How to make a sticky popup for chrome extension

I am new to chrome extension. And I need to create an extension That Appends the external class to the selected tag.

ex:- page

<h1>extension</h1> need to add an external class to the DOM when this tag mouse is clicked on the element. (should not be an empty element)

For that, I am creating a chrome extension. In order to do this, I need to have a fixed popup or Canvas. Like the CSS Selector use.

I want my popup.html to close only when I click on the close button, not when I click on the element of the page. I can not find a way to stick the popup. it is closed every time I clicked on the web page.

this is my manifest code

{
"manifest_version": 2,
"name":"Spider_Extension",
"version":"1.0",
"permissions": [
    "tabs"
  ],
"content_scripts":[{
    "matches":[
        "<all_urls>"
    ],
    "js":["Content.js"]
}],

"background": {
    "scripts": ["background.js"]        
  },
"browser_action":{
    "default_icon":"get_started16.png"
 }
}

this is my background.js

function button_clicked(tab){
let msg={
   txt:"hello"

}
console.log(tab)
chrome.tabs.sendMessage(tab.id,msg);



}

this is my popup.html

<html>
​
<head>
   <title>Spider extension</title>
   <link rel="stylesheet" href="styles.css">
</head>
​
<body>
​
   <label for="cars">Select Level:</label>
   <select id="levels">
       <option value="l1">L1</option>
       <option value="l2">L2</option>
       <option value="l3">L3</option>
       <option value="l4">L4</option>
   </select>
   <br>
   <button id="addLevel" style="margin:5px;">Add level</button>
   <br>
   <button id="break" style="margin:5px;">Break</button>
   <br>
   <button id="deselect" style="margin:5px;">Deselect</button>
   <br>
​
   <button id="nd__close">X</button>
​
   <script src="popup.js"></script>
</body>
​
</html>

can anyone guide me for a solution ?

thank you in advance