Raw Source
ni554n / Facebook - Fix Double Scrollbar on Messenger 

// ==UserScript==
// @name                Facebook - Fix Double Scrollbar on Messenger
// @description         Removes the useless additional scrollbar that apprears on the fullscreen Facebook Chats / Messenger.
// @version             1.1

// @namespace           io.github.ni554n
// @match               https://*.facebook.com/*
// @grant               GM_addStyle
// @run-at              document-idle

// @updateURL           https://openuserjs.org/meta/ni554n/Facebook_-_Fix_Double_Scrollbar_on_Messenger.meta.js
// @supportURL          https://github.com/ni554n/userscripts/issues
// @license             MIT

// @author              Nissan Ahmed
// @homepageURL         https://ni554n.github.io/
// @contributionURL     https://paypal.me/ni554n
// ==/UserScript==

/* Migration Notice */

console.log(`
 █████  ████████ ████████ ███████ ███    ██ ████████ ██  ██████  ███    ██ 
██   ██    ██       ██    ██      ████   ██    ██    ██ ██    ██ ████   ██ 
███████    ██       ██    █████   ██ ██  ██    ██    ██ ██    ██ ██ ██  ██ 
██   ██    ██       ██    ██      ██  ██ ██    ██    ██ ██    ██ ██  ██ ██ 
██   ██    ██       ██    ███████ ██   ████    ██    ██  ██████  ██   ████ 
                                                                           
                                                                           
`)

console.log(
`Thanks for using "Facebook - Fix Double Scrollbar on Messenger" userscript downloaded from OpenUserJS.

You are seeing this message because due to a change introduced in OpenUserJS,
future updates to this script will no longer be vaiable.

If you like to get updates in the future, please open your userscript manager program,
and remove the "Facebook - Fix Double Scrollbar on Messenger" script.

Then head over to this Github link and install this script again.
https://github.com/ni554n/userscripts/blob/master/facebook/fix-double-scrollbar#installation

It's very easy to do and will take only 3-4 clicks.

Sorry for the inconvenience!`
);


const css = `
:root {
  overflow-y: auto !important;
}`;

GM_addStyle(css);