contentScript.js

let shown = false;
window.onscroll = (evt) => {
    const commentEl = document.getElementById("comments");
    const bounding = commentEl.getBoundingClientRect();
    
    const height = (window.innerHeight || document.documentElement.clientHeight);
    if (height >= bounding.top) {
        if (!shown) alert("There be dragons here!");
        shown = true;
    }
};

manifest.js

{
    "name": "YouTube Dragons",
    "version": "1.0",
    "description": "There be dragons here!",
    "content_scripts": [
        {
          "matches": ["https://*.youtube.com/*"],
          "js": ["contentScript.js"]
        }
    ],
    "manifest_version": 2,
    "permissions": [
        "activeTab"
    ]
  }