slack-expand-unread.js

setInterval(() => {
  for (const sel of [
    ".c-button",
    ".p-ia__view_header__button_text",
    ".p-unreads_view__show_newer",
  ])
    for (const e of document.querySelectorAll(sel))
      if (e.innerText && e.innerText.toLowerCase().includes("new message"))
        e.click();
}, 1000);