lobste.rs top level comment search using the root level comment input. If you're using the bookmarklet, save it as a bookmark and then click it once when you want to search top-level comments. Otherwise just copy the contents into console.

00 - lobsters top level comment search.js

javacript:void((() => {
  const topLevelComments = [...document.querySelectorAll('ol.comments > li > div')].slice(1),
    hide = (node) => { node.closest('li').style.display = 'none' },
    show = (node) => { node.closest('li').style.display = null },
    reset = () => { topLevelComments.forEach(show) };

  window.query = (query) => {
    // Convenient for using it as an event handler
    if (query && query.target) { query = query.target.value } 

    // If it's empty string or falsy value, just reset everything
    if (!query) return reset() 

    // Stole this from the hn search.js console snippet:
    // https://gist.github.com/kristopolous/19260ae54967c2219da8#file-hn_seach-js-L26
    // just turns strings into case-insensitive Regexes
    query = query.test ? query : new RegExp(query.toString(), 'i') 

    topLevelComments.filter((el) => !query.test(el.innerText)).forEach(hide)
  }

  // Hijacking the top comment textarea because I am lazy and it's a nicer interface than going into console
  document.getElementById('comment').onkeyup = window.query 
})())

01 - bookmarklet version.js

javascript:void((()=>{const e=[...document.querySelectorAll("ol.comments > li > div")].slice(1),t=e=>{e.closest("li").style.display="none"},l=e=>{e.closest("li").style.display=null};window.query=(n=>{n&&n.target&&(n=n.target.value),n?(n=n.test?n:new RegExp(n.toString(),"i"),e.filter(e=>!n.test(e.innerText)).forEach(t)):e.forEach(l)}),document.getElementById("comment").onkeyup=window.query})())