: top left;
  transform: rotate(-90deg) translate(-100%);
}