waitForElement.js

const waitForElement = (/** @type {string}*/selector) => {
  return new Promise((resolve) => {
    const observer = new MutationObserver(() => {
      const target = document.querySelector(selector)
      if (target) {
        observer.disconnect()
        resolve()
      }
    })
    observer.observe(document, { childList: true, subtree: true, attributes: false })
  })
}