code: function fixRootPageScroll (shell, {sourcePageId, targetPageId} = {}) {
    let page = window.MIP.viewer.page
    if (sourcePageId === page.pageId) {
      document.documentElement.classList.add('mip-no-scroll')
      page.getElementsInRootPage().forEach(e => e.classList.add('mip-hide'))
    }
    if (targetPageId === page.pageId) {
      document.documentElement.classList.remove('mip-no-scroll')
      page.getElementsInRootPage().forEach(e => e.classList.remove('mip-hide'))
      shell.restoreScrollPosition()
    }
  }
docs: Disable scrolling of root page when covered by an iframe
  NOTE: it doesn't work in iOS, see `_lockBodyScroll()` in viewer.js