gecko-dev/browser/base/content/test/sidebar/browser_sidebar_switcher.js

registerCleanupFunction(() => {
  SidebarUI.hide();
});

function showSwitcherPanelPromise() {
  return new Promise(resolve => {
    SidebarUI._switcherPanel.addEventListener("popupshown", () => {
      resolve();
    }, {once: true});
    SidebarUI.showSwitcherPanel();
  });
}

function clickSwitcherButton(querySelector) {
  let sidebarPopup = document.querySelector("#sidebarMenu-popup");
  let switcherPromise = Promise.all([
    BrowserTestUtils.waitForEvent(window, "SidebarFocused"),
    BrowserTestUtils.waitForEvent(sidebarPopup, "popuphidden"),
  ]);
  document.querySelector(querySelector).click();
  return switcherPromise;
}

add_task(function* () {
  // If a sidebar is already open, close it.
  if (!document.getElementById("sidebar-box").hidden) {
    ok(false, "Unexpected sidebar found - a previous test failed to cleanup correctly");
    SidebarUI.hide();
  }

  let sidebar = document.querySelector("#sidebar-box");
  yield SidebarUI.show("viewBookmarksSidebar");

  yield showSwitcherPanelPromise();
  yield clickSwitcherButton("#sidebar-switcher-history");
  is(sidebar.getAttribute("sidebarcommand"), "viewHistorySidebar", "History sidebar loaded");

  yield showSwitcherPanelPromise();
  yield clickSwitcherButton("#sidebar-switcher-tabs");
  is(sidebar.getAttribute("sidebarcommand"), "viewTabsSidebar", "Tabs sidebar loaded");

  yield showSwitcherPanelPromise();
  yield clickSwitcherButton("#sidebar-switcher-bookmarks");
  is(sidebar.getAttribute("sidebarcommand"), "viewBookmarksSidebar", "Bookmarks sidebar loaded");
});