async __calculateHeight(contentNode) {
    contentNode.style.setProperty('max-height', '');
    // @ts-ignore
    // eslint-disable-next-line no-promise-executor-return
    await new Promise(resolve => requestAnimationFrame(() => resolve()));
    return this._contentHeight; // Expected height i.e. actual size once collapsed after animation
  }
Calculate total content height after collapsible opens
@param {HTMLElement} contentNode content node
@private