sidebarAction.isOpen()

let gettingIsOpen = browser.sidebarAction.isOpen(
  details // object
)

browser.sidebarAction.isOpen({}).then((result) => {
  console.log(result);
});

async function checkWindow(windowId) {
  const result = await browser.sidebarAction.isOpen({ windowId });
  console.log(`window: ${windowId} status: ${result}`);
}
browser.windows.getAll().then((all) => {
  for (const { id } of all) {
    checkWindow(id);
  }
});